POPULATION INVERSION IN HOLLOW CATHODE, PULSED AND POSITIVE C0LUI4N, DISCHARGES.byC o lin S idney W i l le t t , B.Sc , G rad, I n s t , P.A th e s i s subm itted fo r th e degree o f D octor o f Ph ilosophy o f th e U n iv e rs ity o f LondonD epartm ent o f P h y s ic s , Royal Holloway C o lleg e , E n g le f ie ld Green, S u rre y .Departm ent o f P h y s ic s , U n iv e rs ity o f York, H eslin g to n ,York,December, 1966.ProQuest Number: 10096739All rights reservedINFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted.In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed,a note will indicate the deletion.uest.ProQuest 10096739Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author.All rights reserved.This work is protected against unauthorized copying under Title 17, United States Code.Microform Edition © ProQuest LLC.ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346- 2 -A b s tra c tF i r s t ly , a su rvey i s g iven o f e x c i ta t io n p ro c e s se s o c c u rrin g in gasd is c h a rg e s , to g e th e r w ith an a n a ly s is o f s e le c t iv e e x c i ta t io n p ro c e sse s ingaseous o p t i c a l raasers.Hollow cathode d isc h a rg e s have been in v e s t ig a te d a s e x c i ta t io nmedia fo r gas l a s e r s .Evidence i s g iven fo r th e e s ta b lish m e n t o f a r e la t io n s h ip betweenth e 6 3 2 .8nm helium -neon l a s e r d isch a rg e and t r a n s i t i o n ’weak to s t ro n g ’d is c h a rg e s . An optimum e le c tro n tem pera tu re fo r o s c i l l a t i o n on th e 6 3 2 .8nml i n e i s in d ic a te d .In a p u lsed h e liu m -io d in e d isc h a rg e , o s c i l l a t i o n i s re p o r te d on fo u rnew l a s e r t r a n s i t i o n s , one an in te rc o m b in a tio n t r a n s i t i o n . S im ultaneouso s c i l l a t i o n f o r th e f i r s t tim e i s re p o r te d on h y p e rf in e components o f th eio n is e d io d in e l i n e s a t 7 0 3 . 3 , 6 3 8 . 3 , 6 1 2 . 7 , 6 0 6 . 9 , 5 7 6 .0 and 3 6 7 .8nm.1 3 ! 3H yperfine i n t e r v a l s o f th e 6p and 6s l e v e l s have been e s ta b l is h e d ,g iv in g agreem ent between c a lc u la te d and observed h y p e rfin e components o f l i n e s a t 6 3 8 . 3 , 6 0 6 .9 and 3 76 .Onm. These le v e ls a re shown to be s tro n g lyp e r tu rb e d .An anomalous behav iou r i s re p o r te d on h y p e rfin e o s c i l l a t i o n s on th e 638.31101 l i n e , which can be a t t r i b u t e d to n o n -eq u ilib riu m in th e p o p u la tio n o f h y p e rf in e su b le v e ls o f th e 6s l e v e l , due to p re s s u re /c u r r e n t changes.R e su lts su p p o rt th e p ro p o sa l th a t charge t r a n s f e r i s th e s e le c t iv e e x c i ta t io n mechanism in th e h e liu m -io d in e l a s e r .- 3 -C onten tsA b s tra c t 2N otes re g a rd in g s e c t io n s , d iagram s, re fe re n c e s e t c . 6N om enclature 6C hapter 1In tro d u c t io n 91.1 The problem s d e a l t w ith in t h i s work 101 .2 G eneral c o n s id e ra t io n 111 .3 O s c i l l a t io n req u irem en ts 12C hapter 2 S e le c tiv e e x c i ta t io n p ro cesse s in gas d isc h a rg e s2 .1 T h e o re tic a l tre a tm e n t 132 .2 Resonance e x c i ta t io n energy t r a n s f e r 172 .3 Charge t r a n s f e r 172 .4 Atom-molecule c o l l i s io n s , d is s o c ia t iv ee x c i ta t io n t r a n s f e r 202 .3 E le c tro n im pact 21Diagrams 24C hapter 3 E x c i ta t io n p ro c e sse s in d isch a rg e s used a so p t i c a l maser mediaIn tro d u c t io n 263*1 The glow d isch a rg e 273 .2 Rf d isc h a rg e s 323 .3 The Hollow Cathode D ischarge (H .C.D.) 373 .4 P u lsed d isc h a rg e s 423*3 S im i la r i ty o f th e n e g a tiv e glow o f th e glow 47d isch a rg e and the a f te rg lo w o f a p u lsed d isc h a rg e .»Diagrams ^0- 4 -PageC hap ter 4 S p e c if ic O s c i l l a to r Systems In tro d u c t io n4 .1 Atora-atom, e x c i ta t io n t r a n s f e r c o l l i s io n m asers 314 .2 Charge t r a n s f e r c o l l i s io n m asers 634 .3 Atom-molecule l a s e r s 674 .4 E le c tro n im pact - pu re noble gas m asers 69Diagrams 76C hap ter 3 P o p u la tio n in v e rs io n in th e n eg a tiv e glow o f aHollow Cathode D ischargeIn tro d u c t io n 813 .1 S p e c if ic Systems 833 .2 The neon-alum inium , m etal vapour system 883 .3 E xperim ental A pparatus, neon-aluminium system 973 . 4 E xperim ental p ro ced u re , neon-alum inium system 993 . 3 R e su lts 1013 .6 Gain and o s c i l l a t i o n requ irem en t 1033 . 7 The s p u tte re d p o p u la tio n in th e H.C.D. 1043 .8 I n te n s i ty v a r ia t io n o f th e A1 I I 3 9 0 .Onm l in ein v a r io u s system s IO73 .9 Summary, neon-aluminium m etal vapour system 1093.10 Experimental apparatus IO93 .1 1 E xperim ental procedure 1123 .1 2 H elium -neon, and neon a t low p re s su re 1133 .1 3 R e su lts and D iscu ssio n s I I 8Diagrams 121- 5 -PageC hap ter 6 P o p u la tio n In v e rs io n in p o s i t iv e column d isch a rg e sIn tro d u c t io n I 336.1 P o p u la tio n in v e rs io n problem s 1356 .2 D ischarge p ro p e r t ie s o f p o s i t iv e column andr f l a s e r d isc h a rg e s 1376 .3 The r e la t io n s h ip between th e 63 2 .8nm helium -neon l a s e r d isch a rg e and t r a n s i t i o n d isc h a rg e s 139Diagrams and ta b le s l43C hap ter 7 P o p u la tio n In v e rs io n in p u lsed d isc h a rg e s -th e H elium -iod ine systemIn tro d u c t io n l467 .1 E xperim ental A pparatus l487 .2 E xperim ental d e t a i l s and p rocedure 1517 .3 T reatm ent o f R e s u lts and C onclusions I 60T ab les and diagram s 172Summary 194F u tu re Work 195Acknowledgements 199A ppendices: C hapter 4 . 2015 . 2067 . 209R eferences 212A ppendices: A, B, C and D.- 6 -Note re g a rd in g s e c t io n s , d iagram s, re fe re n c e s e t c .S e c t io n s . Each c h a p te r i s d iv id ed in to s e c tio n s and su b -se c tio n s r e fe r re d to th u s : - 2 .2 , 3*3»1 e t c , w ith o u t th e words c h a p te r o r s e c t io n e t c . E q u a tio n s , These a re numbered s e r i a l l y w ith in each c h a p te r and a re quoted in p a re n th e s is th u s (2 .1 ) , (4 .1 6 ) w ith o u t th e word c h a p te r .Diagrams and T ab les . These a re c o l le c te d a t th e ends o f th e c h a p te rs , ( a p a r t from F ig , 3*3), and r e f e r r e d to a s F ig , 1 .2 , 4 .3 , Table 6 .1 e t c . R e fe re n c e s . These a re numbered s e r i a l l y in each c h a p te r , a re c o l le c te d a t th e end o f th e th e s i s and ta b u la te d under each c h a p te r .N om enclature. The fo llow ing summarises th e n o ta tio n used th roughout th e t e x t where an ex p la n a tio n i s no t g iv en .G en era l., ga in c o e f f ic ie n t (cm ^)^21’E in s te in A c o e f f i c ie n t , t r a n s i t i o n 2 —*-D, diam eter o f d isch a rg e tubeA E, energy d isc rep an cy in e le c tro n v o l t sa , e le c tro n ic chargeeV, e le c tro n v o l tE, e l e c t r i c f i e ld s t re n g thf , frequency o f o s c i l l a t i o nS i 1 S2 * s t a t i s t i c a l w eights l e v e ls 1 and 2®o’f r a c t io n a l g a in (d is ta n c e )" ^ a t l i n e c e n treG, c a r r i e r gas atomh , P lanck c o n s ta n t- 7 -K, change in p ro ro g a tio n v e c to r o f in c id e n t e le c tro nk , Boltzmann co n s ta n tK.E, k in e t ic energyatom mean f r e e p a thy\ e le c tro n mean f re e p a th7\Q, w avelength a t l i n e c e n tre, d if fu s io n le n g th o f c o n ta in e r ,m, mass o f e le c tro nM, mass o f atomn , r a t e o f e x c i ta t io n o f le v e lN , e le c tro n c o n c e n tra tio ne ’atom d e n s ity le v e ls 1 and 2"\) , frequency o f t r a n s i t io np , gas p re s su rer , c r i t i c a l r a d iu s , (B ates and Damgaard) a t which p o te n t ia l® i s w ith in 1 p e r cen t o f asym pto tic form.R, ra d iu s of d isch a rg e tubeR^, R^, i n i t i a l and f i n a l s t a t e wave fu n c tio n sT, a b so lu te tem peratu reT^, average e le c tro n tem pera tu reX, , r a d ia t iv e l i f e t im eV, v e lo c ityV, p o te n t ia l in v o l t sCT, s tan d a rd d e v ia tio nd if fe re n c e in phase from maximum in t e n s i ty— 8 —S p e c tro sc o p ic , and c o l l i s i o n a l n o ta t io n .E le c tro n , eN e u tra l atom, 0 ; e x c ite d atom , 0 ; m e ta s ta b le atom , 0* ,^tIo n , 1 ; e x c ite d io n , 1 ,Photon , (p(Example o f c o l l i s io n r e a c t io n , ( r e f . 2 (1 1 ) ) , lO /O l /Ql(j> ,in d ic a t in g sp a rk l i n e enhancem ent.)* *He , Ne , m e ta s ta b le helium and neon atom r e s p e c t iv e ly .fH , e x c i te d helium atom .N e u tra l l i n e , — I ; io n is e d l i n e , — I I .W avelengths g iven in nanom etres (nm) and m icrons w ith o u t th e p r e f ix 7\ •- 9 -CHAPTER 1In tro d u c t io nT his th e s i s covers work c a r r ie d ou t in th e p e r io d I 963 to 196% and in c lu d e s p a r t o f one o f th e most f e r t i l e , and p robab ly th e most r a p id ly moving p e r io d in P h y s ic s , which fo llow ed th e dem onstra tio n o f o p t i c a l maser a c t io n in ruby by Maiman (1) in I 96O. In th e wake o f t h i s dem onstra tio n has fo llow ed th e o b se rv a tio n o f harmonic g e n e ra tio n , s tim u la te d Raman em issio n , and n o n - lin e a r e f f e c t s produced by th e ex trem ely la rg e in t e n s i t y o f t h i s o p t i c a l l y pumped s o l id s t a t e o p t i c a l m aser.With l e s s s p e c ta c u la r r e s u l t s than th o se shown by th e ruby l a s e r , th e gaseous o p t i c a l maser was developed in I 96I by Javan , B ennett and H a r r io t t(2 ) . In l i e u o f th e megawatt power c a p a b i l i t i e s o f t h e i r fo re ru n n e r , th e gaseous o p t i c a l maser p o sse sse s v i r tu e s o f s p e c t r a l p u r i ty , beam d i r e c t io n a l i t y and s p a t i a l coherence enorm ously s u p e r io r to th o se o f th e s o l id s t a t e l a s e r . At p r e s e n t , con tinuous o s c i l l a t i o n has been ach ieved in gas l a s e r s in n e u t r a l , s in g ly and doubly io n ise d s p e c ie s , on over 100 t r a n s i t i o n s ran g in g from th e n ea r u l t r a - v i o l e t in to th e m iddle in f r a - r e d and beyond. As w e ll a s con tinuous o p e ra tio n , o s c i l l a t i o n on over 700 t r a n s i t i o n s has been observed under t r a n s ie n t o r p u lsed c o n d itio n s a t peak powers o f up to 200kW.A lthough much o f th e work on l a s e r s has been te c h n o lo g ic a l, th e b o rd e r l in e between technology and pure p h y s ic s has been such th a t an in c re a s e d u n d ers tan d in g o f atom ic p h y s ic s has fo llow ed each te c h n o lo g ic a l advance. Perhaps more im p o rta n t, one has been shown g lim pses- 10 -o f what one does n o t know, in an a re a where th e p h y s ic i s t , chem ist, e l e c t r i c a l en g in ee r and s p e c tro s c o p is t can m eet, co nverse , and a l l c o n t r ib u te .Because o f 1) th e r a p id i ty w ith which advances have been made in t h i s newly r i s e n f i e l d o f O p tic a l P h y s ic s , and 2) th e co m p e titiv en e ss o f th e work, p u b lic a t io n s have o f te n been incom plete o r frag m en ta ry . T his has n e c e s s i ta te d th e ra p id a s s im i la t io n o f new id e a s and te c h n iq u e s , and has s tim u la te d th roughou t th e se a rc h fo r c l a r i f y in g l in k s w ith o th e r , and e a r l i e r work.Up to now, work in t h i s new a re a has le d to s t r ik i n g advances. A c o n s o lid a tio n p e r io d would appear now to be re q u ire d , in which a b e t t e r u n d e rs tan d in g i s sought o f th e p ro cesse s which one so s u c c e s s fu lly m a n ip u la te s . With th e s o p h is t ic a te d exp erim en ta l te ch n iq u es now a v a i la b le , i t would seem an a p p ro p r ia te tim e to re p e a t a number o f experim en ts, conducted - w ith l im ite d te ch n iq u es - du rin g th e 1930 to 1950 p e r io d in th e background a re a o f O p tic a l P h y s ic s .1 ,1 The problem s d e a l t w ith in t h i s work.The work covered in t h i s th e s i s can be d iv id ed in to fo u r main s e c t io n s .In th e f i r s t s e c t io n . C hapters 2 - 4 , fo llo w in g th e g en e ra l c o n s id e ra tio n s and o s c i l l a t i o n req u irem en ts o f C hapter 1 , e f f o r t has been d ir e c te d tow ards a n a ly s in g a number o f gaseous o p t ic a l m asers and c la s s i f y in g th e atom ic p ro c e sse s th a t occur in them. I t i s on ly a t t h i s p o in t th a t th e m ajor s e le c t iv e e x c i ta t io n p ro cesse s a re becoming d isc e rn a b le ,- 11 —In the second s e c t io n , covered in C hapter 5 , acco u n ts a re g iven o f th e experim ents c a r r ie d ou t on hollow cathode d isc h a rg e s d u rin g the f i r s t p a r t o f th e p r o je c t . At th e commencraent o f th e work t h i s type of d isch a rg e had n o t been used a s an o p t ic a l maser medium, and i t was co n sid e re d th a t i t s use m ight y ie ld in fo rm a tio n on the im portance o f re so n a n t energy t r a n s f e r and m olecu lar p ro c e sse s in ach iev in g p o p u la tio n in v e rs io n .Throughout th e developm ent o f gaseous o p t ic a l m asers and in th e l i t e r a t u r e r e p o r ts o f 'enhancement* in i n e r t gas-vapour d isc h a rg e s (see C hapter 2 ) , th e e x ac t p a r t p lay ed by helium i s n o t c l e a r . In t h i s p r o je c t , t h i s le d to ,( i ) a s tu d y o f c e r t a in a s p e c ts o f th e helium -neon l a s e r , (C hapter 6 ) , and( i i ) experim ents be ing c a r r ie d ou t on a h e liu m -io d in e system o p e ra tin g under p u lsed c o n d itio n s (C hapter 7 )•1 .2 G eneral C o n s id e ra tio nExcept in some c a se s where la rg e o p t i c a l g a in s a re r e a l i s e d , o p t i c a l m asers have g e n e ra lly been used a s o s c i l l a t o r s and n o t a m p l i f i e r s .B a s ic a lly a maser o s c i l l a t o r d i f f e r s l i t t l e from tho se proposed by Schawlow and Townes (3 ) , and c o n s is ts o f an o p t i c a l re s o n a to r which su p p o rts th e o s c i l l a t i o n frequency and p ro v id es th e p o s i t iv e feedback n ece ssa ry fo r the b u ild up o f o s c i l l a t io n s from spontaneous em ission in to th e p a r t i c u l a r r e s o n a to r mode, and th e maser medium which p ro v id es th e p o p u la tio n in v e rs io n n ece ssa ry fo r maser a c t io n .- 12 -1 .3 O s c i l l a t io n req u irem en tsA gaseous d isc h a rg e medium s u p p lie s th e s tim u la te d em ission to b u ild up co h eren t o s c i l l a t i o n s from th e u su a l dominant spontaneous em ission on a t r a n s i t i o n . I f th e g a in on one p ass between th e re s o n a to r m irro rs i ss u f f i c i e n t to compensate fo r th e lo s s , th e i n t e n s i t y in th e re s o n a to r mode in c re a s e s u n t i l s tim u la te d em ission b u ild s up and competes w ith spontaneous em ission fo r atoms whose c e n tre f re q u e n c ie s a re w ith in a n a tu r a l l i n e w id th o f th e mode (4 ) .The req u irem en ts fo r o s c i l l a t i o n were f i r s t p u t forw ard by Schawlow and Townes (3 ) , in which th e y equated th e power produced by s tim u la te d em ission in th e c a v i ty mode to th e r a t e a t which energy decayed. The same r e s u l t i s o b ta in ed by eq u a tin g the g a in and lo s s p e r u n i t le n g th . The g a in by s tim u la te d em ission , and lo s s to a b so rp tio n , and i t s dependence on l i n e shape, i s c a lc u la b le u s in g c l a s s i c a l m ethods. In most case s o f low p re s s u re maser system s th e prim ary source o f l i n e broaden ing i s th e therm al motion o f th e atom s•Follow ing th e tre a tm e n t o f M itc h e ll and Zeraansky (3 ) , when th e n a tu ra l l i n e w idth and p re s s u re b roadening Doppler w id th ,th e ex p ress io n fo r th e f r a c t io n a l g a in /u n i t d is ta n c e (g^) a t th e c e n tre o f a Doppler broadened l in e i s g iven by:*^i/^i)(N3/A'6p)............................ ( i . i ) .I t i s ev id en t from ( l . l ) th a t to o b ta in ga in ” ^ l^ ^ l ^ ^ ' t h i sim poses l im i ta t io n s on th e l i f e t im e req u irem en ts of th e le v e ls s u i ta b le fo r cw m aser o p e ra tio n . The most fav o u rab le case occurs when only th e upper- 13 -s t a t e i s e x c i te d , and th e low er le v e l i s on ly p o p u la ted by r a d ia t iv e cascade in th e maser t r a n s i t i o n .The d e n s ity o f th e low er s t a t e s a t i s f i e s th e e q u a l i ty^21’ r e p re s e n ts th e t o t a l decay r a teo f th e low er t r a n s i t i o n . A s tead y s t a t e s o lu t io n e x i s t s so th a t ^2^ ^ 2 ^ ^ I '^ ^ l’ bhe t o t a l decay r a te o f th e low er s t a t e obeys th e in e q u a l i ty ,^ A^^ " Eor o b ta in in g maser o s c i l l a t i o n t h i s c o n d itio n i s n o ts u f f i c i e n t ; enough in v e rs io n must e x i s t so t h a t th e o p t ic a l ga in in a s in g le p ass exceeds th e lo s s e s p e r s in g le p a s s ; f o r sm all lo s s e s th e o s c i l l a t i o n c o n d itio n becomes0 ^ / 9 2 ) Wj )> ^jA2, iJha/ny'2 .........The s te a d y s t a t e p o p u la tio n s a re determ ined by th e l i f e t im e s ( T, ) and ( T%) o f th e le v e l s and th e r a t e o f e x c i ta t io n (r^ and (n^) to th e se l e v e l s , o rI t can be seen from (1 .3 ) th a t even i f the l i f e t im e s and s t a t i s t i c a l w eigh ts a re fav o u rab le , p o p u la tio n in v e rs io n cannot be ach ieved u n le ss th e e x c i ta t io n r a t e to the upper l e v e l i s s u f f i c i e n t .Of c o n s id e ra b le im portance , a re th e l i f e t im e s Y ^ "^2 th e twomaser l e v e l s . In a gaseous d isch a rg e environm ent i t i s th e e f f e c t iv e l i f e t im e s which determ ine th e o s c i l l a t io n and g a in c o n d itio n s . These e f f e c t iv e l i f e t im e s can d i f f e r a p p re c ia b ly from p u re ly r a d ia t iv e ones, due to th e combined p ro c e sse s o f spontaneous decay, resonance tra p p in g and i n e l a s t i c c o l l i s io n s , ( 4 ) .— 14- —For a c o n s id e ra tio n o f th e d e te rm in a tio n o f e f f e c t iv e l i f e t im e s the re a d e r i s r e f e r r e d to "Measurement o f E x c ited S ta te R e lax a tio n R a te s" , by B ennett e t a l . (7 ) .T his th e s i s w i l l now be con fin ed to a tre a tm e n t and o b se rv a tio n s on th e way in which p o p u la tio n in v e rs io n i s r e a l i s e d in a d isc h a rg e .— 15 —CHAPTER 2S e le c tiv e e x c i ta t io n p ro c e sse s in gas d isc h a rg e s2 .1 T h e o re tic a l tre a tm e n tA number o f a n a ly t ic a l s tu d ie s o f p o p u la tio n in v e rs io n in gaseous d isc h a rg e s have been made, (Ward ( l ) , Javan (2 ) , F a b rik a n t (3) and Basov e t a l . ( 4 ) ) .In a l l c a s e s , th e s tu d ie s s t a r t w ith assumed c ro s s - s e c t io n s fo r e le c t r o n ic , o r c o l l i s i o n a l and d e -e x c i ta t io n p ro c e s se s , and an assumed! M axwellian e le c tro n energy d i s t r i b u t io n .L i t t l e p r a c t i c a l b e n e f i t has ensued from th e se c a lc u la t io n s and the degree o f agreem ent w ith ex perim en ta l s tu d ie s has n o t i n s t i l l e d confidence in t h i s approach to th e complex p ro c e sse s o f th e l a s e r system , in which one p la y s o f f one non-therm al e q u ilib r iu m p ro cess a g a in s t an o th e r in o rd e r to ach iev e p o p u la tio n in v e rs io n . S ince such c a lc u la t io n s up to 1962. (3)» had n o t even enab led advance p re d ic t io n s o f t r a n s i t io n s on which o s c i l l a t i o n cou ld be o b ta in ed , i t has become c le a r th a t more d a ta on c o l l i s io n c ro ss s e c t io n s o f a l l ty p es i s r e q u ire d . A lso th e assumed u n iv e rs a l a p p l i c a b i l i t y o f M axwellian e le c tro n energy d i s t r i b u t io n s to d isch a rg e s needs to be determ ined , b e fo re any th e o r e t i c a l tre a tm e n t o f c o l le c t iv e d isch a rg e p ro c e sse s i s more th an mere e x e rc is e .I t i s p o s s ib le , however, to unders tan d in a q u a l i t a t iv e way th e o p e ra tio n o f gaseous d isch a rg e o p t ic a l m asers by c o n s id e rin g th e dominant- 1 6 -p ro c e sse s t h a t have been u sed , and th o se t h a t appear to be used , to produce p o p u la tio n in v e rs io n .F ig . 2 .1 , and 2 . 2 , show sc h e m a tic a lly th e very g e n e ra l 4 and 3 le v e l s i tu a t io n s o b ta in ed in cw gas l a s e r system s, fo r n e u t r a l and io n l a s e r s r e s p e c t iv e ly .T ra n s it io n p r o b a b i l i t i e sT h e o re tic a l c a lc u la t io n s o f pure r a d ia t iv e t r a n s i t i o n p r o b a b i l i t i e s have been made by B ates and Damgaard (6) u s in g th e C e n tra l F ie ld approx im ation . For atom ic system s w ith s in g le e le c tro n s o u ts id e c lo sed e le c tro n s h e l l s , th e method o f B ates and Damgaard y ie ld s r e s u l t s in ex trem ely good agreem ent w ith experim en t, and in c lo s e r agreem ent th an r e s u l t s c a lc u la te d from th e more complex H artree-F ock d i f f e r e n t i a l e q u a tio n s .I n e l a s t i c c o l l i s io n sT h e o re tic a l tre a tm e n ts o f e l a s t i c and i n e l a s t i c c o l l i s io n s have been made by Massey and Burhop (7 ) , Townsend (8 ) , and Mott and Massey (9 ) ; and a tre a tm e n t o f i n e l a s t i c c o l l i s io n s between heavy p a r t i c l e s has been made by B ates (1 0 ) . F u l l rev iew s o f th e se th e o r e t ic a l tre a tm e n ts , to g e th e r w ith experim ents a re g iven in th e work o f H asted (1 1 ), (1 2 ), McDaniel (1 3 ) î and in Massey ( l 4 ) . and B ates and McCarrol (13 )•A number o f predom inant c o l l i s io n mechanisms have been observed in maser system s th u s f a r ; th e se in c lu d e :(1) Atom-atora, atom -ion e x c i ta t io n t r a n s f e r(2) Charge t r a n s f e r ,(3) A tom -m olecule, and(4) E le c tro n im pact c o l l i s io n s .- 17 -2 .2 R esonant e x c i ta t io n energy t r a n s f e r C o ll is io n p ro c e sse s o f th e typex ' + Y -4 » X + y ' - A E (0 0 /0 0 * ) .............(2 .1 )Iin which energy i s t r a n s f e r r e d from th e s t a t e X o f one atom to an o th e rIY have been s tu d ie d th e o r e t i c a l ly by Massey and Burhop (7 )•Theory and experim ent (16) shows th a t t h i s type o f c o l l i s i o n ; -(a ) i s h ig h ly re so n a n t fo r n e a r ex ac t energy co in c id en c e , w ith a c ro s s*10 2s e c t io n la r g e r th a n 100 x 10 cm ; w ith A E o f th e o rd e r o f a few* l6 2kT, c ro ss s e c t io n s o f app rox im ate ly 10 cm a re r e a l i s e d . For A E\ 0 .1 eV th e c ro s s s e c t io n s a re n e g l ig ib le a s shown in F ig . 2 .3( a f t e r Mott and M assey).(b) has a l a r g e r c ro s s s e c t io n when th e Wigner Spin Rule i s obeyed, th an when s p in i s no t conserved . However, t h i s r u le i s n o t always obeyed.2 .3 Charge t r a n s f e rR epresen ted in i t s s im p le s t form by a c o l l i s io n o f the typeX+ + Y X + Y+ + A E (10/01) (2 .2 )A E a s in (2 .1 ) i s th e change in energy d u rin g th e c o l l i s i o n .The energy dependence o f th e c ro s s - s e c t io n fo r a p a r t i c u la r charge t r a n s f e r r e a c t io n depends m arkedly on th e magnitude o f A E , a s d iscu ssed in M assey 's ( l l ) , (17) n ea r - a d ia b a t ic h y p o th e s is . The c ro s s s e c tio n fo r a c o l l i s io n in which an e le c t r o n ic t r a n s i t i o n o ccu rs , i s sm all a t gas k in e t ic c o l l i s io n v e l o c i t i e s u n le s s | A E) i s sm a ll. T his i s because th e g rad u a l c o l l i s io n enab les th e atom ic e le c tro n s to change w ith th e v a ry in g c o n d itio n s .-1 8 -As th e r e l a t i v e v e lo c i ty r i s e s , th e c ro s s s e c t io n in c re a s e s and reac h es a maximum when th e tim e o f in te r a c t io n (a /v ) i s com parable w ith th e tim e o f t r a n s i t i o n h / (A E | , where a i s th e range over which the i n t e r a c t io n o ccu rs , and v i s th e r e l a t i v e v e lo c i ty o f th e in t e r a c t in g p a r t i c l e s .Thus a IA E | /h v « ^ 1 ; (2 .3 )when IA e | i s sm all o r z e ro , th e maximum c ro s s s e c t io n occurs a t low en ergy . jA e | , however, i s a fu n c tio n o f th e s e p a ra t io n o f the c o l l id in g p a r t i c l e s , and i t i s custom ary to use ) A e ^ | , th e va lue o f j A Ej a t i n f i n i t e s e p a ra t io n . A E can a ls o be dom inated by Coulomb o r p o la r i s a t io n fo rc e s and c o r re c t io n s must be made to g ive a mean energy d e fe c t averaged over th e in te r a c t io n p a th ( l 8 ) .The p r o b a b i l i ty fo r e x c i ta t io n d ec re a se s r a p id ly a s v d e c re a se s , so t h a t fo r low e n e rg ie s , th e c ro s s s e c tio n i s sm all u n le ss A E i s re s o n a n t.For a re so n a n t r e a c t io n , th e m agnitude o f th e c ro ss s e c t io n , evidenced by 'enhancement* o r p r e f e r e n t i a l e x c i ta t io n ap p ea rs to be dependent on th e type o f l e v e l e x c ite d and n o t sim ply on th e energy d e fe c t . D uffendack and Thomson (19) found th a t t r a n s i t i o n s from t r i p l e t le v e ls o f th e spark spectrum o f copper were enlianced in neon more th an t r a n s i t io n s from s in g le t l e v e l s , a lth o u g h some s in g le t l e v e ls were c lo s e r to th e neon io n ground s t a t e th an t r i p l e t l e v e l s , (a s shown in F ig . 2 .4 ) , in a n ea r re so n a n t c o l l i s io n o f th e type"2 /2 'Ne' ^ + C u ( \ , , ) — Ne ( \ ) + Cu^(^P) + A E(10/01 ) .............(2 .4 ) .- 1 9 -S im ila r ly , D uffendack and Thomson observed th e 10/01 /O lÿ p ro cess in go ld and s i l v e r , in a helium a r c ; in each o f th e se c a se s th e re i s a predom inance o f t r i p l e t enhancement in accordance w ith th e Wigner Spin R u le . In th e case o f s i l v e r , a lth o u g h t r a n s i t i o n s a re enhanced from t r i p l e t l e v e ls w ith in O.leV o f th e helium io n ground s t a t e , o th e r t r a n s i t i o n s from le v e ls approx im ate ly 2eV away a re a ls o enhanced.Gran and D uffendack (20) have observed s im i la r enhancements in th e s , p , f and d le v e ls in th e io n spectrum o f le a d , b u t in which th e Wigner Spin Rule i s n o t obeyed.Non re so n a n t c o l l i s io n sA ccording to ( 2 .3 ) , th e maximum c ro s s s e c tio n s w i l l occur a t h ig h e n e rg ie s fo r n o n -reso n an t c o l l i s io n s ; L ip e le s e t a l . (2 1 ), however, have r e c e n t ly re p o r te d th a t in t h e i r s tu d ie s o f o p t ic a l e x c i ta t io n w ith low energy io n s la rg e maximum c ro s s s e c tio n s o f th e o rd e r o f 10 cm^ a re observed a t low e n e rg ie s , c o n tra ry to e x p e c ta t io n . These o b se rv a tio n s can be re c o n c ile d w ith (2 .3 ) by n o tin g th a t in g e n e ra l th e energy le v e ls o f th e m olecu lar complex formed du rin g th e c o l l i s io n a re s tro n g ly dependent on th e in te r n u c le a r s e p a ra t io n . Thus acco rd in g to L ip e le s e t a l , th e r e l a t i v e v e lo c i ty (v) and th e in te r a c t io n range a re fu n c tio n s o f the in te r n u c le a r s e p a ra t io n .SummaryI t can be seen th a t in charge t r a n s f e r c o l l i s io n s , th e energy d isc rep a n cy need n o t be a s sm all a s th a t re q u ire d fo r th e r e a l i s a t i o n o f la rg e c ro s s s e c tio n s in atom—atom resonance energy t r a n s f e r c o l l i s io n s .- 20 -2 .4 Atom-molecule c o l l i s io n s , d is s o c ia t iv e e x c i ta t io n t r a n s f e r .These c o l l i s io n s , which have n o t been s tu d ie d e x te n s iv e ly , tak e th e formM* + AB M + a ' + B (o r B*)................................ ( 2 .3 ) .In t h i s type o f c o l l i s io n more than two atoms a re in v o lv ed , so t h a ta d e s c r ip t io n o f th e r e a c t io n i s n o t s im p le , and th e o r e t ic a l e x p la n a tio n sbased on th e i n t e r a c t io n cu rves become too com plica ted fo r even th e sim ple case in v o lv in g a d ia tom ic m olecu le . For t h i s reaso n no q u a n t i ta t iv e th e o ry e x i s t s , though i t i s p o s s ib le to d is c u s s th e c o l l i s io n q u a l i t a t iv e ly ,tI t i s assumed th a t r e l a t i v e l y long range in te r a c t io n s between M and AB cause v e r t i c a l t r a n s i t io n s between th e ground s t a t e and e x c i te d ,I Ire p u ls iv e s t a t e s o f AB . AB th en d is s o c ia te s to g iv e , in g e n e ra l, one e x c i te d atom and one ground s t a t e atom . S ince each s t a t e o f th e p ro d u c tsI I I tA and B, o r A and B w i l l c o r r e la te w ith se v e ra l r e p u ls iv e AB cu rv es , and s in c e th re e atoms a re in v o lv ed , th e re i s no s tro n g resonance req u ire m en t. C ross s e c tio n s o f approx im ate ly 10 cm^, w ith A E o f 1 -2 eV have been no ted by B ennett (2 2 ) .I f th e p ro d u c ts o f th e d is s o c ia t iv e e x c i ta t io n t r a n s f e r a re d ia tom ic r a d ic a l s th e y may have r o ta t io n a l and v ib r a t io n a l a s w e ll a s e le c t ro n ic e x c i ta t io n , and p o p u la tio n in v e rs io n in r o ta t io n a l /v ib r a t io n a l l e v e ls can r e s u l t (23 )•- 21 _2 .5 E le c tro n im pactIn a gaseous d isch a rg e s e v e ra l c o l l i s io n mechanisms a re p re s e n t . E le c tro n im pact c o l l i s io n s upon atoms a re by f a r th e most common. Not a l l o f th e c o l l i s io n s le a d to changes o f s t a t e . For p o p u la tio n in v e rs io n to o ccu r, th e i n e l a s t i c c o l l i s io n s a re o f p rim ary i n t e r e s t , and e le c tro n im pact e x c i ta t io n has y ie ld e d more l a s e r o s c i l l a t io n s th an any o th e r s e le c t iv e e x c i ta t io n p ro c e s s .The c ro s s s e c t io n ^ ° r e x c i ta t io n from th e ground s t a t e (1) to an e x c i te d s t a t e (2) by e le c tro n im pact i s g iven by th e Born approx im ation (2 2 ), (24) a sU k (2 .6 ) ,where th e in c id e n t e le c tro n energy E^, and E^ i s th e e x c i ta t io n energy th re s h o ld ,I (2 .7 ) .The term on th e r i g h t hand s id e o f the p r o p o r t io n a l i ty in (2 .7 ) i s p ro p o r t io n a l to th e e l e c t r i c d ip o le m a trix elem ent fo r a r a d ia t iv e t r a n s i t i o n between an e x c ite d s t a t e and th e ground s t a t e . To a f i r s t app ro x im atio n , a t e le c tro n e n e rg ie s where e le c tro n exchange can be n e g le c te d ,^ 1 2 . ^ A l l • • • • • (2 .8 )where i s th e E in s te in t r a n s i t i o n p r o b a b i l i ty .- 22 -IT his Oe/O e p ro cess has i t s o n se t a t th e th re sh o ld energy , and i s e i th e r n o n -re so n an t o r b ro ad ly re so n a n t up to ap p rox im ate ly 100 - I 50 v o l t s , (o r 10 tim es th e e x c i ta t io n p o te n t i a l ) . A f te r p a s s in g th e maximum, th e p r o b a b i l i ty cu rves f a l l away s lo w ly .The above only ho lds fo r e x c i ta t io n o f atoms fo r which s p in - o r b i t co u p lin g i s n e g l ig ib le . For h e a v ie r and more complex atoms L-S coup ling i s n o t a good app rox im ation , and j - j coup ling i s obeyed.E le c tro n exchange c o l l i s io n sE le c tro n exchange c o l l i s io n s can a ls o r e s u l t in e x c i ta t io n in which s p in m u l t ip l i c i ty i s changed. Except in heavy atom s, t r a n s i t io n s in v o lv in g a change o f m u l t ip l ic i ty a re dom inated by e le c tro n exchange.Such c o l l i s io n s a re very im p o rtan t c lo se to th re sh o ld in th e case o f t r a n s i t i o n s in v o lv in g no change in th e az im u tha l quantum number, and have e x c i ta t io n fu n c tio n s w ith much sh a rp e r maxima th a n fo r o p t i c a l ly allow ed t r a n s i t i o n s . F ig . 2 .5 ( a f t e r B ennett (22 )) g iv e s a q u a l i t a t iv e i l l u s t r a t i o n o f th e energy dependence o f i n e l a s t i c e x c i ta t io n c ro s s s e c t io n s fo r (a ) o p t i c a l ly allow ed t r a n s i t io n s and (b) fo r e x c i ta t io n between le v e ls o f th e same p a r i t y .I o n is a t io n by e le c tro n im pact.C a lc u la t io n s fo r io n i s a t io n u s in g th e Born approx im ation quantum th e o ry a re s im i la r to tho se fo r e x c i ta t io n , and y ie ld s im ila r curves to- 23 -th o se o f e x c i ta t io n by e le c tro n im pact (K ie f fe r and Dunn (2 5 ) ) , Above th re s h o ld th e io n is a t io n fu n c tio n in c re a s e s l i n e a r ly w ith energy to a maximum s e v e ra l tim es th e io n is a t io n energy and th en f a l l s o f f g rad u a lly above t h i s .A part from work on helium and n itro g e n , th e review a r t i c l e o f K ie f fe r and Dunn shows th a t l i t t l e work has been p u b lish e d on io n is a t io n in to e x c i te d f i n a l s t a t e s . I t i s o f some re le v an ce to io n l a s e r s to know i f io n i s a t io n fu n c tio n s e x h ib i t re so n an ces .24 -2I t L a s e r T r a n s i t i o nR a d i a t i v e D e c a yaS.( M e t a s t a b l e )C o l l i s i o n sRaD i f f u s i o n( G r o u n d S t a t e )Ideal four-level system representative of m ost cw gas lasers.F ig . 2 .1LASER4sST impoctVacuum uve“ impoctArThree-level excitation model proposed by Gordon to explain the behavior of the small-bore cw Ar+ laser characteristics.F ig . 2 .225F ig . 2.30.00001 0,0001 0,001 0,01 0,1 Energy difference of initial & final states in voltsCalculated crosa-section for transfer of charge between two atoms as a function of the resonance defectLUzLUOz and i s e x c i te d in th e n e g a t iv e glow plasm a, The m a jo r i ty o f p a r t i c l e s e je c te d from th e ca th o d e a c c o rd in g to von H ippel ( 3 4 ) a re a to m ic . However, Honig (35) has shown tha tw hen 6OO eT p o s i t i v e io n s o f argon bombard a s i l i c o n c a rb id e c a th o d e , io n s p e c ie s S i^ , SiCg^^ SigC^ and SlgC^"** a re e m itte d in m easu rab le q u a n t i t i e s .Wehner and co -w orkers (35 ) - (3 9 ) , have d e te rm in ed s p u t t e r in g y ie ld s f o r low e n e rg y , 100 - 6OO eT i n e r t gas io n s , and have found th a t th e r e i s a p e r io d ic dependence on a tom ic num ber, s im i la r to th e r e c ip r o c a ls o f th e h e a t o f s u b l im a tio n . Peaks in y i e ld o ccu r a t m anganese, co p p er, s i l v e r and g o ld in th e p e r io d ic t a b l e .The e x a c t mechanism o f th e s p u t t e r in g p ro c e ss i s n o t y e t f u l l y u n d e rs to o d , b u t th e a c c e p te d e x p la n a tio n i s th a t*( i ) momentum i s t r a n s f e r r e d from th e in c id e n t io n to th e ca th o d es u r f a c e , and atoms, io n s , and m o lecu les a re e m itte d w ith v a ry in g v e l o c i t i e s from th e c a th o d e , (Townes, ( 4 0 ) ) .( i i ) e v a p o ra tio n o f atoms o ccu rs u n d er th e io n bombardment.Use as a s p e c tro s c o p ic so u rceB ecause o f th e s p u t t e r in g a c t io n , and low e l e c t r i c f i e l d s away from th e ca th o d e edge o f th e n e g a t iv e glow , th e H.C.D. has been u sed e x te n s iv e ly as a so u rce o f l i n e s p e c t r a o f m ost o f th e e lem en ts ( 2 4 ) , and i s o n ly su rp a s se d by th e atom ic beam and th e l a s e r in th e p ro d u c tio n o f s p e c t r a h av in g sm all l i n e - w id th s . I t s main advan tage ov er o th e r- 41 -ty p e s o f d is c h a rg e so u rce i s in e x c i t in g th e h ig h m e ltin g p o in t m e ta ls , f o r w hich th e s p u t t e r in g a c t io n can p roduce th e n e c e s s a ry atom ic vapour w ith o u t th e n e c e s s i ty f o r h ig h te m p e ra tu re ( 4 1 ) .E f f e c t o f c a r r i e r gasThe ty p e o f s p e c t r a e m itte d i s a f f e c te d c o n s id e ra b ly by th e c a r r i e rg a s .Helium p roduces more * s p a rk ' s p e c t r a th a n any o f th e i n e r t g a s e s .I t i s b e l ie v e d th a t t h i s i s due to i t s h ig h e x c i t a t io n and io n i s a t i o n p o t e n t i a l s . Helium i s a ls o a l e s s e f f e c t i v e s p u t t e r e r th a n o th e r g a s e s , (S to c k e r ( 4 2 )), so t h a t th e ca th o d e m a te r ia l vapour p re s s u re i s low , and P enn ing ty p e c o l l i s i o n s a re l i k e l y to p redom inate o v e r e l e c t r o n e x c i t a t i o n / i o n i s a t i o n c o l l i s i o n s . The low e l e c t r i c f i e l d s i n th e H.C.D. a ls o make i t a f a v o u ra b le so u rc e f o r th e e x c i t a t io n o f sp a rk s p e c t r a w ith th e e x c i t a t i o n l im i te d by charge t r a n s f e r (lO /O l ) , o r ( lO * /l 0) c o l l i s i o n s (Sawyer (4 3 ))»The ty p e o f e x c i t a t i o n p roduced i s n o t o n ly a fu n c t io n o f th e e x c i t a t i o n p o t e n t i a l s o f th e c a r r i e r g a s . M itc h e ll (44) found t h a t a rgon gave a r c - l i k e s p e c t r a , b u t xenon gave more io n s p e c t r a , u s in g an ^ n c a th o d e , a lth o u g h th e e x c i t a t i o n p o te n t i a l o f xenon i s l e s s th a n th a t o f a rg o n . E xam ination o f th e d a ta g iv e n i n (43) shows th a t th e r e i s a maximum o f e x c i t a t i o n observ ed in a H .C .D ., e x p la in a b le as r e s u l t i n g from th e (lO /O l* ) p ro c e s s , w hich w ith helium as a c a r r i e r g a s , ap p ea rs more p ro b a b le th a n a m e ta s ta b le - m e ta s ta b le c o l l i s i o n p ro c e s s , i n th e e x c i t a t i o n o f m e ta l s p e c t r a .- 42 -3*4 P u lse d d is c h a rg e sP u lse d gas d is c h a rg e s w ere f i r s t u sed as l a s e r e x c i t a t i o n so u rces by Boot and C lu n ie (4 5 ) , who o b ta in e d o s c i l l a t i o n i n th e n e a r i n f r a - r e d in th e a f te rg lo w o f a h ig h p r e s s u r e , h ig h - c u r r e n t , p u ls e d d is c h a rg e .T his means o f e x c i t a t i o n has en ab led l a s e r s to be developed w ork ing up in to th e t r i p l y io n is e d s p e c ie s u n d er p u ls e d c o n d i t io n s , and h as le d su b se q u e n tly to th e r a p id developm ent o f h i ^ power cw and th e new t r a n s i e n t l a s e r sy s te m s .E x c i ta t io n p ro c e s s e s a t breakdownI f a h ig h p o t e n t i a l i s a p p l ie d to a d is c h a rg e tu b e v ia i n t e r n a l e l e c t r o d e s , i n p a r a l l e l w ith a c a p a c i to r , th e gas b re a k s down a t a p a r t i c u l a r v o l t a g e . T h is v o l ta g e depends on th e gas p re s s u re and th e s e p a r a t io n betw een th e e l e c t r o d e s , i . e . E /p . P lo ts o f th e v a r i a t i o n o f breakdow n v o lta g e w ith p re s s u re and s e p a r a t io n o f th e e le c t r o d e s , form th e fam ilia r P aschen breakdown c u rv e s . P ro v id ed th e e l e c t r i c f i e l d i s u n ifo rm and th e p r e s s u r e - e le c t r o d e s e p a r a t io n (pd) i s l e s s th a n 150 t o r r cm, P a s c h e n 's Law i s obeyed.At breakdown th e c u r r e n t r i s e s r a p id ly to a v a lu e d e te rm in ed by th e r e s i s t a n c e i n th e c i r c u i t and th e f i n i t e power o f th e s o u rc e . A f te r th e i n i t i a l r a p id r i s e i n c u r r e n t , th e e l e c t r i c f i e l d f a l l s u n t i l th e c u r r e n t re a c h e s a p a r t i c u l a r v a lu e , a f t e r w hich th e m a in ta in in g p o te n t i a l i s in d e p en d en t o f th e c u r r e n t . The c u r r e n t th e n decays e x p o n e n t ia l ly a c c o rd in g to th e tim e c o n s ta n t o f th e c i r c u i t . In such a d is c h a rg e th e c u r r e n t d e n s i ty can approach ^00 Amp cm , as a r e s u l t o f th e c o n s id e ra b le i o n i s a t i o n t h a t o c c u rs .- 43 "*The norm al c a p a c i to r d is c h a rg e i s n o t a v e r s a t i l e d is c h a rg e as f a r as a n a ly s i s o f a d is c h a rg e i s co n ce rn ed , s in c e i t i s n o t p o s s ib le to i s o l a t e p ro c e s se s w hich o ccu r i n th e a f te rg lo w o f th e i n i t i a l breakdown p u ls e , from th o s e w hich o ccu r d u r in g th e d ecay in g c u r r e n t p u ls e . By use o f a d e la y l i n e , how ever, i n c o n ju n c tio n w ith a th y r a t r o n s w itc h and a p u ls e t r a n s fo rm e r , as shown in F ig . 7 .2 i t i s p o s s ib le to p roduce q u a s i- cw r e c ta n g u la r d is c h a rg e c u r r e n t p u ls e s o f up to 100 msec d u r a t io n .In th e p u ls e d p o s i t i v e column d is c h a rg e th e e l e c t r i c and s p e c t r a l p r o p e r t i e s a re d e te rm in e d by th e d im ensions o f th e d is c h a rg e tu b e , and th e e m iss iv e p r o p e r t i e s o f th e c a th o d e .To th e w r i t e r 's know ledge no e le c t r o n energy d i s t r i b u t i o n d e te rm in a tio n s have been made in a p u ls e d d is c h a rg e , b u t i t i s b e l ie v e d th a t th e av e ra g e e le c t r o n energy i s h ig h because o f th e la rg e E/p a t breakdow n, o f th e o rd e r o f 200 V cm"" ^ t o r r ”^ , and d u r in g th e la r g e s u s ta in in g p e r io d , when th e r a t i o o f th e e l e c t r o n d r i f t v e lo c i ty to random v e lo c i ty T^/V — 0. 2 - 0 .3 i n some g a s e s . The r e s u l t s o f Cheo and Cooper ( 4 6 ) on p u ls e d carb o n d io x id e and n i t r o g e n l a s e r d is c h a rg e s show t h a t e l e c t r o n s a re o f h ig h energy on ly d u rin g th e tim e to gas breakdow n.Beyond breakdow n, B reusava (47) has shown th a t i n th e ca se o f a l lth e g ases she in v e s t ig a t e d , th e l a r g e e l e c t r i c f i e l d decayed a t a r a t ed e te rm in ed by th e d ia m e te r o f th e d is c h a rg e tu b e ; th e i n i t i a l v o lta g edrop b e in g s te e p e r in th e s m a lle r d is c h a rg e tu b e . The tim e decay o fth e la r g e e l e c t r i c f i e l d was ap p ro x im a te ly 25 m icroseconds from i n i t i a lbreakdown o f th e g a s .- 44 •E x c i ta t io n p ro c e s s e s in th e a f te rg lo w o f a p u ls e d d is c h a rg e .I t i s i n s t r u c t i v e to c o n s id e r a f te rg lo w p ro c e s s e s i n h e liu m , s in c e he lium i s commonly u sed as a b u f f e r g a s , and s in c e i t i s m a in ly w ith he lium th a t second k in d c o l l i s i o n a l energy t r a n s f e r p ro c e s s e s , p ro d u c in g p o p u la t io n in v e r s io n , have been p o s tu la te d f o r l a s e r sy s tem s .In th e a f te rg lo w o f a p u ls e d d is c h a rg e in h e liu m , th e r e i s a sh a rp in c re a s e in i n t e n s i t y , in atom ic s p e c t r a 5 - 10 m icroseconds a f t e r c e s s a t io n o f th e p u ls e , w ith th e peak i n t e n s i t y v a ry in g w ith th e p a r t i c u l a r e x c i te d l e v e l (4 8 ) . B iond i and H o ls te in (49) have found t h a t v i s i b l e l i n e s p e c t r a a re e m itte d from h ig h le v e l s t a t e s n « 3 , 4 , 5 and 6 a l l w ith in 1 .5 eT o f th e he lium a tom ic i o n i s a t io n p o t e n t i a l . They a ls o d e te rm in ed t h a t th e sq u a re r o o t o f th e r a d ia t io n i n t e n s i t y e m itte d from th e a f te rg lo w fo llo w e d c lo s e ly th e decay o f e l e c t r o n d e n s i ty , in d i c a t in g th a t th e e m iss io n was by a re c o m b in a tio n p ro c e s s . This was su b se q u e n tly v e r i f i e d by m icrowave s tu d ie s made by A nderson (5 0 ) .The m ost im p o rta n t d e io n is a t io n p ro c e s se s in helium a re s t a t e d by Pakhomov e t a l . (48) to be%-He^ + 2e — > He + e . . . . . (3 * 9 ),(The p ro c e ss He"*" + e — > He , has a much low er reco m b in a tio n r a t eth a n ( 3 . 9 ) ; (Loeb (51).)and ^He + He —^ 2He + h\S . . . . » (3 .1 0 a )H " + h-vi ........ (3 .lot).P ro c e s se s 5 .9 and 3 .1 0 a le a d to d e - e x c i ta t io n w ith th e p ro d u c tio n o f a tom ic l i n e s and p ro c e s s 3 .10b to th e appearance o f m o le c u la r bands.- 45 ~Pakhamov has shown t h a t a t d is c h a rg e w a ll te m p e ra tu re o f 77^K th e i n t e n s i t y o f th e a f te rg lo w o f m o le c u la r he liu m hands i s low , so th a t r e a c t io n ( j . lO h ) i s n e g l ig ib l e .A f te r a l l c u r r e n t has c e a se d , th e f lu x o f e n e r g e t ic e l e c t r o n s i s n e a r ly z e ro , and th e re m a in in g e l e c t r o n d e n s i ty i s composed o f f r e e e le c t r o n s p roduced by r e a c t io n s o f th e ty p e ; -(a ) below 5 t o r rHe* (2^3 ) + He* (2^3) —3- He* + e ( j . l l )and (b ) above 5 t o r rHe’* (2^S) + He* (2^S) + He He* + He + e .......... ( 3 .1 2 ).E le c t r o n s , a tom ic and m o le c u la r io n s a re th e r e f o r e p re s e n t f o r rec o m b in a tio n to o c c u r .A d d itio n a l p ro c e s se s p ro d u c in g m o le c u la r io n s and e le c t r o n s ( 5 1 ) a r e th e ;Hornbeck - M olnar p ro c e ssHe + (e +')>23 eT and < 2 4 .2 eT) —^ He* + e ............. (3 .1 3 )He* + He —> He* + e + K .E. (3 .1 4 )and th e H o lt - B iond i p ro c e ssHe"^ + 2He He^ + He + K .E. (3 .1 5 )(w hich o ccu rs a t h ig h e r p r e s s u r e s ) .The d i s s o c ia t io n energy o f He^ i s betw een 2 .2 and 3 . I ©T ( 5 1 ) . To accoun t+ ' 'f o r th e d is s o c ia t iv e - r e c o m b in a t io n He^ + e — He + He, where He i san e x c i te d s t a t e o f helium (n = 3 , 4 , 5 o r 6 ) , th e m o le c u la r he lium io n H e^ can n o t be i n i t s lo w es t energy s t a t e , b u t m ust be an e x c i te d- 4-6 -m o le c u la r io n , w hich l i e s w ith in . 1 ,5 eT o f th e a tom ic io n i s a t io n p o te n t i a l o f h e liu m , and n o t a t 2 4 .5 - ( 2 .5 o r 3 . I ) eT.IThe He from re c o m b in a tio n and d i s s o c ia t io n i s e x c i te d to w ith in0 .5 - 0 .3 eT o f th e he lium io n i s a t io n p o t e n t i a l , th e r e a c t io n b e in gHe^ + e —^ He ( ^ 2 3 e?) + He .......... ( 3 .16)*F u r th e r , fo l lo w in g Loeb ( 5 1 ) , to acco u n t f o r t h i s r e a c t io n , th e He^ ini t s e x c i te d v ib r a t i o n a l s t a t e m ust l a s t lo n g enough to recom bine w ith7 8helium atom s, a t c o l l i s i o n f r e q u e n c ie s o f 10 - 10 s e c . T his meansth a t th e e x c i te d v ib r a t i o n a l He^ io n s can s u f f e r a p p ro x im a te ly 10^1 ^c o l l i s i o n s w ith o u t l o s in g t h e i r energy o f v ib r a t io n . S tu d ie s o f such v ib r a t i o n a l s t a t e s e x c i te d to w ith in O.3 eT o f th e atom ic io n i s a t i o n p o te n t i a l in d i c a te t h a t th e chance o f energy lo s s i s o f th e o rd e r o f lO"^ p e r c o l l i s i o n . T his means t h a t th e e x c i te d he lium m o le c u la r io n i s m e ta s ta b le (He^ ) , T h is p ro c e s s , a c c o rd in g to B iond i (52) has been e s ta b l i s h e d 'beyond any q u e s t i o n ', by th e o bserved D oppler b ro ad en in g o ffth e a tom ic l i n e s from He . However, i t i s now q u e s tio n e d , fo llo w in g th e work o f F erguson e t a l . (53)»The o v e r a l l p ro c e s s , th e r e f o r e in th e a f te rg lo w o f a p u lse d helium* zd is c h a rg e i s one o f d e - e x c i ta t io n o f He (2 S) atoms by double im pact o r t r i p l e im pact depend ing on th e p re s s u re as in ( 3 . I I ) o r ( 3 . 1 2 ) , g iv in g(a ) m e ta s ta b le m o le c u la r helium io n s (He* )(b ) e x c i te d he lium atom s, e x c i te d to w ith in O.3 - 0 ,5 eT o f th e h e lium io n ground s t a t e .T h is i s j u s t th e energy ran g e in w hich s e le c t iv e e x c i t a t io n occurs in~ 47 -h elium -m ercu ry and h e liu m -io d in e l a s e r s , and in w hich a decay in t e r v a ls im i la r to th e decay o f e x c i te d helium a tom ic l e v e l s i s o b se rv ed .More r e c e n t work by C o ll in s and R o bertson (5 4 ) , on a f lo w in g heliuma f te rg lo w , has shown a t ex trem es o f h ig h io n i s a t i o n and low n e u t r a ld e n s i ty , t h a t i t i s c o l l i s i o n a l - r a d i a t i v e reco m b in a tio n o f He*, t h a td e te rm in e s th e r a t e o f a tom ic spon taneous em issio n in th e 50- 2 5 0 /u s e cp e r io d in a he liu m a f te rg lo w and th a t th e r a t e o f e m issio n fo llo w s t h a t + +o f He and n o t He^ , even a t p re s s u re s as h ig h as 20 t o r r .3 .5 S im i l a r i t y o f th e n e g a t iv e glow o f th e glow d is c h a rg e and th e a f te rg lo w o f a n u ls e d d is c h a rg e .A nderson (55) has shown t h a t th e n e g a t iv e glow o f a p u ls e d d is c h a rg e can be quenched by th e a p p l ic a t io n o f microwave r a d i a t i o n .More im p o r ta n t , he found t h a t em issio n from th e anode s id e o f th e n e g a t iv e glow was quenched, w h ile t h a t on th e ca thode s id e was enhanced . These r e s u l t s su g g e s t t h a t re c o m b in a tio n i s th e im p o rta n t p ro c e ss deep in th e n e g a t iv e glow , s in c e t h i s i s th e on ly p ro c e ss le a d in g to th e fo rm a tio n o f r a d i a t i v e s t a t e s t h a t would be a f f e c te d by th e m icrow aves.H urt ( 1 4 ) i a in v e s t ig a t io n s on th e n e g a tiv e glow has found th a t em iss io n from atoms peaked on th e ca thode s id e o f th e glow , w ith th e peak f o r m o le c u la r em ission , te n tim es g r e a t e r th a n in th e p o s i t i v e c o lu m n ,o c c u rr in g tow ards th e anode s id e . To accoun t f o r th e m o le c u la r em issio n i n th e n e g a t iv e glow , Emeleus and D uffendack ( 5 6 ) p roposed a th r e e body c o l l i s i o nHe*(2^S) + 2He He^ ( 2 ^ ^ ) + He (3 -1 ? ) .— 4 ^ ”U sing a f lo w in g h e lium a f te rg lo w , C o ll in s and H urt ( 5 7 ) de te rm in ed th a t th e o b se rv ed m o le c u la r em issio n was d e f i n i t e l y th e r e s u l t of rec o m b in a tio n o f th e m o le c u la r io n . They found th a t th e m o le c u la r em iss io n had th e same p re s s u re dependence as th e m o le c u la r io n , w hich was v e ry d i f f e r e n t from th e p re s s u re dependence o f th e a tom ic io n o r th e He (2^S) m e ta s ta b le .As in th e he lium a f te rg lo w , m o le c u la r em iss io n i s enhanced a t h ig h e r p re s s u re s in th e n e g a t iv e glow . Above s e v e ra l t o r r , th e m o le c u la r io n i s th e dom inant io n , (K err ($ 8 ) , Hornbeck and M olnar (5 9 ) , P he lp s and Brown ( 6 0 ) ) . A no ther s t r i k i n g s i m i l a r i t y betw een th e a f te rg lo w and th e n e g a t iv e glow m o le c u la r em iss io n i s t h a t th e r e l a t i v e i n t e n s i t i e s o f m o le c u la r bands a re th e same, i n d i c a t in g t h a t e x c i te d m o lecu les a re form ed by th e same p ro c e s s i n th e two c a s e s .R ecen t work by F erg u so n , F e h se n fe ld and Schm eltekopf (5 3 ) , ( b r i e f l y m en tioned i n 3 *4 ) on f lo w in g he lium a f te rg lo w s ap p ea rs to con firm th e c o n c lu s io n s reac h ed by C o ll in s and R obertson (54) on th e mechanism p ro d u c in g th e atom ic em iss io n in th e helium a f te rg lo w . They ap p ea r to show c o n c lu s iv e ly t h a t ;( i ) th e a tom ic em issio n i s due to c o l l i s i o n a l reco m b in a tio n o f th e+ * / \ a tom ic io n by th e He + 2e —>■ He + e ,p ro c e s s (3*9) o£Pakhomov e t a l . ( 4 8 ) .( i i ) th e m o le c u la r em issio n in th e l a t e a f te rg lo w i s due toc o l l i s i o n a l - r a d i a t i v e reco m b in a tio n o f th e Heg^ io n .- 49 -I f t h i s i s th e c a s e , th e p re se n c e o f th e h ig h ly e x c i te d m e ta s ta h le m o le c u la r he liu m io n , n e c e s s a ry to e x p la in th e p ro c e ss e s ta b l is h e d 'beyond any q u e s t io n ' by B io n d i i s q u e s t io n a b le . I t i s hoped th a t th e s i t u a t i o n w i l l be made c l e a r e r in th e n e a r f u tu r e fo llo w in g f u r t h e r f lo w in g a f te rg lo w e x p e rim e n ts .r- 50 -dark space A.A ston-, cath(Kle“ , Faraday-I I Ii t .catholic 1 glowlayer ,I II 'boundaryIanode. . ' , anode glowpositive column j *' J l_>d -----1■ ■■ 1 ^ 1 -------------------------------------Spatial distribution of dark and luminous zones, electric field X , space charge densities p+ and p~, and current densities j * and j~ in a glow discharge (schematically).F i g . 3 . 1>*•uTimeVariation o f the aoeraee electron ener^ with time, in helium at X l p ^ O eV lcmm m H g: (a) high frequency; (ï) Uw frequency. (From A. VON E n g e l and W. L. H a r r i e sF i g . 3 . 2- 5 1 -CHAPTER 4 S p e c if ic O s c i l l a to r SystemsIn tro d u c t io nIn t h i s c h a p te r , p a r t i c u la r system s have been chosen fo r a n a ly s is to i l l u s t r a t e th e s e le c t iv e e x c i ta t io n p ro cesse s d iscu ssed under th e s ix head ings in C hapter 2 .4 .1 Atom-atom, e x c i ta t io n t r a n s f e r c o l l i s io n m asersThere have been fo u r s u c c e s s fu l m asers in which p o p u la tio n in v e rs io nhas been produced predom inan tly by e x c i ta t io n t r a n s f e r c o l l i s io n s . These a re th e helium -neon, m ercu ry -z in c , helium -kryp ton and neon-xenon system s.4 .1 .1 Helium-Neon SystemThe f i r s t l a s e r o s c i l l a t i o n , on f iv e t r a n s i t io n s in th e n ea r i n f r a re din neon was o b ta in ed by Javan ( l ) in 1961. The f iv e t r a n s i t io n s a rebetween th e 2s and 2p le v e ls o f l e v e ls in n e u t r a l neon.I t can be seen from F ig . 4 .1 th a t th e re i s an energy co inc idence betweenth e helium (2^S) m e tas tab le l e v e l , and th e fo u r neon 2s l e v e l s . A s im i la r1 'energy co in c id en ce e x i s t s between th e helium (2 S) m e tas tab le l e v e l and th e fo u r neon 3s l e v e l s .F ig . 4 .2 shows in d e t a i l th e energy co in c id en ces fo r th e helium (2^S) m e ta s ta b le s a t 19*8 eV above th e ground s t a t e .- 52 -As s ta t e d in s e c t io n 2 .1 , la rg e c o l l i s io n c ro s s s e c tio n s a re observed♦fo r c o l l i s io n s o f th e type A + B -----> A+B(excited) + A E .............(4 .1 )where A E i s l e s s th an a few kT and where th e Wigner Spin Rule i s n o t v io la te d (2 ) .A ll th e Ne 2s energy le v e ls a re w ith in 124? cm"^ o f th e He (2^8) l e v e l ,so th a t re so n a n t energy t r a n s f e r i s to be expected between th e helium (2^S)m e ta s tab le atoms and neon ground s t a t e atom s, r e s u l t in g in e x c i ta t io n toth e 2s l e v e l s and d e -e x c i ta t io n o f th e helium m e tas tab le atom s.The r e a c t io n i s : -He* (2^8) + Ne ----^ Ne (2 s , , =) + He (^8 ) +ZlE . . . . (4 .2 ).oThe 2 s , . _ l e v e ls correspond to th e * l e v e ls r e s p e c t iv e ly in L85 ,4 , 5 ^ 0 ,1 , 2n o ta t io n , so th a t s p in co n se rv a tio n i s obeyed in t h i s r e a c t io n . The 2s£^ ^l e v e l i s o p t i c a l ly connected to th e ground s t a t e v ia a s tro n g f a r ÜV t r a n s i t i o n so th a t a la rg e e le c tro n im pact c ro ss s e c tio n fo r the r e a c t io nNe + (e + K E) ---- > Ne (2 s .) + e (4 .3 )r e s u l t s in s e le c t iv e e x c i ta t io n o f th e 2s^ le v e l o c c u rrin g .Assuming th a t the neon p re s su re i s h igh enough fo r r a d ia t io n tra p p in g o f th e f a r UV photons to occur th e 2S|^ le v e l p o p u la tio n i s m ain ta ined and can decay on ly r a d ia t iv e ly to th e 2p s t a t e .L ife tim e c o n s id e ra tio n sThe e f f e c t iv e l i f e t im e s o f th e 2s le v e ls o f neon have been measured by B ennett (3) and o f th e 2p le v e ls by B ennett (4 ) , KLose (5 ) , Landenburg (6)and G r i f f i t h s (? )•Table 4 .1 g iv e s th e e f f e c t iv e l i f e t im e s from r e f . (3) o f th e two s t a t e s .The 2s v a lu e s correspond to 1 t o r r o f neon.- 53 •*T able 4.1 R ad ia tiv e l i f e t im e s fo r th e 2p and 2s (Paschen n o ta tio n ) le v e ls o f neon, in nanoseconds.2s3 2s^ 2Pi 2pg ^"3 2P4 '^3 2P6 2P, 2pg 2P9 2p^Q96 160 98 110 8 10 13 12 11 13 13 16 17 20The e f f e c t iv e l i f e t im e s d i f f e r a p p re c ia b ly from th e p u re ly r a d ia t iv e ones e x h ib ite d by an i s o la te d atom and given by th e method o f B ates and Daragaard (8), s in c e c o l l i s io n a l d e -e x c i ta t io n r a t e s can be com parable to r a d ia t iv e decay r a t e s a t p re s s u re s o f a few t o r r .From l i f e t im e c o n s id e ra tio n s a lo n e , g iven s u f f i c i e n t e x c i ta t io n o f th e 2s l e v e l s , i t shou ld be p o s s ib le to o b ta in p o p u la tio n in v e rs io n on a l l o f th e t h i r t y a llow ed 2s - 2p t r a n s i t i o n s , and th u s o s c i l l a t io n should be o b ta in a b le in a pure neon d isc h a rg e . A lthough o s c i l l a t io n on 2s - 2p t r a n s i t i o n s was f i r s t observed in a m ix tu re o f helium and neon, o s c i l l a t i o n has s in c e been re p o r te d in pu re neon by B ennett (9) and Chebotayev (1 0 ) .Assuming th a t th e e le c tro n energy d i s t r i b u t io n was M axwellian,Javan (11) c a lc u la te d t h a t th e r e a c t io ne + Ne ( i s ) ---- ^ Ne (2p) + e . . . . . (4 .4 )and resonance tra p p in g o f th e 2p — — I s photons were second o rd e r p ro c e s se s , and th a t in v e rs io n would no t be u p se t between th e 2s and 2p l e v e l s .At h ig h p re s su re s th e se second o rd e r p ro cesses were subseq u en tly found to be o f c o n s id e ra b le im portance, n e c e s s i ta t in g o p e ra tio n a t low p re s s u re s , where th e assum ption o f a M axwellian e le c tro n d i s t r ib u t io n i sq u e s tio n a b le .- 54 -The a d d i t io n o f an im p u rity (argon) to reduce th e Ne (1s) m e ta s ta b le s , d id n o t ach iev e th e e f f e c t d e s ire d by red u c in g t h i s second o rd e r p ro c e ss , as th e upper s t a t e p o p u la tio n r a t e was reduced . (Argon was an u n fo rtu n a te cho ice in t h i s r e s p e c t a s i t has a h ig h io n is a t io n e f f ic ie n c y .) The a d d it io n o f hydrogen o r oxygen,by Chebotayev (10) in l a t e r work u s in g hollow cathode d isc h a rg e s has enabled o s c i l l a t io n in neon on a number o f 2s - 2p t r a n s i t i o n s to be o b ta in ed , by reducing the Ne ( i s ) m e ta s tab le c o n c e n tra tio n .N oting th e re so n a n t energy t r a n s f e r p ro cess o f (4 .2 ), Javan (1 2 ), sug g ested u s in g a helium -neon m ixture in which o s c i l l a t io n in th e d isch a rg e a f te rg lo w m ight be observed .E xperim ental s tu d ie s by Javan and B ennett (13) showed th a t in heon th e 2s le v e ls decayed w ith a tim e v a r ia t io n s im ila r to th e He(2^8) m e ta s ta b le s , and l a s e r o s c i l l a t i o n was su b seq u en tly observed in th e a fte rg lo w a t f iv e w avelengths ( in m icrons)1 .1 1 8 0 (2s^ - 2p^) ; 1 .1 3 2 3 (2s^ - 2p^);1 .1 6 0 (2s^ - 2p^) ; 1 .199 (2s^ - 2P2 );1 .2 0 7 (2 Sc - 2pg) .The s t ro n g e s t o s c i l l a t io n occu rred on the 2s^ — 2p^ ^ t r a n s i t i o n . T his was c o n tra ry to e x p e c ta tio n a s s e le c t iv e e x c i ta t io n o f th e 2s^ le v e l i s no t expected in c o l l i s io n s w ith He(2^8) m e ta s tab le s s in c e th e Wigner 8p in Rule i s v io la te d , ----- th e 2s le v e l in L8 coup ling being a s in g le t ( P^) l e v e l .“ 55 -The re a c t io n He(2^S) + Ne > Ne(2s2) + He + A E (4 .3 )was th e re fo re a p p a re n tly o f g r e a te r im portance th an ( 4 .2 ) .A E i s on ly 313 cm ^ f o r th e 2s_ l e v e l , and 468 cm ^ fo r th e 2s_l e v e l , so th a t i t ap pears th a t the sm a lle r energy d isc rep an cy i s moreim p o rtan t than th e sp in co n se rv a tio n req u irem en t, u n le ss LS co u p lin g f a i l s in th e Ne(2s) l e v e l .The 2s^ le v e l i s s tro n g ly o p t i c a l ly connected to th e ground s t a t e , sot h a t a la rg e c ro ss s e c tio n fo r d i r e c t e x c i ta t io n from th e ground s t a t e i s tobe expected in th e re a c t io n :e + Ne ---- > Ne (28^) + e (4 .6 ) .S tu d ie s by B ennett ( l4 ) have shown th a t n o t only a re re a c tio n s (4 .3 ) and (4 .6 ) o f equal im portance fo r the optimum p re s s u re s used in th e in f r a - r e d , r f helium -neon l a s e r (P^^ approx . 1 t o r r , approx . 0 .1 to r r ) bu t th a t r e a c t io n (4 .6 ) was enhanced by th e p resence o f helium . E v id en tly the p resen ce o f helium en ab les a h ig h e r e le c tro n c o n c e n tra tio n to be o b ta in ed w ith o u t a decrease in e le c tro n te m p era tu re . An e x p lan a tio n o f t h i s i s p roposed in C hapter 6 . Cw o s c i l l a t i o n has been ob ta in ed now on 23 o f th e 30 p o s s ib le t r a n s i t io n s between th e 2s and 2p le v e ls o f neon.In v e rse r a d ia l dependence o f g a in .Gain measurements on th e 1.1323 micron l i n e by B ennett (9) showed th a t th e s in g le pass g a in v a r ie d in v e rs e ly a s th e d isch arg e tube d iam ete r, f o r d iam ete rs ran g in g from 7 - 3 0 mm. A s im i la r r a d ia l g a in r e la t io n s h ip has been o b ta in ed on th e 2 .02 and 3 .3 m icron l i n e s in xenon, and on th e o p t i c a l ly pumped cesium t r a n s i t i o n a t 7 .1 8 2 m icrons (13)*- 56 -B ennett (13) has shown th a t th e only mechanism which y ie ld s a 1 / r gain dependence in th e helium -neon l a s e r i s th a t o f atom-atom c o l l i s io n a l d e - e x c i ta t io n o f th e m e ta s ta h le /q u a s i-m e ta s ta b le N e (ls ) l e v e l s . A dominant lo s s mechanism dependent on d if fu s io n to the w a lls (u s u a lly assum ed), r e s u l t s in a l / r ^ dependence. Using known d if fu s io n c o n s ta n ts and e s tim a te d c ro s s - s e c t io n s fo r helium -neon c o l l i s io n s , a t r a n s i t io n from ac o l l i s io n to d if fu s io n lim ite d ca se , should occur a t a bore o f approx im ate ly5 mm, w ith in a f a c to r n o t g r e a te r th an 3*In a l l tre a tm e n ts , th e r a d ia l dependence o f th e e le c tro n tem p era tu re , and i t s e f f e c t on th e s e le c t iv e e x c i ta t io n o f th e upper maser le v e l has been n e g le c te d ; t h i s i s consid ered in C hapter 6 .A ttem pts to in c re a se th e atom-atom c o l l i s io n a l r e la x a t io n r a t e o f th e Ne( is ) l e v e l b o tt le n e c k by th e a d d itio n o f argon has le d only to a re d u c tio n in g a in , and i t has been presumed th a t the Penning type re a c tio nHe (2^S) + M ^ Ar"^ + He + e G<4.7),r e s u l t in g in th e d e s tru c t io n o f He(2^S) m e ta s ta b le s ,is la rg e compared w ithHe(2^S) + Ne —^ Ne(2s2) + He + e o f (4 .3 )*The p resence o f even sm all q u a n t i t i e s o f argon in a glow d isch arg e has been shown by D orgelo, A ltin g and Boers ( l6 ) to reduce the e le c tro n te m p era tu re , so th a t as w ell a s d e s tro y in g He(2^S) m e ta s ta b le s , th e r a t e o f p ro d u c tio n o f t r i p l e t m e ta s tab le s and Ne(2 s2 ) atoms by h igh energy e le c tro n s w i l l be reduced , and low er g a in w i l l ensue.- 57 -The a d d i t io n o f hydrogen, which i s known from th e d is p e rs io n work of Ladenburg, and D orgelo (17) to be e f f e c t iv e in d e s tro y in g N e(ls ) m e tas tab les has been used a s a su c c e s s fu l r e la x a t io n c o n s t i tu e n t in a neon maser during th e l a s t y e a r by Chebotayev (1 0 ), b u t as y e t th e re have been no r e p o r ts of th e a d d i t io n o f hydrogen to a helium -neon system . The c ro ss s e c t io n o f6 .1 X 10 cm ,^ (Benton ( l8 ) ) o r 1*7 x 10 cm^ (Je s se and Sadausk is (19 )) fo r th e d e s tru c t io n o f He(2^S) by hydrogen m olecules i s com parable to th a t o f 6 .6 X 10 cm^ (Benton and R obertson (20)) fo r t h e i r d e s tru c t io n by argon atom s, so t h a t u n le ss th e e le c tro n tem peratu re i s m ain ta ined , l i t t l e b e n e f i t w i l l r e s u l t from th e a d d it io n o f hydrogen.The g a in measurements of B ennett ( l 4 ) . . which proved th a t a p r a c t i c a l amount o f ga in was o b ta in a b le by d i r e c t e le c tro n im pact e x c i ta t io n on th e 28^ l e v e l o f neon, le d to an in v e s t ig a t io n o f o th e r pure noble gas system s, and to o b ta in in g o s c i l l a t i o n su b seq u en tly in them. The a d d i t io n o f helium to th e o th e r noble gas system s, where re so n a n t energy t r a n s f e r p ro cesse s do n o t occur w ith helium m e ta s ta b le s , has a ls o r e s u l te d in in c re a se d g a in .O s c i l l a t io n in th e v i s ib l e .Rigden and White (21) observed th a t th e - 2p^ t r a n s i t io n a t 6 3 2 .8nm in neon was s tro n g ly enhanced in a d isch a rg e in a helium -neon m ixture and concluded th a t s e le c t iv e e x c i ta t io n was o ccu rrin g through th e re a c tio nHe (2^S) + Ne — Ne (^s^) + He + A E . . . .{ n ^ 4 .7 ) .T h is was n o t the f i r s t o b se rv a tio n o f s e le c t iv e e x c i ta t io n o f th e 33^ le v e l o f neon, a s enhancement on th e vacuum UV t r a n s i t io n - 2p^- 58 -a t 60«004nra in neon in a condensed d isch a rg e in a helium -neon m ix tu re , had p re v io u s ly been re p o r te d in 1938 by Suga (2 2 ) . Suga concluded th a t re so n a n t energy t r a n s f e r from helium (2^S) m e ta s ta b le s to neon ground s ta t e atoms was re sp o n s ib le fo r th e enhancement.F ig u re 4 .3 g iv e s th e energy co inc idences fo r helium (2^S) m e ta s ta b le s in neon and shows th a t th e 33^ le v e l of neon l i e s 386 cmT^ (approx im ate ly 2 kT) above th e s in g le t helium m etas tab le le v e l and i s f u r th e r from i t than th e 35^ o r th e 3s^ ^ l e v e l s . The 38^ le v e l i s a s in g le t l e v e l , so th a t sp in i s conserved in r e a c t io n (4 .7 ) in v o lv in g th e s in g le t helium m e tas tab le and a ground s t a t e neon atom. U nlike re a c t io n (4 .3 ) in which th e Wigner Spin r u le i s n o t obeyed, in t h i s case th e la rg e r energy d iscrep an cy appears to be o f l e s s im portance than sp in c o n se rv a tio n . However, th e re a re a d d i t io n a l rea so n s fo r th e s e le c t iv e e x c i ta t io n o f th e neon 38^ l e v e l . The 3s le v e l i s o p t i c a l ly connected to th e ground s t a t e by th e s tro n g vacuum UV t r a n s i t i o n a t 60.004nm and has a la rg e c ro s s - s e c tio n fo r e le c tro n im pact e x c i ta t io n , in a s im ila r way to th e 2s^ le v e l of neon.The r e a c t io n ,e + Ne —>“ He (3 8 2 ) + e . . . . . (4 .8 ) •can th e re fo re be a s im portan t in s e le c t iv e ly e x c i tin g th e le v e l , as r e a c t io n (4 .6 ) i s in e x c i t in g th e 2S2 l e v e l of neon. I t subsequen tly ap p ea rs t h a t d i r e c t e le c tro n im pact e x c i ta t io n of the ^^2 1®^® ^com parable to th e re so n an t energy t r a n s f e r p rocess of ( 4 .7 ) .- 59 -A lthough o s c i l l a t i o n could n o t be o b ta in ed in th e la rg e bore d isch arg e tu b e s used fo r th e f i r s t in f r a - r e d m asers, Rigden and White (21) were s u c c e s s fu l in o b ta in in g cw o p e ra tio n a t 632 .8nm on th e - 2p^ ^ t r a n s i t io n in neon by u t i l i s i n g sm a lle r bore tu b es and d i f f e r e n t optimum p re ssu re s*In t h i s o p e ra tio n a t 6 32 .8nm, i t has n o t been p o s s ib le to determ ine w ith any c e r t a in ty th a t a re so n a n t energy t r a n s f e r p ro cess i s a c tu a l ly inv o lv ed in th e s e le c t iv e e x c i ta t io n o f the 38^ l e v e l .The l i f e t im e o f th e He(2 S) m e ta s tab le s i s very s h o r t under th e o p e ra tin g c o n d itio n s o f th e 63 2 .8nm helium -neon l a s e r , so th a t enhancement i s n o t observed in th e a fte rg lo w (Suzuki (2 3 )) , and i t has n o t been p o s s ib le to use th e e le g a n t tim e-re so lv e d co incidence techn ique o f B ennett ( l4 ) to determ ine i f th e 6 3 2 .8nm spontaneous em ission has th e same decay r a t e a s th e He(2 S) m e ta s tab le decay r a t e .In v e rse r a d i a l dependence o f ga inRigden and White (24) have shown th a t o s c i l l a t io n on th e 63 2 .8nm t r a n s i t i o n e x h ib i ts a s im ila r in v e rse r a d ia l ga in dependence to th a t o f th e 1 .1 3 2 3 m icron m aser. As w ell a s in c re a s in g th e d if fu s io n lo s s o f Ne ( I s ) m e ta s ta b le s , a re d u c tio n in th e bore o f a d isch arg e tube r e s u l t s in an in c re a s e in th e e le c tro n tem p era tu re , so th a t th e c o n tr ib u tio n to upper l e v e l p o p u la tio n by d i r e c t e le c tro n im pact e x c i ta t io n can assume g re a te r s ig n if ic a n c e .— 6o —A d d itio n a l l a s e r t r a n s i t io n s from th e 3s le v e ls in Neon.O s c i l l a t io n from th e 38^ le v e l to other.^ le v e l s has now been observed, by su p p re ss in g th e s tro n g 6 32 .8nm (382 - 2p^) t r a n s i t i o n , on seven o th e r l i n e s . There a re two om issions, 3s^ - 2p^ and 3 S2 - 2p^g. At th e tim e o f w r i t in g , a lth o u g h t r a n s i t io n s from th e 3s^ and 3 th e 2p group have been shown by Zarowin and S c h if f (23) to show some g a in , o s c i l l a t i o n has s t i l l to be re p o r te d on them.Bloom e t a l . (26) were th e f i r s t to r e p o r t o s c i l l a t io n from th e 382 to th e 3p l e v e l s , (38^ - 3p^) a t 3*39 m icrons. The g a in on th i s t r a n s i t i o n (more th a n 20 db p e r m etre) i s s u f f i c i e n t to s a tu r a te th e medium, and can p rev en t o s c i l l a t i o n a t 632 .8nm by d e p le tin g th e p o p u la tio n o f th e common 382 upper l e v e l .A lthough o s c i l l a t io n has no t been ob ta in ed on th e 3 s^ , ^ l e v e lsto th e 2p l e v e l s , o s c i l l a t io n between the 3S|^ and 3s^ le v e ls and the 3p group have been observed (P a te l e t a l .( 2 7 )*Resonant energy t r a n s f e r between He(2 S) m e ta s tab le s and th e 4 f le v e ls in neon, which l i e approx im ate ly 4 kT above the m e tas tab le l e v e l , i s a ls o co n s id e red re sp o n s ib le fo r s e le c t iv e ly p o p u la tin g th e 4 f le v e ls and so en ab lin g o s c i l l a t i o n to be ob ta ined on s ix 4 f — 3^ t r a n s i t io n s a t 1 .8 m icrons (2 8 ) .4o1*2 Mercury - Zinc SystemB efore o s c i l l a t i o n a t 1 .1 m icrons in neon had been re p o r te d , Ablekov e t a l . (2 9 ) re p o r te d o b ta in in g co n sid e ra b le ga in in a m ercury-zinc d isch arg e- 6 l -a t 6 3 6 . 2nm, co rrespond ing to th e 4 - 4 t r a n s i t i o n in z in c . Thes e le c t iv e e x c i ta t io n mechanism proposed being''2Hg (7 + Zn —> Hg + Zn (4 ) - A E . (4 ,9 )where A E = I 33 cm” ^ .M ercury atoms in th e 7 s t a t e a c t a s donors in a re so n a n t energyt r a n s f e r p ro c e ss w ith ground s t a t e z inc atoms to g ive e x c i ta t io n to th e1 —1 34 le v e l o f z in c , 133 cm above th e 7 s t a t e . Decay o f th e low er4 P^ le v e l ta k e s p la ce through UV r a d ia t io n a t 213»8nra. The l i f e t im e s o f- 8 —Qth e se le v e l s a re in th e r a t i o o f 2 .3 x lO" / 1 . 7 x 10~ and so s a t i s f y th e o s c i l l a t i o n l i f e t im e req u irem en t.In s p i t e o f th e re p o rte d o p t ic a l a m p lif ic a tio n o f 10 a t 6 3 6 . 2nm o s c i l l a t i o n has n o t been re p o rte d in t h i s system . I t would appear th a t th e e a r l i e r exp erim en ta l r e s u l t s were m is in te rp re te d .(P r iv a te Communication, Prokhorov (1964), work i s about to recommence on t h i s system ; no re p ly g iven to query about o s c i l l a t i o n . )4 . 1 .3 H elium -krypton and neon-xenon system sO s c i l l a t io n in th e v i s ib le was ob ta in ed in th e a f te rg lo w o f p u lsedh e lium -k ryp ton and neon-krypton d isch a rg e s by Dana and Laures (30) in 1964.In each ca se , helium and neon a re b u f fe r gases a t h igh p re s su re s (3 to 10 t o r r ) and only sm all q u a n t i t i e s (10*"^ - 10“^ to r r ) o f k ryp ton and xenon a r e n ece ssa ry to s u s ta in o s c i l l a t i o n .The e x c i ta t io n mechanisms, in which sp in i s conserved, a re :He (2^S) + Kr ^ He(''s^ ) + Kr (6s ^P) + A e (4.10),Ne ( I s ) + Xe "^(^S^/2^ —^ Ne(^S^) + Xe + (7s ^P) + A E (4.11).- 62 -The He (2^S) m e tas tab le le v e l i s approx im ate ly 2000 cm ^ (0*23 eV)+ 4above th e Kr ( 6s P) le v e ls and re so n a n t energy t r a n s f e r occurs between th e helium and k ryp ton l e v e l s . The Kr ( 6s ^P) le v e ls decay in t r a n s i t i o n s having s tro n g o s c i l l a t o r s tre n g th s p re d ic te d on th e b a s is o f LS co u p lin g , and o s c i l l a t i o n can be o b ta in e d .S im ila r ly , in th e neon-xenon m ix tu re , re so n an t energy t r a n s f e r occurs between th e Ne ( l s ) le v e ls and th e 7s ^P le v e ls o f s in g ly io n is e d xenon.A e i s app rox im ate ly 1200 cm ^ (0 .13 eV) and p o p u la tio n in v e rs io n o ccu rs in4 * 2 4 4th e a f te rg lo w between th e 7s P and 3d S le v e ls , and th e 6p P and 6p D* 2 4 ^4and 3d S le v e l s , and th e 6p P and 6p D le v e l s . O s c i l la t io n occurs on lyin th e a f te rg lo w as s ta t e d , w ith spontaneous em ission la s t in g 23 - 30 m icroseconds. I t w i l l be no ted th a t th e above re a c t io n s fo r an atom -ion c o l l i s io n a re re so n a n t w ith energy d isc re p a n c ie s o f up to 0 .23 eV, so th a t th e requ irem en t f o r resonance in v o lv in g io n s i s n o t a s r e s t r i c t i v e a s fo r re so n a n t atom-atom c o l l i s io n s in which A E must be o f the o rder o f a few kT, presum ably because p o la r i s a t io n occurs du rin g th e c o l l i s io n .The low er l a s e r le v e ls fo r th e re p o r te d l i n e s in k rypton a re th e upper l e v e ls f o r l a s e r l i n e s re p o r te d in pure k ryp ton by B ridges and C heste r (31) so t h a t r a p id d ep opu la tion o f th e low er le v e ls can occur. O s c i l la t io n on th e low er le v e l l a s e r l i n e s o b ta in ed in low p re ssu re p u lsed k ryp ton d isc h a rg e s (31) was no t observed in the h ig h p re s su re helium -kryp ton d isch a rg e s o f Dana and L a u re s ,so th a t th e s e le c t iv e e x c i ta t io n appears alm ost c e r ta in ly to be o f a c o l l i s i o n a l ty p e .The s tro n g e s t l i n e s in bo th k ryp ton and xenon obey th e s e le c t io nr u le s A J = A e = 0 .- 63 -4 .2 Charge t r a n s f e r c o l l i s io n m asersA number o f su c c e s s fu l l a s e r s have been observed in which the s e le c t iv e e x c i ta t io n o f th e upper l a s e r le v e l appears to be th rough a charge t r a n s f e r , o r e le c tro n exchange c o l l i s io n .In only two case s has th e mechanism producing p o p u la tio n in v e rs io n been reaso n ab ly determ ined a s being due to a charge t r a n s f e r c o l l i s io n .These two a re th e h e liu m -io d in e and helium -m ercury system s.4 .2 .1 H elium -Iodine SystemThe f i r s t l a s e r in which charge t r a n s f e r was proposed a s th e s e le c t iv e e x c i ta t io n p ro c e s s , was th e h e liu m -io d in e system re p o r te d by Jenson and Fowles (3 2 ) .O s c i l l a t io n was o b ta in ed by them in th e a fte rg lo w o f a p u lsed helium - io d in e d isc h a rg e , from th re e le v e ls in s in g ly io n ise d io d in e , w ith in O.3 I eV o f th e helium io n ground s t a t e . The proposed upper s t a t e p o p u la tin g mechanism w as:-He^ + I ^ ^ 3 /2 ^ — ^ (^8^) + I* (6 p ^ ) + A e . . . . (4 .1 2 ),where A E was between 0 .21 - O.3I eV.The 6p le v e ls which were p r e f e r e n t i a l ly e x c ite d a re a l l t r i p l e t l e v e l s . A lthough th e re a re s in g le t le v e ls which l i e c lo s e r to th e helium io n ground s t a t e than th e t r i p l e t l e v e ls , o s c i l l a t i o n does no t occur on t r a n s i t io n s from them . Fowles and Jenson (33) suggested th a t t h i s was a case in which th e Wigner Spin s e le c t io n ru le a p p l ie s . There i s a t r i p l e t l e v e l , however, n e g le c te d by Fowles and Jenson which i s w ith in 0 .2 eV o f th e helium io n- 64. -grt)und s t a t e from which th e re i s a s tro n g analogous l a s e r t r a n s i t io n in i s o e le c t r o n ic doubly io n ise d xenon, y e t o s c i l l a t i o n i s n o t observed from t h i s t r i p l e t (6p l e v e l .T y p ica l o p e ra tin g c o n d itio n s were a few t o r r o f helium and 0 .2 t o r rvapour p re s su re o f io d in e , in a 3mm d iam ete r, 1.2m lo n g , co ld cathode p u lsed d isc h a rg e . O s c i l la t io n was a lso observed on two t r a n s i t io n s in a neon-io d in e d isch a rg e when r f e x c i ta t io n was used to g e th e r w ith h igh c u r re n t p u ls e s . These two t r a n s i t io n s have analogous l a s e r t r a n s i t io n s in s in g ly io n is e d c h lo r in e and doubly io n ise d xenon, and th e s e le c t iv e e x c i ta t io n i s ex trem ely l i k e l y to be by a two s ta g e , e le c tro n impact, ty p ic a l io n l a s e r p ro c e s s .A f u l l a n a ly s is o f t h i s system i s g iven in C hapter 7, to g e th e r w ith r e s u l t s o b ta in ed in re s e a rc h undertaken on th i s system .4 .2 .2 Helium-M ercury SystemL aser a c t io n in gaseous io n s was f i r s t observed in io n ise d m ercury, byB e ll (3 4 ) , who o b ta in ed o s c i l l a t i o n in th e a fte rg lo w o f a p u lsed helium -m ercury d isc h a rg e , on green (3 6 7 . 7nm), orange (6l3 .0nm ), and a number o f in f r a - r e d t r a n s i t i o n s in s in g ly io n ise d m ercury.S ince th e f i r s t re p o r te d o s c i l l a t i o n , o s c i l l a t io n has been re p o r te d a t o th e r w aveleng ths, w ith o th e r i n e r t gas-m ercury m ix tu res , and in pure mercuryby a number o f w orkers (33) - (39)•The fo llo w in g th re e s e le c t iv e e x c i ta t io n mechanisms have been su g g ested fo r t h i s h igh g a in , (g re a te r than 20 p e r c en t/m e tre ) system ."•65 -(1) Bloom, B e ll and Lopez (33) suggested th a t d i r e c t e le c tro n im pact e x c i ta t io n from th e ground s t a t e popu la ted th e upper l a s e r le v e ls o f th e 367*7 and 6l3*Onm t r a n s i t io n s . They accounted fo r the hollow beam form ation which th ey observed , under c e r t a in e x c i ta t io n c o n d itio n s , and th e a b i l i t y to change from o s c i l l a t i o n a t one wavele n g th to an o th e r by a p p a re n tly minor changes in t h e i r p u lse r i s e tim e and c u r r e n t , a s being due to changes in th e e le c tro n energy d i s t r i b u t io n . F ig . 4 .4 shows th e t r a n s i t io n s f i r s t observed by Bloom, B e ll and Lopez (33)*(2 ) Dyson (4o) has shown f a i r l y co n c lu s iv e ly th a t in th e case o f th e 6l 3 *Onm l i n e , th e s e le c t iv e e x c i ta t io n in h is d isch arg e system was t h a t o f charge t r a n s f e r in v o lv in g th e helium ground s t a t e io n , the r e a c t io n suggested beingHe’*‘(^S^y2 ) + Hg('^S^) He(^S^) + làg A E o . . . . (4 .13 )- l 4 2where A E = 0 .1 3 eV, w ith a c ro s s - s e c tio n o f 1 .3 x 10 cm com parable to th e Penning io n is a t io n c ro ss s e c tio n fo r He(2^S) + Hg o f 1 .40 X lO 'l^ cmi(3) Suzuki (4 l) , u s in g a tim e re so lv e d sp e c tro sc o p ic g a tin g tech n iq u e , showed th a t th e in t e n s i ty o f the 6l 3 . 0nra l i n e was p ro p o r tio n a l to th e p ro d u c t o f th e d e n s i t ie s o f m e tas tab le helium and m e tas tab le m ercury atoms in th e d isch a rg e system th a t he was u s in g . He concluded th a t the s e le c t iv e e x c i ta t io n mechanism was th a t o r ig in a l ly proposed by Paschen (42) to ex p la in enhancement o f th e 6l3.0nm l in e observed in a helium -m ercury hollow cathode d is c h a rg e :-— 66 —He (2^8) + Hg * (^P) — He + Hg * (7^ P^y^^ + (e + KE) (4 .1 4 ) .—14 2An e s t im a te 'o f t h i s c ro ss s e c tio n i s approx im ate ly 10 cm • F ig . 4 ,5 shows th e energy r e la t io n between th e m e tas tab le s t a t e s o f m ercury and helium .The e x c i ta t io n co n d itio n s used by Suzuki and Dyson d i f f e r e d . Dyson used a d isch a rg e tube o f 2 .3 cm bore and Suzuki one of 6mm b o re , so th a t d if f e re n c e s in r e s u l t s might be expected . Although Suzuki used sh o r t e x c i ta t io n p u ls e s o f 0 .5 m icroseconds which a re considered no t to g ive la rg e m e ta s tab le c o n c e n tra tio n s (H asted (4 3 )) , h is s e le c t iv e e x c i ta t io n i s s t i l l a t t r i b u t e d to m e ta s ta b le -m e te s ta b le c o l l i s io n s .A ccording to B ridges and C hester (4 4 ), B e ll and Bloom have re c e n t ly o b ta in ed o s c i l l a t i o n a t 479*7, 567*7 and 6l 5 *Onm in a pure mercury d isch a rg e (as y e t u n p u b lish e d ) , so th a t i t i s ev id en t th a t a number o f s e le c t iv e e x c i ta t io n p ro c e sse s a re invo lved in the mercury system s, which in c lu d e charge t r a n s f e r , d i r e c t e le c tro n im pact e x c i ta t io n and m e ta s tab le -m e ta s tab le c o l l i s i o n a l energy t r a n s f e r .F u r th e r evidence th a t e le c tro n im pact e x c i ta t io n i s of co n s id e ra b le im portance in th e helium -m ercury system , i s th a t o s c i l l a t io n has now been observed in o th e r Group 2a elem en ts, - cadmium and z inc - on id e n t ic a l t r a n s i t i o n s to tho se on which o s c i l l a t io n i s observed in m ercury. On only a few o f th e se t r a n s i t io n s in cadmium or z inc does c o l l i s io n a l energy t r a n s f e r appear l i k e l y .- 6? -Given in Appendix 4,1 a re th e e q u iv a le n t l a s e r t r a n s i t io n s in z in c , cadmium and m ercury, to g e th e r w ith l i s t s o f th e n e u t r a l and io n ise d le v e ls in v o lv ed in th e l a s e r t r a n s i t i o n s . In d ic a te d a l s o , a re th e p o s s ib le m e ta s ta b le -m e ta s ta b le , ion-atom , re so n a n t type c o l l i s io n s re sp o n s ib le fo r th e s e le c t iv e e x c i ta t io n .4 .3 Atom-molecule l a s e r s .At l e a s t fo u r gas system s have e x h ib ite d maser a c t io n on d is s o c ia t io n by c o l l i s io n s w ith m e ta s tab le atoms o f an o th er g a s . These in c lu d e : -(a) Ne - 0^ ; Ar - 0^ , He o r Ne - CO o r CQ^, He o r Ne - NO o rN^O g iv in g o s c i l l a t i o n in atom ic oxygen a t 844 .6nm (P a te l e t a l . (43 , 46)).(b) He o r Ne - CO or CO^, g iv in g o s c i l l a t io n in n e u tra l atom ic carbon in th e in f r a - r e d (43, 4 6 ) .(c ) He - SFg, o r SF^ g iv in g o s c i l l a t io n in n e u t ra l atom ic su lp h u r(43, 4 6 ) .(d) Argon - B r^, g iv in g o s c i l l a t io n a t 84 4 .6nm, in an unknown s p e c ie s , most l i k e ly atom ic bromine (45, 4 6 ).4 .3 .1 Ne - 0^ , Ar - 0^ SystemsI t i s on ly in th e se two m asers th a t a q u a n t i ta t iv e d e s c r ip t io n can be g iven o f th e e x c i ta t io n p ro cess in v o lv ed , a s c o l l i s io n c ro ss s e c tio n s a re known on ly fo r energy t r a n s f e r from neon o r argon m e tas tab les to oxygen.For a com prehensive account o f th e se system s th e re a d e r i s r e f e r r e d to P a te l (4 7 ) .- 68 -The p e r t in e n t energy le v e ls of neon and argon , th e oxygen molecule and atom ic oxygen a re shown in F ig . 4 .6 a f t e r B ennett e t a l . (4 8 ). Masera c t io n i s o b ta in ed a t 844 .6nm on th e 3p ------ 3 s t r a n s i t i o n o f atomicoxygen.In th e Ne - 0^ m aser, c o l l is io n s o f oxygen m olecules w ith neon m e ta s ta b le s , r e s u l t in m olecular d is s o c ia t io n , le a d in g to ground s t a t e(2^P) and e x c ite d (3^P) oxygen atoms being produced.5 3R a d ia tiv e decay occurs from th e 3 P le v e l to th e 3 S le v e l , and thento th e atom ic ground s t a t e g iv in g r a d ia t io n a t around l40.0nm . The3 8r a d ia t iv e l i f e t im e o f th e 3 Pg le v e l i s approx im ately 3 x 10 s e c , and th e3 -Q3 8 l e v e l approx im ate ly 10 s e c , so th a t th e t r a n s i t io n p r o b a b i l i t i e s a res u i ta b le fo r o s c i l l a t i o n . The low atom ic ground s t a t e oxygen p o p u la tio nen su res t h a t r a d ia t io n tra p p in g does no t l im i t t h i s system a s i t does ino th e r atom ic system s.The e x c i ta t io n p ro cess i s : -Ne * (^P^, ^P^) + 0^ -4 » 0 (3p ^P) + 0 (2^P) + Ne .......... (4 .1 3 ).The Ar - 0^ system i s s l i g h t ly d i f f e r e n t from th a t o f Ne - O^, in th a ti t in v o lv e s a two s te p p ro cess le a d in g to th e p r e f e r e n t ia l e x c i ta t io n o f th e3^P le v e l o f atom ic oxygen. Argon m e ta s tab le s have in s u f f ic i e n t energy tog iv e d i r e c t e x c i ta t io n to th e 3^P le v e l , b u t a re e f f i c i e n t in g iv in g1 1d is s o c ia t io n to th e m e tas tab le 2 8 and 2 D le v e ls , a s fo llow skr* ( I s ) + 0^ 0 *(2 ^8 , 2 ^D) + 0(3^P) + Ar (4 .l6 ) ,o r 0*(2^D) + 0*(2^D) + Ar (4 .17).“ 69 “The second r e a c t io n g iv in g one e x c ite d and one ground s t a t e atom i s themore u su a l form o f d is s o c ia t iv e energy t r a n s f e r .The m e ta s tab le oxygen atoms a re then e x c ite d by e le c tro n im pact to 3th e 3 P l e v e l , r e s u l t in g in p o p u la tio n in v e rs io n o c c u rr in g . The o p e ra tin g c o n d itio n s fo r bo th system s a re s l i g h t ly d i f f e r e n t , b u t r e s u l t in s im ila r p o p u la tio n in v e rs io n s .4 .4 E le c tro n Im pact, - Pure noble gas masersI t was th e ga in r e s u l t s o f B ennett (9) on th e Bs^ - 2p^ ^ t r a n s i t i o n in neon a t 1 .1 3 2 3 m icrons which showed th a t a p r a c t ic a l amount o f g a in was o b ta in a b le in a system th a t r e l i e d on e le c tro n im pact e x c i ta t io n a lone fo r s e le c t iv e e x c i ta t io n .Subsequen tly P a te l e t a l . (49) have produced cw maser o s c i l l a t io n on l4 nob le gas t r a n s i t i o n s , and have extended cw o p e ra tio n out to 133 m icronsin th e nob le g a ses ; most o f th e t r a n s i t io n s being in neon.4 .4 .1 N eu tra l l a s e r sThe e le c tro n c o n f ig u ra tio n o f neon, argon, k ryp ton and xenon a re6 2 , 3, 4 5s im i la r ; th e ground s t a t e i s (np) , where n = ^ and ^ r e s p e c t iv e ly .In each g as , th e re a re s tro n g ly allow ed o p t ic a l t r a n s i t io n s from the le v e lsin th e h ig h e r (np)^ ms and (np)^ rad c o n f ig u ra tio n s , and so th e maine x c i ta t io n occurs through r e a c tio n s o f th e type ,e + (np)^ —9^ (np)^ ms + e ( 4 .l8 ). . 6 _____^ (np)^ rad + e (4 .19 )and e + (np; — ^where m = n + l , n + 2 e tc as shown in F ig . 4 .7 .- 70 -For th e p re s su re s a t which th e se m asers a re opera ted 0 ,01 -0 .2 t o r r , in up to 40ram bore tu b e s ) , th e s tro n g ly allow ed UV t r a n s i t io n s to th e ground s t a t e a re r a d ia t io n trap p ed , and th e s and d s t a t e s can only decay through t r a n s i t io n s to th e low er ly in g e x c ite d p s t a t e s . These a re marked a s MASER in F ig . 4 .7 .The r a d ia t iv e l i f e t im e s of le v e ls (np)^ mp c o n f ig u ra tio n s a re o f the -8o rd e r o f 10 sec due to t r a n s i t io n s o f th e type mp - (n + l ) s . Thus th e se le v e ls a re s u i ta b le l a s e r le v e ls and p o p u la tio n in v e rs io n s can occur between th e s and p , and th e d and p le v e l s .In s p i t e o f t h i s , i t i s found th a t more maser t r a n s i t io n s a re observed in neon and argon from t r a n s i t io n s between th e h ig h e r ly in g s , p , d and f l e v e l s , th a n between th e s and p , and the d and p le v e ls .I t i s on ly in th e case o f the 3s - 4p t r a n s i t io n s , where th e He (2 S) m e ta s ta b le s a re co n s id ered re sp o n s ib le fo r s e le c t iv e ly e x c i t in g th e 3s l e v e l , t h a t th e p o p u la tin g mechanism i s known. The only ex p lan a tio n seems to be th a t a la rg e p a r t o f th e e x c i ta t io n to the h igh ly in g s t a t e s a r i s e s from cascade t r a n s i t i o n s , o r through e le c tro n - io n recom bination .As s ta t e d in s e c tio n 4 .1 .1 , i t has been found th a t the a d d itio n o f la rg e q u a n t i t i e s o f helium to an r f d isch a rg e in xenon in c re ased th e o p t ic a l g a in on a number o f t r a n s i t io n s (P a te l e t a l . (30))» b u t p a r t i c u la r ly a t 2 .026 and 3 .3 0 8 m icrons. This in c re a se in gain has been a t t r ib u t e d to an in c re a s e d e le c tro n d e n s ity , o r an in c re a se d tem pera tu re , o r b o th .A isenburg (3 1 ) u s in g a m u lti-p ro b e tech n iq u e , has observed an in c re a se in th e e le c t ro n d e n s ity b u t no a p p re c ia b le change in the e le c tro n .- 71 -tem p era tu re when helium was added to an r f d isch a rg e in xenon under la s in g c o n d itio n s .In a dc d isch a rg e in xenon th e a d d itio n o f helium only r e s u l t s in a s l i g h t in c re a s e in g a in , and a t th e h ig h e r c u r re n ts n ece ssa ry , c a ta p h o re s is occurs and th e g a in i s g re a t ly reduced (B ridges, (3 2 ) ) .4 .4 .2 Ion l a s e r s .O s c i l l a t io n in gaseous io n s was f i r s t observed by B e ll (34) in s in g ly io n is e d m ercury, in th e a f te rg lo w o f a p u lsed , h igh c u r re n t d isc h a rg e .W h ils t in v e s t ig a t in g th e e f f e c t o f noble gas b u f fe rs on th e v i s ib le mercury l a s e r io n l i n e s , o s c i l l a t i o n was observed in s in g ly io n ise d argon . Even sm all t r a c e s o f argon were s u f f i c i e n t in which to s u s ta in o s c i l l a t io n (B ridges (3 3 ), Convert e t a l . (34) (33))»U nlike th e a f te rg lo w o s c i l l a t io n in m ercury, o s c i l l a t io n in argon was observed du rin g th e e x c i ta t io n p u ls e . Quasi-cw o p e ra tio n , lon g er than anytim e c o n s ta n ts o f rea so n ab le atom ic p ro cesses was re p o rte d in io n ise d argon by B ennett e t a l . (3 6 ) who o b ta in ed o u tp u t powers o f w a tts fo r about a m ill is e c o n d .By u t i l i s i n g sm all bore tu b e s , g iv in g in c re a se d c u rre n t d e n s ity , cw o p e ra tio n was ach ieved by Gordon e t a l . (37) 011 most of th e s tro n g s in g ly io n is e d arg o n , k ryp ton and xenon l i n e s .O s c i l l a t io n has now been ob ta in ed in over 20 elem ents on over 230 t r a n s i t i o n s , in s in g ly , doubly and t r i p l y io n ise d sp e c ie s , ex tend ing over th e UV, v i s ib l e and IR reg io n s o f th e spectrum (Gheo and Cooper (3 8 ), B ridgesand C h este r (3 9 ))•-7 2 -The s e le c t iv e e x c i ta t io n in th e m a jo rity o f th e se t r a n s i t io n s i s by e le c t ro n im pact, and i s c h a ra c te r is e d by two ty p es o f p ro c e ss ,( i ) th e sudden p e r tu rb a tio n p rocess o f B ennett e t a l . (3 6 )( i i ) th e two s te p p ro cess o f Gordon e t a l . ( 6 0 ) .( i ) Sudden p e r tu rb a tio n p ro cessT his f i r s t p ro cess has been used to i n t e r p r e t p u lsed low p re s su re d is c h a rg e s , in which th e re i s a preponderance o f p - s and p - d l a s e r t r a n s i t i o n s . B ennett (3 6 ) has shown th a t the sudden p e r tu rb a t io n p ro cess r e s u l t s in m ainly e x c ite d s t a t e s in th e io n core hav ing th e same p a r i ty as th e io n ground s t a t e . Having th e same p a r i t y a s th e ion ground s t a t e , r a d ia t iv e decay can occur to low er le v e ls which can be re la x ed by s tro n g vacuum UV t r a n s i t io n s to th e ground s t a t e . This p o p u la tin g mechanism only appears ' dominant in p u lsed d isch a rg es w ith h igh E /p , and sh o r t p u lse d u ra tio n , The sudden p e r tu rb a tio n trea tm en t has been extended to h ig h e r d eg rees o f io n is a t io n than the f i r s t , in which t h i s p ro cess must occur w ith e x c i ta t io n , no t from the n e u tra l ground s t a t e , b u t from th e io n ground s t a t e o f th e p reced ing degree o f io n is a t io n . This i n t e r p r e ta t io n has been borne ou t by the o b se rv a tio n s o f Gheo and Cooper (6 1 ) , who r e p o r t th a t doubly io n ise d l in e s only appear a f t e r s a tu r a t io n o f th e s in g ly io n ise d l i n e s , c o n s is te n t w ith a two s te p p ro c e s s .“ 73 -(ii) Two-step processT his p ro cess was proposed to e x p la in th e sm all bore cw argon io n l a s e r , and i s a two s ta g e p ro cess :Ar (3p^) + e —> Ar + (3p) + 2e (4 .2 0 ),then Ar"*"(3p) + e —^ Ar (4p) + e (4 .2 1 ).The ev idence to suppo rt t h i s p ro cess i s th a t th e spontaneous em ission r a t e o f the upper argon io n le v e ls v a r ie s a s th e square o f th e d isch a rg e c u r re n t .A nother two s te p p ro cess has been proposed by B ennett and L ichen (6 2 ), which i s as fo llow sAr (3p^) + e — Ar (3p^ 4s) + e . . . . . (4 .22 ),fo llow ed by Ar (3p^ 4s) + e —^ Ar ^ (4p) + 2 e (4 .2 3 ).In t h i s p ro c e s s , th e f i r s t s te p produces m etastab le argon atoms in th e 3p^ 4s le v e l ; th e second e le c tro n im pact produces the s e le c t iv e e x c i ta t io n to th e Ar ^ (4p) le v e l .4 .4 .3 T ra n s ie n t L asersThe f i r s t t r a n s ie n t l a s e r was re p o rte d by Fowles and Jenson (6 3 ) who3 2 1observed a h igh g a in l a s e r t r a n s i t io n 6p ?s - 6p a t 7 2 2 . 9nm in th espectrum o f n e u t r a l le a d . The low er l a s e r le v e l has th e same c o n f ig u ra tio na s th e le a d ground s t a t e , and i s m e ta s tab le . The l a s e r t r a n s i t io n i s a ls o fo rb id d en fo r e l e c t r i c d ip o le r a d ia t io n in LS coup ling , though i t i s observed a s a f a i r l y in te n s e l i n e in the a rc spectrum o f le a d .- 7 4 -T ra n s ie n t o s c i l l a t i o n has a ls o been re p o r te d by P i l t c h e t a l . (64) in n e u t r a l manganese, and r e c e n t ly o s c i l l a t io n has been observed in n e u tra l atom ic copper vapour a t 310.54nra (38 db/m, 2 kW peak power) and a t 378.215nra (42 db/m, 0 .6 kW peak pow er), by W alter e t a l . (6 3 ) .Physical properties of the new transient lasersA ll th e t r a n s i e n t l a s e r s a re ty p i f ie d by th e le a d and copper system s, th e energy le v e l diagram s o f which, l a s e r t r a n s i t io n s and e x c i ta t io n p a th s a re shown in F ig s . 4 .8 and 4 .9 and a re c h a ra c te r is e d by th e fo llo w in g :(a ) th e upper l a s e r le v e l i s a resonance le v e l , having an o p p o site p a r i t y to th e ground s t a t e , w ith th e e x c i ta t io n prov ided by e le c tro n -a to m c o l l i s io n s .(b) th e low er l a s e r le v e l i s m etastab le having the same p a r i ty as th e ground s t a t e .(c) th e t r a n s i t i o n p r o b a b il i ty o f th e l a s e r t r a n s i t io n i s sm a lle r than th a t o f th e e x c i ta t io n t r a n s i t io n (approxim ately 10^ sec ^ ) , b u t l a r g e r th an th e re la x a t io n t r a n s i t io n (approx im ately 1 sec ^ ) .Experimental conditions( i ) To o b ta in s u f f i c i e n t vapour p re ssu re o f s u i ta b le elem ents h ightem p era tu res have been used in th e la s e r s re p o rte d so f a r . The tem p era tu res used have ranged from 800°C in the case o f le a d to 1300°C in th e case o f copper, g iv in g vapour p re ssu re s o f 0 .1 to0 .3 t o r r .-7 5 -( i i ) C a r r ie r gases of helium or neon a t p re s s u re s o f 1 - 3 t o r r ,enab le th e d isch a rg e to be c a r r ie d from th e e le c tro d e s to th em e ta l l ic vapour re g io n , and p rev en t m e ta ll ic vapour from being d e p o s ite d on th e B rew ster angled windows.( i i i ) The p u lse r i s e time has to be s h o r te r than the r a d ia t iv e l i f e t im eo f th e l a s e r t r a n s i t io n to enable s u f f ic ie n t p o p u la tio n in v e rs io nto b u ild up . To ach ieve t h i s , th e p u lse e x c i ta t io n has taken th e form o f a sim ple c a p a c ito r d isch arg e through in t e r n a l e le c tro d e s , o r by th e d isch a rg e o f a delay l i n e , charged to about 7*5 kV th rough a sp ark gap in s e r ie s w ith th e d isch arg e tu b e .— 76 —H,\ 'i -( 2 f " )COL LISIOHINFRAREDH A S E R- |412e * I M P A C TM E L I U U N E O N l îe -X e cn cr-y levels. T he dom inant excitation pat;'.sfor ih e red and infrared maser transitions are shown.F i g . 4 . 1- 77 -He2 %N e313cm155 cm-1584cm^I 195cm-12 s 22 s 32 s 42S5IiJ L lHelium (2 3) m etastable energy level coincidences in Neon.He336cm-1357.cm"'2 % .450cm-1Ce583 s 23 s 385cm-13 s 43 s 5CM1Helium (2^3) m etastable energy level coincidences in Neon,F i g . 4 . 2F i g . 4 . 3- 78 _1 5 "9 3 9 6 ilOs 11179S'es4ei7346110583.I0--5 -F l g . 4 . 4P a r t ia l energy le v e l diagram of Hg I I showing la s e r tr a n s i t io n s , ( a f te r Bloom e t a l . (3 5 )).2Q55He(2^) — 19.77Hel?S) —eV103815.25:5 F. 13.87; 7*P, 11.85 i 7*S758 : 6*P,— — 000:6*36 Pi5.434.66 6'PoF ig . 4.5aoo.__Hg_e,3^Energy re la tio n between s ta te s of mercury and m etastable s ta te s of he lium ,(afte r Suzuki,(4I ) ,- 79 -fiiiEnergy levels” pertinent to the neon-oxygen and argon-oxygen masers.» The dom inant excitation paths in these two systems are indicated by the arrows.F i g , 4 . . 6(O p )* lOMZATION LIMIT (np)*(n»2)dAm+I)dlnprtn+3)«(np£(2i£2,PLevel*OLeveli~\^ )(o«l)P 12 Level!14 Level#e IM P AC TGround S ta teSchematic indication of electron configurations pertinent to the pure neon, argon, krypton, and xenon masers. The dom inant excitation paths are indicated by the arrows.F i g . 4 . 75 -,4 ->a>2 -1 ■80Pb.*Pl 6s26p7s7229 nm LASER LINE□2 6s2 6p2o 2 83*3 nmq:u Re s o n a n c e lineLÜ_ JLÜ- ^Pq 6s2 6p2Energy level diagram for the lead laser.Fig. 4.84 .>01Cu.3/2 ^ 1 /2 i r10 ir> <3/21/21/23d10fs)4p 2p*=Fig. 4.93d9 4s2 2 d3d^0(1s)4s hEnergy level diagram for the copper laser.- 81 -CHAPTERP o p u la tio n In v e rs io n in th e n eg a tiv e glow plasma o f a Hollow Cathode D ischargeIn tro d u c t io nThe re so n a n t, atom-atom , energy t r a n s f e r p ro c e ss , in I 963 b e lie v e d to be th e predom inant s e le c t iv e e x c i ta t io n p ro cess in th e helium -neon l a s e r , in d ic a te d th a t t h i s p ro cess would be th e s o r t o f p ro cess to seek fo r o th e r l a s e r system s.The n e g a tiv e glow Hollow Cathode D ischarge (H.C.D.) has th e fo llow ing p ro p e r t ie s which made i t worthy o f c o n s id e ra tio n as a l a s e r medium.(a) Atom-atom p ro cesse sThe l i t e r a t u r e in d ic a te d th a t atom-atom c o l l i s io n a l p ro cesse s were o f co n s id e ra b le im portance, in the lo w -f ie ld , e lec tron -beam m ain ta ined plasm a o f th e H.C.D.(b) Narrow lin e -w id thUnder s u i ta b le c o n d itio n s , th e H.C.D. can g ive extrem ely narrow l i n e s , having been used e x te n s iv e ly fo r h y p erfin e s t r u c tu re a n a ly s is by S chu le r (1 ) , Tolansky (2) and R oesle r and De Noyer (3 ) . The f r a c t io n a l ga in in a l a s e r (g^) i s in v e rs e ly p ro p o r tio n a l toassuming th e l i n e i s Doppler broadened (Heavens ( 4 ) ) , a s i s shown by ( 1 .1 ) . C onsiderab le advantage i s , th e re fo re , to be expected in u s ing a source th a t p rov ides narrow em ission l i n e s .- 82 —(c ) E le c tro n c o n c e n tra tio nThe e le c tro n c o n c e n tra tio n i s la rg e r in th e n e g a tiv e g low -region o f a glow d isch arg e than in th e p o s i t iv e column, and i s la rg e r by two o rd e rs o f magnitude in the H.C.D, (Badareu and Wachter ( 3 ) ) . F u r th e r , i t i s b e lie v e d th a t a non-M axwellian e le c tro n energy d i s t r i b u t io n b ia sed to h igh en e rg ie s occurs in th e H.C.D. a t th e cathode edge o f the n eg a tiv e glow plasm a, (F ra n c is ( 6 ) ) .(d) R ad ian t in t e n s i tyThe r a d ia n t in t e n s i t y in th e H.C.D. i s many tim es la rg e r th an th a t in th e p o s i t iv e column o f th e glow d is c h a rg e . (P opovici and Somesan ( ? ) ) .(e) S p u tte r in g mechanismThe s p u t te r in g a c t io n o f th e H.C.D. p ro v id es an atom ic vapour of th e m a te r ia l o f which th e cathode i s c o n s tru c te d , w ithou t the need fo r h ig h tem p era tu res to p rov ide s u f f i c i e n t vapour p re ssu re in which to o b ta in l a s e r o s c i l l a t i o n . By a s u i ta b le choice o f cathode m a te r ia l and c a r r i e r g as , p o p u la tio n in v e rs io n through re so n an t energy t r a n s f e r c o l l i s io n s appeared p o s s ib le . T h is , in 1963 would have extended l a s e r in v e s t ig a t io n s in to the non-gaseous e lem en ts, and p o s s ib ly in to s h o r te r w avelengths than were then a v a i la b le . (P u lsed o s c i l l a t io n has now been ob ta in ed in non-gaseous elem ents by B e ll e t a l . (8 ) ; P i l t c h e t a l . (9 ) ; and S i l f v a s t e t a l . (10) u s in g h igh tem peratu re te c h n iq u e s .)-83 -( f ) M olecular p ro cesse sThe n eg a tiv e glow o f a glow d isch arg e i s a recom bination-dom inated plasm a, (H urt, (1 1 ) ) , M olecular p ro cesse s a re o f co n sid e ra b le im portance in such a plasm a and a re th e re fo re o f i n t e r e s t in b a s ic l a s e r s tu d ie s .(g) Plasma u n ifo rm ityNo s t r i a t i o n s o r i n s t a b i l i t i e s have been re p o rte d in electron -beam m ain ta ined , f i e l d f re e p lasm as, and th e same i s to be expected in th e H.C.D. (B irg e r-P e rsso n (1 2 ) ) .3 .1 S p e c if ic system sThe H.C.D. was in v e s t ig a te d a s a means o f o b ta in in g p o p u la tio n in v e rs io n and cw l a s e r o s c i l l a t i o n in ;(1) m e ta l l ic elem ents which have low vapour p re s su re s and h igh m e ltin g p o in ts .(2) th e io n sp e c ie s o f a gas m ixture.(3) pure neon; o r in a helium -neon m ix tu re , a t low p re ssu re a t 6 32 .8nm,3 .1 .1 P o p u la tio n in v e rs io n in m etal vapours R esonant energy t r a n s f e r r e a c t io n s o f the typeM + G —^ M + G — ^ E . . . . . (3 . 1 )where M h ere in d ic a te s a m etal atom, and G a c a r r i e r gas atom, were co n s id e re d , fo r A® 0 .1 eV, in which la rg e c ro ss s e c tio n s fo r energyt r a n s f e r occur (see F ig . 2 .4 from Mott and Massey (1 3 )) .S u ita b le com binations were determ ined , o f th e i n e r t gases helium , neon and argon , and o f elem ents from which cathodes o r cathode l i n e r s could be- 8 4 -made and in which c lo se energy co in c id en ces o ccu rred between m etastab le l e v e ls o f th e i n e r t gas and th e cathode elem ent.The m e ta s tab le le v e ls o f th e i n e r t gases a re h ig h e r than th e io n is a t io n p o te n t ia l s o f o th e r e lem en ts, so th a t r e a c t io n (3*1) becomes th e f a m i l ia r Penning r e a c t io nM + G* + G + (e + A a ) ( o o '/ l 'o e ) ( 3 .2 ) .In t h i s r e a c t io n an e le c tro n i s a v a i la b le to remove any excess p o te n t ia l energy and re so n a n t r e s t r i c t i o n s a re n o t a s s t r in g e n t a s fo r n o n - io n is in g , atom-atom e x c i ta t io n t r a n s f e r c o l l i s io n s .Assuming th a t p a r t i c u la r le v e ls would be s e le c t iv e ly e x c ite d by t h i s p ro c e s s , w ith A E o f th e o rd e r o f a few kT, t r a n s i t io n s were determ ined from Atomic Energy L eve ls ( l4 ) and examined fo r l a s e r s u i t a b i l i t y .P o s s ib le re so n a n t energy com binations o f th e i n e r t gases and m ercury, w ith v a r io u s cathode m a te r ia ls a re g iven in Appendix 3*1»Because o f th e h igh s p u t te r in g y ie ld o f neon (13) and in fo rm atio n a v a i la b le on th e c o n c e n tra tio n o f neon m e ta s tab le s in a H.C.D. ( l6 ) , com binations o f neon and cathode m a te r ia ls were &iven s p e c ia l a t t e n t io n .F ig s . 3 .1 - 3 .3 show e l e c t r i c d ip o le r a d ia t io n t r a n s i t io n s w ith s u i ta b le l i f e t im e s fo r th e upper and low er le v e ls in which th e upper le v e l has a c lo se energy co incidence w ith a neon I s m etastab le l e v e l .The neon-aluminium com bination was chosen fo r experim en ta l s tu d ie s fo r th e fo llo w in g re a so n s : ( i ) t r a n s i t io n p r o b a b i l i t i e s a re c a lc u la b le fo rio n is e d aluminium w ith s u f f i c i e n t accuracy u s in g th e B ates and Damgaard C e n tra l F ie ld approx im ation , and ( i i ) neon can g ive a h ig h s p u t te r in g y ie ldo f alum inium .- 85 "3*1.2 P o p u la tio n in v e rs io n in gas m ix tu resC o n s id e ra tio n o f th e r e p o r t o f p o p u la tio n in v e rs io n in a m ercury-zinc system ( l a t e r u n su b s ta n tia te d )b y Ablekov e t a l . (1 ? ) , and v a r io u s a sp e c ts o f th e o p e ra tin g co n d itio n s of th e helium -neon p o s i t iv e column and r f e x c ite d l a s e r s le d to an exam ination o f th e H.C.D. a s an e x c ite d medium fo r gas, and gas m ixture l a s e r s . Three system s were examined ( i ) helium -m ercury;( i i ) h e liu m -io d in e ; and( i i i ) helium -neon; and neon; a t low p re s s u re s .In bo th mercury and io d in e , th e spectrum l in e s show ex ten siv e h y p e rfin e s t r u c tu r e , and i t was o f co n s id e ra b le i n t e r e s t to examine such s t r u c tu r e du rin g l a s e r o s c i l l a t i o n .( i ) H elium -m ercuryL aser o s c i l l a t io n was f i r s t observed in the io n s p e c ie s , in mercury, in a m ix ture o f helium and mercury under p u lsed e x c i ta t io n c o n d itio n s by B e ll ( l 8 ) . O s c i l l a t io n occurred on two v is ib l e l i n e s and on a number o f l i n e s in th e in f r a - r e d . The behaviour o f th e l a s e r ou tpu t was such to in d ic a te th a t s a tu r a t io n was n o t o ccu rrin g so th a t cw o p e ra tio n might be p o s s ib le in a s u i ta b le so u rc e .D uring an experim ent designed to determ ine i f mercury was an e f f e c t iv e c a r r i e r gas in producing s p u tte r in g and to check w hether the A1 I I l in e a t 39 0 .Onm (see 3 .8 .3 . ) was enhanced in a H.C.D. w ith a c a r r i e r gas o th e r than neon, i t was n ecessa ry to id e n t i f y s tro n g spectrum l in e s in the reg io n o f 4-OOnm.— 86 —In t h i s id e n t i f i c a t i o n i t was found th a t th e s tro n g l i n e s o r ig in a te d from h ig h ly e x c ite d mercury io n l e v e ls , above th e le v e ls from which la s e r t r a n s i t i o n s occu r. I t was a sm all s te p then to check fo r th e l i n e s on which o s c i l l a t i o n had been o b ta in ed in th e range 300 - 800nm, in th e H.C.D. spectrum . In t h i s ran g e , a l l the l i n e s on which o s c i l l a t io n had been o b ta in ed under p u lsed co n d itio n s were observed in th e H .C .D ., w ith i n t e n s i t i e s com parable w ith th o se o f Hg I l i n e s . A l a t e r l i t e r a t u r e sea rch re v e a le d th a t th e se l i n e s had been observed and examined by McLennon (1 9 ), Naude (2 0 ), and Mrozowski (21) in a H.C.D. in mercury and helium -m ercury m ix tu re s . In view o f t h i s , a number of experim ents were c a r r ie d ou t to see i f i t was p o s s ib le to m a in ta in a plasm a in a long m etal hollow cathode , w ith a le n g th o f g r e a te r than one m etre , n ecessa ry fo r o b ta in in g a p r a c t i c a l amount o f g a in .S tu d ie s were a ls o made o f m u lti-ca th o d e hollow cathode d isc h a rg e s .( i i ) H elium -iodineP u lsed l a s e r o s c i l l a t io n was f i r s t o b ta in ed by Fowles and Jenson (2 2 ), on 12 v i s ib l e and near in f r a - r e d l i n e s in th e n e u tra l and io n spectrum o f io d in e . O s c i l la t io n occurred only in th e a f te rg lo w .In s tu d ie s o f halogen e x c i ta t io n u sin g argon , neon and helium , B erez in and Yanovskaya (23) observed th a t the in t e n s i ty of c e r ta in io n ise d io d in e l i n e s was c o n s id e ra b ly enhanced when a he liu m -io d in e m ixture was e x c ite d in a H.C.D. Three of the l i n e s X s 498 .6 , 321.6 and 3 4 0 .?nm which were found to be enhanced in th e w avelength range 440.0 - 3 7 0 . Onm (Table 7 .1 ) , have e x h ib ite d p u lsed la s e r o s c i l l a t io n . The maximum in t e n s i ty o f th e se io n ise d l i n e s in th e H.C.D. occurred a t a p re ssu re o f 17 t o r r .— 87 —The h ig h p re ssu re o p e ra tio n o f th e H.C.D. o f B erez in and Yanovskaya and the r e l a t i v e l y h ig h p re ssu re o f o p e ra tio n (a few t o r r o f helium ) o f Fowles and Jen so n , in d ic a te d th a t the h e liu m -io d in e system was a lm ost c e r ta in ly a re so n a n t c o l l i s i o n a l system w ith atom-atom, a tom -ion , or atom -m olecule c o l l i s io n s b e ing re sp o n s ib le fo r th e s e le c t iv e e x c i ta t io n o f th e upper l a s e r l e v e l s . S ince th e upper l a s e r le v e ls , w ith one ex ce p tio n , a re w ith in O.3 I eV o f th e helium io n ground s t a t e , Fowles and Jenson suggested th a t the s e le c t iv e e x c i ta t io n v;as one o f th e charge exchange type (10/01 ) . The h igh g a in in t h i s system does su g g est th a t th e e x c i ta t io n i s v ia a re so n a n t p ro c e s s . However, energy t r a n s f e r p ro cesse s have only been co n sidered re so n a n t when th e energy d isc rep an cy i s l e s s than 0 .1 eV, and la rg e c o l l i s io n c ro s s s e c tio n s a re , by th e Massey 'n e a r -a d ia b a tic h y p o th e s is ', no t expected fo r slow io n s .Dr. R. S tebb ings (p r iv a te communication) has shown th a t a la rg e c o l l i s io n c ro s s - s e c t io n occurs in th e slow m olecule, 0^ + c o l l i s io n bu t on ly when resonance i s o b ta in e d . Presumably p o la r i s a t io n o r Coulomb fo rc e s , which could accoun t fo r th e la rg e A E fo r re so n an t ion-atom c o l l i s io n s , a re n o t r e le v a n t to 3 body ion-m olecu le c o l l i s io n s .A s e r i e s o f experim ents were c a r r ie d ou t to see i f cw la s e r o s c i l l a t io n could be ach ieved in a h e liu m -io d in e H .C .D ., and to t r y to determ ine the energy t r a n s f e r mechanism invo lved in th e s e le c t iv e e x c i ta t io n o f c e r ta in io n ise d io d in e energy l e v e l s . These experim ents a re covered in 3*11»(ii#) Helium-neon and neon a t low p re s s u re .T his system i s inc luded a s a se p a ra te trea tm en t in 3 .1 2 , to g e th e r w ith exp erim en ta l work and r e s u l t s .3*2 The neon-alum inium , m etal vapour system3 .2 .1 In o rd e r to ach ieve cw l a s e r o s c i l l a t io n (1 .3 ) shows th a t l i f e t im e s o f th e upper and low er le v e ls must be s u i ta b le , and th a t th e r a t e of e x c i ta t io n to th e upper le v e l must be s u f f ic ie n t to enable th e g a in to be g re a te r than th e o v e ra l l system lo s s e s .The in v e s t ig a t io n o f th e neon-aluminium system c o n s is te d o f two p a r ts(a) C a lc u la tio n of th e t r a n s i t io n p r o b a b i l i t i e s o f th e proposed A1 I I l a s e r t r a n s i t i o n s .(b) E xperim ental measurement o f th e ab so lu te in t e n s i ty o f th e proposed l a s e r l i n e , under optimum c o n d itio n s , to determ ine th e s tead y s t a t ep o p u la tio n o f th e upper le v e l and r a t e o f e x c i ta t io n to th e upper l e v e l ,3 .2 .2 C a lc u la tio n o f t r a n s i t i o n p r o b a b i l i t i e sThe c a lc u la t io n of t r a n s i t i o n p r o b a b i l i t i e s between le v e ls o f an atom icsystem e n t a i l s th e e v a lu a tio n o f t r a n s i t io n in t e g r a l s fo r th e le v e lsconcerned . B ates and Damgaard (24) have shown th a t a lm ost th e wholec o n tr ib u tio n to th e t r a n s i t io n in t e g r a l s invo lved a r i s e s from th e reg io n o fth e atom fo r which a Coulomb approxim ation fo r th e p o te n t ia l i s v a l id .The in t e g r a l s may th u s be exp ressed , a s in th e case o f the hydrogenatom, in term s o f hypergeom etric fu n c tio n s . The in te g r a l quantum numbers*o f hydrogen a re re p la c e d by th e e f f e c t iv e quantum numbers (n ) .n* = (5 .3 )where ( i s th e energy in Rydbergs fo r e x c i t in g th e n e u tra l atom . T ables fo r e v a lu a tin g the r a d ia l p a r t o f the t r a n s i t io n in t e g r a l have been given by B ates and Damgaard.— 89 “The spontaneous t r a n s i t i o n p ro b a b i l i ty fo r r a d ia t io n from s t a t e A to s t a t e B i s g iven by Condon and S h o rtle y (23) a sA (A , 6 ) = (t4-n"^-v) V 3 k ) . 5(A ) (5 . 6 )where S(A,B) i s th e l i n e s t r e n g th , i s th e t o t a l an g u la r momentum in s t a t e A, and i s th e wave number in cmThe l i n e s t r e n g th may be expressed in term s o f th e in te g r a l o f the r a d i a l p a r t o f th e waVe fu n c tio n in th e i n i t i a l and f in a l s t a t e and in th e form- i / ( 4 - £ 2 _ x') J G lR f (5.7)where 1 i s th e la r g e r o f th e two az im u tha l quantum numbers. Values o fa re o b ta in ed from th e ta b le s o f B ates and Damgaard. W ith l) expressedin Rydbergs, th e t r a n s i t i o n p ro b a b i l i ty may be w r i t te n a sA(A,B) = 2.662 X lO* k"Ll 2 g e c '^ ................‘ (3 .8 )where k depends on th e l i n e o f th e m u l t ip le t considered (g iven in (2 3 ) ) .B a s ic a l ly , in th e B ates and Damgaard C en tra l F ie ld approx im ation , the charge i s con sid ered a s lumped in a sm all r a d iu s , and th a t fo r r l e s s th an r^ th e c o n tr ib u tio n to th e p o te n t ia l i s n e g l ig ib le . B ates and Damgaard ta k e 1 p e r c en t o f coulomb d ep a rtu re a s l i m i t o f r^ — P«po • The method r e s u l t s in th e fo llow ing agreem ent being ob ta ined between c a lc u la te d and experim en ta l d e te rm in a tio n s fo r :( i ) s im p le , l i g h t system s, a c c u ra te on a l l t r a n s i t io n s ; agreem ent w ith in 10 p e r cen t fo r p - s t r a n s i t io n s a s f a r a s atom ic number19, and p - d, d - f t r a n s i t io n s fu r th e r down th e p e r io d ic ta b le .- 90 -(ii) simple, heavy systems, not subject to interference farther downthan atomic number 19, e.g. Cul or Csl, the agreement is towithin about 50 per cent.(iii) complex systems, e.g. the active electron is in a shellcontaining other electrons, fair agreement is shown; even Til,(atomic number 8l) is not expected to differ by a factor greater than 2.Transition probabilities for A1 II transitionsAluminium has an atomic number of 13, and has neutral ground state electron configuration Is^ 2s^ 2p^ 3s^ 3p* Fig. 5.1 shows the proposed A1 II laser transitions, which are:(3p^ “■ ^^^) at 390.Onm, and(3s 3p - 3s^ ) at 167.Inm.For these transitions, in a light complex system, fair agreement is expectedbetween calculated and experimental transition probabilities. Using the method of Bates and Damgaard:cy ^ for the transition A1 II (3P^ ^D^ - 3s 3p ^P^ )^ = 0.64»Carrying out a similar treatment for the lower transitioncT ^ for the transition A1 II (3s^ - 3s 3p ^P^) = 0.l4.From (5 .6), and with the line strength S(A,B) calculated from Condon and Shortley (25), the ratio of the upper transition probability (A^^) tothe lower (A^^) is equal to 1.28/1.79. Since A^^A^^ is less than 1, thetransitions are just suitable for cw oscillation. The effect of thestatistical weights factor (g^g^) in (1.3) marginally reduces the suitability by a factor of 5/3.cw-9 1 -Using (3.8), = 130 X 10^ sec” ,^ and A^ = 162 x 10^ sec.”^3 .2 .3 Measurement of the absolute intensity of the A1 II 390.0nm lineThe absolute intensity is defined as the radiant energy flowing per unit time in unit solid angle, through unit area perpendicular to the beam. This energy may be contained in a unit spectral range, or unit wave numberi.e. I>^ or I-^ . If the energy is distributed over a spectrum line theintegrated intensity is given byI o r ciO (3.9).Generally I,() = I{) (€,o_i 0 6>I -Z 0 5 Q0 40 321522 5 2223Spectrum line positionF ig . 5 .8 , Photographic plate calibration-continuous source.- 126 -0 8 torr0 60 6 4 torr0 5oE 1 6 torr0 4tozoLIMIT OF PLATEP^ÔRTÎÔNÂLÏTy0 3—I<Ü§02oi8 0 too 120 140SPECTRUM LINE POSITION.F ig , 5«9. P r o f i le o f io n ise d alurainium l in e a t 390 .Onm,' Nell 3655nm0 8N e l 3575nm0 6'A l l ' ' 39Q0nmI- 0 40 210 2 00NEON PRESSURE (torr)F ig . 5 .10 . V a ria tio n o f in te n s i ty w ith p re ssu re o f neon,- 127f *ao-H I S- f -%CVJF ig . 5 .1 1 . Hollow cathode d ischarge power supply .— 128 —Anodes7 mm bore I 1 LT>v2cm bore | \:? rPyrex tube Locating dimplesCold FingerOverall Length =125cm Cattwdes: 7 5cm x Bmmdia.Separation 1 5cmmultl-cathoda discharge tubeF ig , 5 . 1 2 . Helium-neon m u lti-ca th o d e d ischarge tu b e .Anode I Mo disci Kovar seal C J / Water cooling jjacket FVrex window7 ^/ CartB14Gone. Spot welded\8mm grsaseless to tube stopcocktKxte 0-570 Bore Nllo K tubeWater ooolad hollow caBioda tube, external type.Fig, 5. 13 . Helium-neon and neon external H.G.D, tube,- 129 -VPyrex disc Araldite sealn , r \AnodeKovar seal ^y/ brazed onTo vacuum steel, 155cm I D Length 30cmsystem T 9 c m O DF ig , 5 . U . Chebotayev tube without water cooling.To vacuum Annular Anode system4 cm dia. pyrex10cm3 c m oxide coated cathode, 1 cm dia. from commercial neon signF ig , 5,15. Negative glow test tube.U lPyrex windowF igs. 5,14. and 5,15. Experimental discharge tnbes,130 -8 |BUb!S 9OU0J0|08V I B U D j c 0 O U 0 j 0 i a yi2 cE2gCOCOoCOc(DQ)ÛXHIFig. 5.16.— 131 —E ü301ü.OXk—OzLU12108005 NEON PRESSURE (torr)0F ig , 5 .1 7 . E x ten t o f n eg a tiv e g low -vs-p ressu re o f neon,- 132 ->%COL_■*— >nL_CO>I -(/)zLUH-ZLUZHELIUM PRESSURE (torr)A Annular d ischargeCathode: 7 5 c m x 0 6cm d iam eterAr15010050CURRENT( mA)Fig# 5#13# Variation of lin e intensity with current,- 133 -nv_CO>h-co zLUI -zLU8765432I*4496nm1Cathode; 7 5cm x 0-6cm diameter00 15010050CURRENT(mA)F ig , 5.19* V a ria tio n o f in te n s i ty o f io n ise d io d in e and helium l in e s w ith c u rre n t (15 t o r r o f he lium ).- 134 -mIPlate 5*1* General arrangement of helium-iodine H.G.D. apparatus showing chopper systems.P la te 5 ,2 . 'Resonant* chopper assembly.- 135 -CHAPTER 6P o p u la tio n In v e rs io n in p o s i t iv e column d isch arg es In tro d u c tio nT his s h o r t ch ap te r d e a ls on ly w ith th e helium -neon l a s e r , and e x p la in s im p o rtan t a s p e c ts o f th e observed e f f e c t s o f th e d isch arg e tube d iam ete r, th e p re s s u re , and the a d d itio n o f helium , on t h i s w ell documented l a s e r system . I t i s no t concerned w ith g iv in g a d e ta i le d d e s c r ip t io n o f p o p u la tio n in v e rs io n mechanisms in th e p o s it iv e column reg io n o f glow d isc h a rg e s , in a whole range o f l a s e r system s.The f i r s t helium -neon la s e r re p o rte d by Javan , B ennett and H a r r io t t (1) a t 1.1523yuM, used an e le c tro d e le s s r f e x c ite d d isch a rg e . T his type o f d isch a rg e was used to avo id s p u tte r in g o f the in te r n a l m irro rs th a t were f i r s t u sed . I t was l a t e r shown by White and Rigden (2 ) , u s in g an e x te rn a l m irro r c a v i ty , th a t o s c i l l a t io n could be ob ta ined in the p o s it iv e column o f a glow d isc h a rg e .6.1 P o p u la tio n in v e rs io n problem s.A number o f th e o r e t ic a l s tu d ie s o f p o p u la tio n in v e rs io n in gaseous d isc h a rg e s have been made in which th e assum ption i s made th a t a M axwellian e le c tro n energy d is t r ib u t io n i s app licab le ,^B aso v , (3)» Javan ( 4 ) . The v a l id i t y o f th i s assum ption i s no t obvious, and i t has been q u es tio n ed by F a b rik a n t (3 ) , Ward (6) and B ennett (? )•“136 •In th e f i r s t helium -neon system s o p era tin g in neon a t l«132^J!m and a t 6 3 2 *8nm, th e dominant mechanisms producing p o p u la tio n in v e rs io n were co n sid ered to be th o se o f re so n an t energy t r a n s f e r c o l l i s io n s in v o lv in g helium m e ta s ta b le s and ground s t a t e neon atoms, a s d escrib ed in C hapter 4 . Subsequen tly o s c i l l a t i o n was ob ta ined a t 1.1323^m in an r f d isch arg e in pure neon by e le c tro n im pact e x c i ta t io n , and in th e in f r a red in a number o f pure noble gases by B ennett (8 ) , However, th e a d d itio n o f helium was found to g ive in c re a se d g a in , (notably 3n a helium-xenon discharge), in noble g a se s , where re so n an t energy t r a n s f e r c o l l is io n s do no t occur, (P a te l e t a l . ( 9 ) ) . At th e same tim e a s th e above, Rigden and White (10) showed th a t th e g a in a t 6 3 2 . 8nm in th e helium -neon l a s e r v a rie d in v e rs e ly as the d isch a rg e tube d iam ete r. This in v e rse d iam eter dependence o f th e s in g le p ass g a in has been a t t r ib u t e d to th e p rocess o f d if fu s io n to th e w a lls and d e - e x c i ta t io n o f th e Ne 1s m e ta s ta b le s , the presence o f which r e s u l t s in e le c tro n e x c i ta t io n to th e 2p le v e ls , and a lso in ra d ia t io n tra p p in g o f th e 2p-1s em ission .B ennett (1 1 ), however, has analysed th e helium -neon l a s e r , and c a lc u la te s th a t th e only p rocess th a t can r e s u l t in a l / r ga in dependence i s th a t o f atom-atom c o l l i s io n a l d e -e x c ita t io n o f the low er l a s e r l e v e ls , and th a t d if fu s io n o f m e tas tab les to th e w a lls r e s u l t s in a l / r ga in dependence. T his observed l / r ga in dependence le d to the design o f sm all bore l a s e r s system s, and le d to s u b s ta n t ia l gains being o b ta in ed . With tu b e s o f d iam ete r about 2mm i t i s d i f f i c u l t to couple in enough r f power and i t i s now custom ary to use dc d ischarges to p rovide th e necessary e x c i ta t io n .-1 3 7 -The o b se rv a tio n s above le d to an exam ination o f th e d isch arg e p ro p e r t ie s o f th e p o s i t iv e column (and r f ) d isch a rg e s , in an a ttem p t to o b ta in a u n ify in g q u a l i ta t iv e ex p lan a tio n o f helium -neon l a s e r d isc h a rg e s , and th e e f f e c t o f helium in gaseous la s e r system s.6 .2 D ischarge p ro p e r t ie s o f p o s i t iv e column and r f l a s e r d isc h a rg e s .The e lem entary th eo ry o f th e p o s it iv e column and r f d isc h a rg e s , in which th e io n is a t io n r a t e and d if fu s io n r a te to th e w a lls a re equated , g iv e s a good q u a n t i ta t iv e d e s c r ip t io n o f the r e la t io n between th e a x ia l and r a d i a l e l e c t r i c f ie ld , p re s su re , tube ra d iu s and th e n a tu re o f th e gas (von Engel (1 2 ) ) .In th e r f d isch a rg e , e le c tro n s ga in energy from the o s c i l l a t in g f i e l d , and in th e p o s i t iv e column, from th e s t a t i c e l e c t r i c f i e ld along th e column. The only way th a t an e le c tro n can ga in s u f f ic ie n t energy from an o s c i l l a t i n g e l e c t r i c f i e l d to enable i t to e x c ite o r io n is e , i s through e l a s t i c c o l l i s io n s . The average k in e t ic energy l o s t per c o l l i s io n (2m /k), where m i s th e mass o f th e e le c tro n and M the mass o f an atom o r m olecule , i s more th an made up by th e gain in energy which can ensue by p u t t in g th e e le c tro n ou t o f phase w ith th e o s c i l l a t in g f i e l d , and in to a c c e le r a t in g phase w ith th e nex t h a l f c y c le .In th e dc d isc h a rg e , c o l l i s io n s , in th e absence o f plasma o s c i l l a t io n s , can on ly r e s u l t in a lo s s o f k in e t ic energy by the e le c tro n s .Two th in g s im m ediately emerge, to produce h igh energy e le c tro n s capab le o f d i r e c t e x c i ta t io n i n : -- 138-(s-) an r f d isc h a rg e , th e e le c tro n c o l l i s io n frequency must be a s h ig h a s p o s s ib le .(b) a dc d isc h a rg e , th e r a t i o o f e le c tro n d r i f t ve loc ity /random v e lo c i ty must be la r g e . This can only be achieved by making the g ra d ie n t o f th e s t a t i c e l e c t r i c f i e l d la rg e .1 . Helium has th e low est io n is a t io n e f f ic ie n c y o f a l l th e gases a t a l l e le c tro n e n e rg ie s , (von Engel (13)) so th a t i t w i l l be the most e f f i c i e n t gas in producing e l a s t i c c o l l i s io n s w h ils t m ain tain ing a h igh e le c tro n tem pera tu re (T ^). The a c tio n o f helium in non-resonan t l a s e r system s, and r f l a s e r d isch a rg es can th u s be exp la in ed . This c o l l i s io n a l in te r p r e ta t io n and i t s im portance in th e 632 .Snm helium -neon la s e r has been borne ou t by th e o b se rv a tio n s o f Toyoda and Yamanaka ( l4 ) who observed th a t in a 60 c . p . s . s in u s o id a l ly modulated helium -neon r f d isch a rg e , o s c i l l a t io n was observed only when the e le c tro n c o l l i s io n frequency was h ig h . W hilst the c o l l i s io n frequency in c re a sed th e e le c tro n d e n s ity remained e s s e n t i a l ly c o n s ta n t .2 . A re d u c tio n in th e bore o f a d ischarge tube r e s u l t s in an in c re a se in th e e l e c t r i c f i e l d th a t i s needed to balance th e charge d if fu s io n lo s s .As a r e s u l t o f the in c re a se in the e l e c t r i c f i e ld , the average energy o f th e e le c tro n s in c re a s e s , and more d i r e c t e le c tro n im pact e x c i ta t io n occurs in the form o f;Ne + e -------- ^ Ne (^^g) + @ . . . . o (6 .1 ),He ("*S ) + e ------^ He (2^S) + e . . . . . (6 .2 ) ,oo r He (^S ) + e ------> He (2^S) + e (6 .3 ).o-1 3 9 -Wada and H e il (15) have re c e n tly shown th a t a t a p re ssu re x d iam eter o f d isch a rg e (pD) o f 0*6 t o r r - ram, in a helium -neon dc d isc h a rg e , the average e le c tro n energy i s 20eV. D irec t e x c i ta t io n to the 3s^ le v e l can th e re fo re be a p p re c ia b le .A f u r th e r exam ination o f th e p ro p e r t ie s o f helium -neon la s e r d isch a rg e s has r e s u l te d in th e e s tab lish m en t o f a r e la t io n s h ip between such a d isch a rg e and 't r a n s i t i o n d is c h a rg e s ', and the f a c t th a t th e re i s an optimum value fo r th e average e le c tro n tem peratu re fo r maximising p o p u la tio n in v e rs io n on th e (^Sg - 2p^) 632 .8nm neon t r a n s i t io n in a helium -neon d isc h a rg e .6 .3 The r e la t io n s h ip between th e 632 .8nm helium -neon la s e r d isch arg e andt r a n s i t i o n d isc h a rg e s .Using th e elem entary theo ry o f the p o s it iv e column in a s in g le gas (von Engel and Steônbeck ( l 6 ) ) , and ex ten sio n s to th e theory by D orgelo, A ltin g and Boers (1?) fo r b in a ry gas m ix tu res. Young (19) has shown th a t th e average e le c tro n energy in th e p o s it iv e column o f a helium -neon d isch a rg e i s a fu n c tio n only o f the p re ssu re -d iam e te r p roduct (pD) and th e m ixture r a t i o o f th e two g a se s . F ig . 6 .1 ( a f t e r Young ( l8 ) ) shows th e v a r ia t io n o f th e average e le c tro n tem peratu re w ith pD and th e m ixture r a t i o . The f u l l l i n e i s th e c a lc u la te d tem pera tu re , th e dashed l in e i s a s t r a i g h t l in e approx im ation to th e c a lc u la te d tem pera tu re . Good agreement i s shown between th e c a lc u la te d and ex perim en ta lly determ ined e le c tro n tem peratu re e s tim a te s o f Labuda and Gordon (19)•- 140-Shown p lo t te d in F ig . 6.1 a re r e s u l t s derived from graphs given by K orolev, O dintsov and M itsa i (20) and r e s u l t s g iven by M ielenz and N efflen (21) fo r optimum 632 .8nm la s e r o u tp u t, fo r dc and r f d isch a rg e s r e s p e c t iv e ly . The pD p roducts a g a in s t the r a t i o of neon p r e s s u r e / to ta l p re s su re a re p lo t te d g iv in g th e re le v a n t e le c tro n tem perature in 10^ °K fo r each d isc h a rg e . The range o f e le c tro n tem perature over whichoptimum p o p u la tio n in v e rs io n on th e 3s^ ......... 2p^ le v e ls occurs i s about8,000°K fo r each type o f e x c i ta t io n in each experim ental d e te rm in a tio n .At helium :neon r a t i o s g re a te r than 3 :1 , or ^ ^ e ^ t o t a l 0 .166 , th e re p o r te d optimum fo r 632.8nm la s e r g a in , (Gordon and White (22)) energy t r a n s f e r from He(2 S) m e ta s ta b le s , o r the p roduction o f h igh energy e le c tro n s v ia s u p e re la s t ic c o l l i s io n s w ith helium m e tas tab les , csin ex p la in th e movement o f p^^^^ D to low er e le c tro n tem p era tu res.The e f f e c t o f reducing th e d iam eter o f a plasma tube , o r reducing the p re s su re i s to in c re a s e th e e le c tro n tem perature in agreement w ith the r e s u l t s o f Bochkova e t a l . (23 ), (2 4 ). This in c re a se i s extrem ely ra p id below ^mm b o re , and p a r t ly ex p la in s the in c re a se in gain experienced w ith sm all bore l a s e r tubes (lO ).In F ig . 6.1 assuming th a t the average e le c tro n tem perature 6 .0 x 10^ °K i s an optimum fo r 632 .8nm o s c i l l a t io n in a ty p ic a l r f helium -neon d isch a rg e , and rem ains c o n s ta n t a t g re a te r than 0 .3 , the pD re q u ire d to m ain ta int h i s e le c tro n tem peratu re in pure neon i s approxim ately 2 .9 - 3 .2 torr-mm; w ith D = 12mm, p = 0 .26 t o r r . This p ressu re corresponds c lo se ly to th e p re s su re o f 0 .2 4 t o r r in a tube of 12mm bore, in an r f d isch a rg e , a t which a t r a n s i t i o n from a 'weak* to a ' s tro n g ' d ischarge in neon occu rs , see- 141-Table 6 .1 , ( a f t e r Bochkova and Razumovskaya (23)). and F ig s . 6 .2 - 6 .3 (2 6 , 2 3 ) fo r v a r ia t io n of e x c ite d atom co n ce n tra tio n s w ith e le c tro n c o n c e n tra tio n in th e se t r a n s i t io n d isch a rg e s , and th e p re ssu re dependence o f th e b r ig h tn e s s o f some helium s p e c tr a l l in e s in tubes of d i f f e r e n t d iam ete rs r e s p e c t iv e ly .To m a in ta in an e le c tro n tem peratu re of 6 .0 x 10^ °K in a pure helium r f d isc h a rg e , r e q u ire s a pD o f approxim ately 7 torr-mm. For a 12mm d iam eter tu b e , p = O.3 8 t o r r . This i s again in reasonab le agreem ent w ith a p = 0 .4 t o r r , D = 12mm, t r a n s i t io n p o in t in pure helium (2 3 ). I t has been re p o r te d by Bochkova and Razumovskaya (23) th a t p o p u la tio n in v e rs io n s occur in s im i la r t r a n s i t io n d isch a rg es in helium . Under th ese t r a n s i t io n d isch a rg e c o n d itio n s , th e e le c tro n mean fre e p a th i s o f th e same magnitude a s the d iam eter o f th e d ischarge tu b e , and appears to be s im ila r to th a t in a 'c o n s t r i c t e d ' glow d isch arg e (von Engel (1 2 )) . Bochkova and Razumovskaya (23) a t t r i b u t e th e se p o p u la tio n changes to d is s o c ia t io n o f io n - m olecu lar com plexes, s in c e the in flu e n c e o f the w alls of th e d ischarge tube when Ae = D, i s to produce a Maxwellian d is t r ib u t io n o f e le c tro n v e l o c i t i e s .C onstan t e le c tro n tem peratu reK orolev e t a l . (20) have shown th a t as long as th e neon p re ssu re - d iam ete r p roduct i s h igh enough to g ive complete r a d ia t io n tra p p in g o f th e 60.004nm vacuum UV t r a n s i t io n , 3^2 “ th e neon ground s t a t e , th e 6 3 2 . 8nm helium -neon la s e r ou tpu t depends only to a sm all e x te n t on th e helium p re s s u re .-142 -R e su lts shown in Table 6 .2 derived from re fe re n c e (2 0 ), show th a t th e p re s s u re -c u r re n t p roduct i s co n s tan t fo r optimum 632.8nm la s e r o u tp u t, when r a d ia t io n tra p p in g i s com plete. This i s c o n s is te n t w ith th e m aintenance o f c o n s ta n t e le c tro n tem perature (von Engel, p r iv . com m unication). I t i s , however, c o n tra ry to th e o b se rv a tio n s o f Labuda and Gordon (19) who have shown, u s in g a microwave n o ise measurement techn ique, fo r a g iven p re s s u re - ra d iu s p ro d u c t, t h a t th e e le c tro n energy i s no t dependent on th e d isch a rg e c u r re n t over a wide range o f p re ssu re s in a ty p ic a l 632 .8nm helium -neon d isc h a rg e .- 14313121110oX9LU0:Z)»—LUCLZLUM-87ZS 6h-oLU51-00*60*2 0-80"60Pure He 'W / 'r o tQ iRjre NeC a l c u l a t e d Straight line approximationa - D C discharge, 7mm bore tube,reference (20)b - R f » 3mm ----------- ( 21)Q _ — • • 5 mm **F ig , 6 .1 , V a r ia tio n o f e le c tro n tem peratu re w ith helium -neon m ixture r a t i o showing 632,Snm Pd optim a.”• 144- —} 7 i P. (mm H ,}Otî®. (mm H g)«•(00«0 -U*p'B‘«4-P ressu re dependence u. the brightness of some helium spec tra l lines in tubes of different d iam eters. 1—3, 2—35, 3—60 mm.Variation of the concentration of excited atoms in the higher energy levels of (a) helium, (b) neon, and (c) argon with the electron concentration in "w eak" and " s tro n g " discharges. The dashed line gives the calculated curve for one of the levels of Ne, He, and Ar. The prin ts on the left re fe r to the "w eak" discharge (small n,), and on the right to the "s tro n g " discharge (large n«).F ig . 6 .2 F ig . 6.3- 145 -GAS Tube diameter(mm)3 0 120 35 0 60 0Helium — 38-20 0 4 0 15 0 10Neon — — 0 2 4 0 10 —Argon — ------- 0 0 6 -------- —Table6‘1 Transition pressures (torr)Total Optimum PressureHe: Ne ratio Pressure Current Current(torr) (mA ) (torr-mA)= 7:1 14 25 3512 30 36DC discharge 10 32 3008 42 3367 mm bore tube 06 52 31 205 82 41Table62 Pressure/optimum current relationship for maximum output at X632-8nm- 14.6 —CHAPTER 7P o p x ila tio a In v e rs io n in p u lse d d isc h a rg e s - th e Helium - Io d in e systemI n tro d u c t io nAs s t a t e d in 5 .1 .2 , l a s e r o s c i l l a t i o n in io n is e d io d in e was f i r s t observed by Jenson and Powles ( l ) in th e a f te rg lo w o f a p u lse d he lium - io d in e m ix tu re . O s c i l l a t i o n on tw elve l i n e s was su b se q u en tly r e p o r te d by them (2 ) u s in g a few t o r r o f helium and 0 .1 t o r r vapour p re s s u re o f io d in e , in a co ld ca thode d isc h a rg e o f bo re and le n g th 1.2m .O s c i l l a t i o n was a ls o observed a t 5 6 2 .5 and 690.4nm in a n e o n -io d in e d is c h a rg e , and a t 69O. i n a k ry p to n - io d in e d is c h a rg e .As a l l th e u p p er l e v e l s , excep t f o r th e 5 6 2 . 5nm l i n e , a re w ith in 0 . 5 1 eV o f th e helium io n ground s t a t e H e*(0), Powles and Jenson ( 2 ) su g g es ted t h a t th e s e le c t iv e e x c i t a t io n , in th e case of th e h e liu m -io d in e system , was by th e fo llo w in g p ro c e s s ,e / ( \ / 2 ) + M \ ) + I*(X)............ ..... (7 .1 )I f th e e x c i te d le v e l o f io n is e d io d in e (x) in t h i s asym m etric charge exchange r e a c t io n i s a t r i p l e t , s p in i s con serv ed , b u t n o t i f i t i s a s i n g l e t . F ig u re 7 . I shows t h a t l a s e r o s c i l l a t i o n i s on ly observed from t r i p l e t l e v e l s .Both he lium and neon m e ta s ta b le s t a t e s have s u f f i c i e n t energy toe x c i te an io d in e io n by a p ro c e ss analogous to t h a t r e p o r te d by Dana andL aures ( 3 ) f o r th e Kr and Xe a f te rg lo w l a s e r s ,He(2^S) + 1^ ( 0 ) — He(^S^) + I ^ ( 6p ) + A E ....... (7»2)H e ( ls ) + I+(o) Ne (^S ) + 1+ ( 6p ' ) + AE, ...... (7.5)- 1 4 7 -th e io d in e io n b e in g form ed e i t h e r by p r io r c o l l i s i o n w ith a n o th e r helium o r neon m e ta s ta b le , o r by p h o to d is s o c ia t io n in th e d is c h a rg e .U sing th e n o ta t io n o f M artin and C o r lis s ( 4 ) :( i ) There a re th r e e s in g l e t le v e l s ( 6p 6p $d ^Dg)w hich a re c lo s e r to th e h e liu m io n ground s t a t e th a n th e u p p er l a s e r l e v e l s , b u t th e se do n o t undergo p o p u la tio n in v e rs io n , im p ly in g th a t s p in c o n s e rv a tio n i s im p o rtan t in th e p o p u la tin g p ro c e s s .( i i ) There i s a t r a n s i t i o n from th e s in g l e t l e v e l 6p ^F^ g iv in g a s tro n gio n is e d io d in e l i n e a t 6O7 . 5nm, analogous to a v e ry s t ro n g l a s e r I I * 1 Vt r a n s i t i o n (5 s - 5p F^) observed in pu re brom ine by K eefe andGraham ( 5 ) , and i s o e l e c t r o n i c w ith a l a s e r t r a n s i t i o n in doubly io n is e d xenon ( 4 6 3 «Tam) ( 6 ) , so t h a t l a s e r o s c i l l a t i o n i s to be ex pec ted on t h i s t r a n s i t i o n i f th e s in g l e t l e v e ls were b e in g p o p u la ted a t a r a t e eq u a l to t h a t o f th e t r i p l e t l e v e l s , and a ls o i f th e s e le c t iv e e x c i t a t io n was by e le c t r o n im p a c t.( i i i ) There i s a ls o a v e ry s tro n g t r a n s i t i o n ( 6s - 6p ^F^) a t5 3 3 . 8nm, from a t r i p l e t l e v e l c lo s e r to th e helium io n ground s t a t e th a n any o f th e u p p er l a s e r l e v e l s , which does not o s c i l l a t e . This l i n e i s equal in i n t e n s i t y to th e 5 4 0 . 7nm l a s e r l i n e , which i s enhanced l ) when u sed w ith he lium u n d er p u lse d c o n d it io n s ; 2) i n th e H.G.D. work re p o r te d in C hap ter 5; and 3 ) i n th e work o f B erez in and Yanovskaya ( ? ) . This l i n e co rresp o n d s to a l a s e r t r a n s i t i o n i n Cl I I a t 490.5Kum ( S) , and by com parison w ith Xe I I I , i s exp ec ted to have a la rg e r e l a t i v e l i n e s t r e n g th . (F ig . 7 . 1 7 ) . F u rtherm ore th e l i n e (5d ^Dg - 6p* ^F^) a t 4 5 6 . 06nm from- 14-8 —I 3th e bp t r i p l e t l e v e l , analogous to a weak l a s e r t r a n s i t i o n a t5 0 5 . 4nm. in Br I I , does n o t o s c i l l a t e under th e c o n d itio n s u se d .Absence o f l a s e r o s c i l l a t i o n on th e se l i n e s would ap p ea r to in d ic a te th a t th e 6p ^F^ l e v e l i s n o t b e in g s e l e c t i v e ly e x c i te d a t th e same r a t e as th e o th e r t r i p l e t l e v e l s o r t h a t t r a n s i t i o n p r o b a b i l i t i e s a re n o t s u i t a b le f o r o s c i l l a t i o n .Table 7 .1 g iv e s a l i s t o f t r a n s i t i o n s i n th e range 480 .0 - 830.0nm„ in io n is e d io d in e w hich have u p p er l e v e ls w ith in ap p ro x im ate ly 0 .5 eV o f th e io n l i m i t o f He ^ (O ). In d ic a te d a re th o se l i n e s which a re enhanced in h e liu m , (E), and th o se on which l a s e r o s c i l l a t i o n has now been o bserved (L).The fo llo w in g s e c t io n s d e s c r ib e th e a p p a ra tu s c o n s tru c te d , and th e experim en ts c a r r i e d o u t , i n th e h e liu m -io d in e system , to de te rm in e th e e x c i t a t io n mechanism r e s p o n s ib le f o r th e p o p u la tio n in v e rs io n , and su b seq u en t l a s e r o s c i l l a t i o n .7 . 1 E xperim en ta l A pparatus7 . 1 . 1 The p u m p in g /f i l l in g vacuum systemThe p u m p in g /f i l l in g system used was t h a t d e sc r ib e d in C hapter 5» w hich was u sed f o r th e H.C.D. in v e s t ig a t io n s . P la te 7*1 shows th e g e n e ra l arrangem ent o f th e a p p a ra tu s .7 . 1 . 2 The P u lse M odulato r.The p u ls e m odu la to r c o n s tru c te d was desig n ed to g iv e h ig h v o lta g e p u ls e s a t up to 4OKY a t up to 25A, a t a v a r ia b le r e p e t i t i o n-149 -r a t e o f 500 - 1 ,5 0 0 c / s . , w ith th e le n g th o f th e c u r re n t p u ls e determ ined by th e d e la y l i n e and th e power c a p a b i l i t i e s o f th e h ig h v o lta g e do power su p p ly c o n s tru c te d .P ig . 7*2 shows th e b a s ic c i r c u i t o f th e m o d u la to r, w hich was b ased on a m o d ified government su rp lu s r a d a r p u lse m o d u la to r, k in d ly g iv en by R .R .E ., M a lv e rn ,fo r t h i s work.The d is c h a rg e c a p a c i to r o r d e la y l i n e i s charged from a dc power su p p ly h av in g a maximum o u tp u t v o lta g e o f lOKV, re so n a n t c h a rg in g o f th e l i n e occu rs th ro u g h a lOH c h a rg in g choke^ and H.V. d iodes a re in c lu d e d in th e c h a rg in g c i r c u i t to d isc o n n e c t th e l i n e o r c a p a c i to r from th e su p p ly when th e v o lta g e reac h es a maximum and i s b eg in n in g to f a l l ( 9 ) . The d isc h a rg e i s t r ig g e r e d by a heavy du ty th y r a t r o n . To i s o l a t e th e d isc h a rg e and to h e lp m atch th e n o n - l in e a r d isc h a rg e to th e d e la y l i n e and to s te p up th e d isc h a rg e v o lta g e a 4*1 b i f i l ar-wound p u lse tra n s fo rm e r i s in c o rp o ra te d in th e c i r c u i t .The d is c h a rg e c u r re n t was m on ito red by means o f a 1 :1 c u r re n t tr a n s fo rm e r on th e e a r th r e tu r n from th e d isc h a rg e tu b e . P ro v is io n was made f o r m o n ito r in g waveforms o f th e ch a rg in g c u r r e n t , th y r a t r o n anode and o u tp u t to th e p u ls e tra n s fo rm e r .7 . 1 . 5 The L ase r tu b eA number o f d isc h a rg e tu b e s w ith d ia m ete rs from 7 to 20mm and from Im to 2 . 5m in le n g th were c o n s tru c te d th ro u g h o u t th e work to be d e s c r ib e d .-150 -A ty p ic a l d i s c h a r g e / la s e r tu b e c o n s is ts o f a fu se d s i l i c a tu b e to which a re s e a le d TmiiL bo re fu se d s i l i c a B rew ster an g led end tu b e s , w ith windows f l a t to A / lO , and p a r a l l e l to 30 se c s o f a r c . Fused s i l i c a was always u sed f o r th e d isc h a rg e tu b e , as t h i s en ab les end tu b e s to be s e a le d d i r e c t l y to i t w ith o u t g raded s e a ls * r e s u l t i n g in g r e a te r m echan ical s t r e n g th . M oreover, fu s e d s i l i c a i s b e l ie v e d to g iv e u n d er d isc h a rg e c o n d itio n s (lO ) a low er co n tam in a tio n r a t e th a n p y rex .I n te r n a l e le c tro d e s from d is c a rd e d f l a s h tu b e s a re mounted in s id e arm s, w ith a p u m p in g /f i l l in g arm a t ta c h e d c lo se to th e la rg e e le c tro d e (c a th o d e ) , where a h ig h e r c o n c e n tra t io n o f im p u r it ie s i s exp ec ted th a n e lsew h ere .The l a s e r tu b e co u ld be i s o l a t e d from th e vacuum system by means o f a la r g e b o re , V iton A, g r e a s e le s s s to p c o c k .7 . 1 .4 The l a s e r c a v i tyN ear C onfocal arrangem entThis c o n s is t s o f two m u l t i la y e r concave m ir ro rs , d i e l e c t r i c - c o a t e d , h ig h r e f l e c t i v i t y , ( ^ e a t e r th a n ^8 p e r cen t over th e range 6OO - 6S0nm) u sed in a n e a r c o n fo ca l c o n f ig u ra t io n . This c o n f ig u ra t io n , a lth o u g h h av in g th e d isad v an tag e o f n o t u s in g th e whole volume o f th e a c t iv e medium, i s c o m p ara tiv e ly easy to s e t up and en ab les mixed, r a d i i o f ouiTvature m ir ro rs to be u sed .The o rth o g o n a l movement m ir ro r mounts shown in F ig . 7»5 p ro v id e a means o f h o ld in g th e c a v i ty m ir ro rs w ith o u t damaging th e d i e l e c t r i c c o a t in g s , and o f a d ju s t in g th e m irro r a lig n m en t. The 40 TPI m icrom eter-151 -screw s p ro v id ed a l l th e c o a rse and f in e ad ju stm en t t h a t was n e c e s sa ry , w ith 45 m inu tes o f a rc p e r tu r n , and a minimum s e t t i n g v a r i a t i o n o f abou t 5 m in u te s .P rism w aveleng th s e le c t io nIn l a t e r work i n th e h e liu m -io d in e system i t was d e s ir e d to s e l e c t s in g le l a s e r t r a n s i t i o n s from amongst a number com peting f o r u p p er l e v e l p o p u la tio n s . A m o d ified v e r s io n o f th e p rism w aveleng th te ch n iq u e su g g es ted by W hite ( l l ) was u se d , as shown in F ig . 7 . 4 . P la t e 7 .2 shows th e p r is m /m ir ro r a rrangem ent.The B rew ste r ang led p rism of fu se d s i l i c a , n » I . 46OI a t A 546nm, g iv in g a p rism a n g le o f 6 8° 50 > w ith a t o t a l c a v i ty lo s s app ro ach in g 0 .5 - 1 p e r c e n t , i s mounted on a r o t a t a b l e t a b le so th a t i t can be p o s i t io n e d f o r minimum d e v ia t io n . A c ro s s s l i d e en ab les one o f th e r e s o n a to r m ir ro rs to be p o s i t io n e d and a d ju s te d in an o f f - a x is p o s i t io n to com plete th e c a v i ty . W ith th e c a v i ty shown in F ig . 7*4A, s in g le w aveleng th o s c i l l a t i o n co u ld be o b ta in e d , when th e w avelengths d i f f e r e d by more th a n abou t 6nm. W ith s m a lle r s e p a ra t io n , s im u ltaneous o s c i l l a t i o n a t two w aveleng ths on d i f f e r e n t p a r t s o f th e c a v i ty m ir ro rs o c c u rre d .7 .2 E xperim en ta l d e t a i l s and p ro c e d u re .7 . 2 .1 C lean ing o f l a s e r tu b e sThe c o n d i t io n o f th e B rew ster an g led end tu b e windows i s c r i t i c a l in most l a s e r o s c i l l a t o r s . F o r t h i s re a so n , c a re was ta k en to avo id-152 -co n d en sa tio n o c c u rr in g on th e end tu b e windows d u r in g g la s s b low ing by- u s in g a d ry in g tu b e and by f la m in g w ith a h o t , b lu e , to r c h f la m e . A f te rassem bly l a s e r tu b e s w ere washed o u t w ith warm Decon 75 and th o ro u g h lyr in s e d o u t w ith d i s t i l l e d w a te r . F in a l ly , to remove any rem a in in g d i s t i l l e d w a te r , th e tu b e s w ere washed ou t r e p e a te d ly w ith m ethyl a lc o h o l and pumped ou t on a two s ta g e gas b a l l a s t e d r o ta r y vacuum pump. P r io r to c o n n e c tio n to th e vacuum system , tu b e s were pumped ou t and c lo se d o f f to p re v e n t in g re s s o f d u s t from th e room.7 .2 .2 O u tg ass in gA c c id e n ta l im p u r i t ie s can have a d e le te r io u s e f f e c t on c o l l i s i o n a l p ro c e s se s o c c u rr in g in a l a s e r d is c h a rg e . ( in th e helium -neon l a s e r , an oxygen c o n tam in a tio n in neon o f 1 p a r t i n 10^ g iv e s a re d u c t io n in g a inof 1 p e r c e n t , and 1 in 10^ g iv e s 100 p e r c e n t r e d u c t io n , (T a y lo r (12%)To ou tg as th e l a s e r tu b e s , e le c tro th e rm a l h e a t in g ta p e s were wound around th e tu b e and e le c tro d e a ssem b lie s and g ra d u a l ly ru n up to above 250°C w ith th e d i f f u s io n pump on. At th e same tim e any exposed g la ssw are o f th e p u m p in g /f i l l in g system was flam ed in a d i r e c t io n tow ards th e pumps.A f te r a minimum of two hours th e h e a t in g ta p e s were removed, helium added to a p re s s u re o f ap p ro x im ate ly 10 t o r r and a p u lse d d isc h a rg e ru n .As im p u r i t ie s were removed from th e e le c tro d e s and w a lls o f th e tu b e , th e l i n e s p e c t r a o f helium changed to band s p e c t r a . \ The tu b e was th e n ev acu a ted as q u ic k ly as p o s s ib le . When t h i s p ro ced u re had been c a r r ie d o u t f o r th r e e o r more t im e s , th e d isc h a rg e would m a in ta in em issio n o f l i n e s p e c t r a .- 153 -A lthough th e system used was n o t bakeable a t th e u su a l tem peratu re o f kOO - 50 0°C, th e o u tg ass in g procedure used proved s a t i s f a c to r y , and la s e r o s c i l l a t io n could be ob ta in ed in a tube a f t e r i t had been l e f t i s o la te d fo r p e r io d s o f a t l e a s t one week.A fte r f i l l i n g w ith helium to the d e s ire d p re s su re , io d in e was allow ed to d if fu s e through from th e io d in e r e s e r v o i r . To m ain ta in l a s e r o s c i l l a t io n i t was found n ecessa ry p e r io d ic a l ly to b leed in io d in e vapour to re p la c e th a t which became absorbed on th e w a lls o f th e tube c lo se to th e e le c tro d e s .7 . 2 .3 R esonator alignm entC avity m irro r alignm ent was ach ieved by means of th e sim ple p in h o le and il lu m in a te d c ro ss method. Alignment was in d ic a te d by the appearance o f a b r ig h t 'moon' when look ing through the p in h o le along the a x is o f the d isch a rg e tu b e . Once th i s b r ig h t 'moon' was seen , i t was c e n t r a l i s e d by minor ad ju stm en ts to bo th m irro rs . This ad justm en t was c a r r ie d out befo re co n d itio n s were op tim ised fo r l a s e r o s c i l l a t io n .O bservation o f o s c i l l a t io nC avity m irro rs peaked a t 632 .8nm were used when o s c i l l a t io n was f i r s t o b ta in ed . O s c i l la t io n was i n i t i a l l y in d ic a te d by a re g u la r d if fu s e yellow p a t te rn which could be seen when th e l a s e r tube was viewed a x ia l ly through a sm all 15,000 LPI tran sm iss io n d i f f r a c t io n g ra t in g . This f a in t p a t te rn developed as th e p u lse v o ltag e was in c re a se d and d isappeared on fu r th e r in c re a s e o f v o lta g e . Optimum ou tpu t was then achieved by c a re fu l ad justm en t of th e helium p re s s u re , th e vapour p re ssu re of io d in e-154 -and th e p u ls e v o l ta g e . I t was l a t e r found th a t g r e a te r o u tp u t and s t a b i l i t y was o b ta in e d i f th e 2 / a sec d e lay l i n e was re p la c e d by a0 .00 6 6 ^ iF E,V. c a p a c i to r , and a s im p le c a p a c i to r d isc h a rg e u sed in s te a d o f one h av in g a f ix e d i n t e r v a l , de term ined by th e d e la y l i n e .O s c i l l a t i o n on seven l i n e s 7 0 3 .3 , 6 5 8 , 5 , 6 1 2 .7 , 5 7 6 .0 , 5 6 7 . 8 , 540 .7 and 4 9 8 .7nm, p re v io u s ly r e p o r te d by Fowles and Jenson,w as observed s u b se q u e n tly w ith a number o f c a v i ty m ir ro rs w ith r e f l e c t i v i t y g r e a te r th a n 98 p e r c e n t ov er th e w aveleng th range 5OO - 700nm ,using a l a s e r tu b e o f 7^ 0. b o re .The 'm i ld n e s s ' o f th e e x c i ta t io n n e c e ssa ry to g ive l a s e r o s c i l l a t i o n , th e s e n s i t i v i t y o f each t r a n s i t i o n to m odu la to r in p u t v o l ta g e , and th e o b s e rv a tio n th a t a s h o r t c u r re n t p u lse gave maximum o u tp u t, su g g es ted th a t th e means by which p o p u la tio n in v e rs io n o ccu rred in t h i s system was d i f f e r e n t from th a t o c c u rr in g i n th e i n e r t gas io n l a s e r sy stem s. I twas d e c id e d , th e r e f o r e , to u se a l a r g e r bo re l a s e r tu b e , where c o l l i s i o n a l reco m b in a tio n p ro c e sse s would be in c re a s e d and w a ll e f f e c t s d im in ish ed , where c a ta fp h o re s is would be red u ced , and in which th e v a r i a t io n o f r a d i a l g a in due to th e e l e c t r i c f i e l d would a ls o be red u ced , (Ahmed and F a i th ( 1 3 ) ) .The u se o f a 126cm lo n g l a s e r tu b e , h av ing a plasm a le n g th o f 100cm and b o re o f 2cm, su b se q u en tly en ab led h ig h e r g a in s o f up to 10 p e r c e n t p e r m e tre , and a new in te rc o m b in a tio n l a s e r t r a n s i t i o n a t 651.6nm to be o b ta in e d ( 1 4 ) . F u r th e r , by m a in ta in in g a h ig h vapour p re s s u re o f io d in e , o s c i l l a t i o n on th e h y p e rf in e components o f a number o f l i n e s was-155 -o b ta in e d f o r th e f i r s t tim e (15), c o n tra ry to th e o b s e rv a tio n s o f Fowles and Jen so n , ( l 6 ) ,P la te s 7*5 &nd 7*4, showing d is p e rs e d l a s e r l i n e s c ro s se d w ith F a b ry -P e ro t f r i n g e s , in d ic a te by th e double and t r i p l e f r in g e system s t h a t s im u ltan eo u s o s c i l l a t i o n i s o c c u r r in g on h y p e rf in e components o f in d iv id u a l l a s e r l i n e s .D uring a tte m p ts to o b ta in l a s e r o s c i l l a t i o n on th e 553-8nm l i n e and c e r t a in o th e r l i n e s , u s in g prism w aveleng th s e le c t io n , o s c i l l a t i o n was o b ta in e d on f u r t h e r new lin e s * 6 8 2 .5nm (6s ^D^ - 6p ^Fg); and; % I z7 1 3 . 9nm (5d D ^ - 6p Dg), and on a number o f h y p e rf in e com ponents, ( 1 7 ) , an a ly se d in 7 *3 *7 . 2 .4 W avelength m easurement and h y p e rf in e components d e te rm in a tio n .(a ) W avelength m easurementM easurements made on spectrum p la te s , o b ta in e d on a H ilg e r and W atts D186 C onstan t D ev ia tio n sp e c tro g ra p h h av in g a l i n e a r d is p e r s io n o f ap p ro x im ate ly 7 «0 i^m/mm a t 6 5 0 . 0nm,were n o t s u f f i c i e n t l y a c c u ra te to en ab le i d e n t i f i c a t i o n to be made to l e s s th a n - l.Onm o f th e l i n e observed a t abou t 6 5 2 . Onm. There a re two p o s s ib le t r a n s i t i o n s in t h i s w aveleng th re g io n ,( i ) (5p^ ^P^ - 6p ^Fg) a t 651.6l8nm , and( i i ) ( t e n t a t i v e c l a s s i f i c a t i o n , 5 .^ ^Dg - 6p ^F^), a t 651.891nm.(M artin and C o r lis s ( 4 ) do n o t c l a s s i f y t h i s 651.9nm l i n e ; and th e c l a s s i f i c a t i o n above was a r r iv e d a t by assum ing th a t th e l a s e r l i n e would be l i k e l y to have an analogous l a s e r t r a n s i t i o n in- 156 -i s o e l e c t r o n i c doubly io n is e d xenon, and th a t i t m ight be from th e 6p 'l e v e l in io n is e d io d in e from w hich o s c i l l a t i o n had n o t been o b serv ed .The X e l l l l a s e r l i n e a t 4 8 7 .Onm (5d - 6p* (6 )* g iv e s th ec o r r e c t wave number i n t e r v a l f o r th e 651.891nm I I I l i n e . )A Beck R ev ers io n S p e c tro sc o p e , No. 3505» was used to d e te rm in e th ew aveleng th o f t h i s l i n e and o th e r l i n e s to w ith in an accu racy o f - O.lnmin th e fo llo w in g way.A s e r i e s o f t h i r t e e n re a d in g s was ta k e n o f th e c lo s e s t l a s e r l i n ea t 658*5nm and on th e unknown l i n e a l t e r n a t e l y i n p a i r s . From th em easurem ents on th e 6 5 8 .5nm l i n e th e in s tru m e n t e r r o r a t t h i s w aveleng thwas de te rm in ed as b e in g 0.63nm . Assuming th e in s tru m e n t e r r o r ands e t t i n g e r r o r s a t th e unknown w aveleng th would be th e same as a t 6 5 8 .5nm*th e unknown w aveleng th i s (652 .18 - 0.63)nm = 65I .6 5 - O.lnm ( c r « 0.05nmon s e t s o f I 3 m easurem en ts). The l i n e i s th u s th e (5p^ - 6 ^Fg)t r a n s i t i o n , a t 651.6l8nm .A lthough t h i s i s a doubly fo rb id d e n t r a n s i t i o n in v o lv in g a changeo f s p in , and b e in g a p - f t r a n s i t i o n , i t recommends i t s e l f becauseI 3th e l i n e behaves in a s im i la r way to o th e r l a s e r l i n e s from th e 6p Fg l e v e l . I t i s l a t e r shown in 7 . 4 to be a l i n e hav in g a low er l e v e l t h a t i s v e ry s t r o n g ly p e r tu rb e d .A nother l i n e a t ap p ro x im ate ly 607.0nm o b ta in e d on ly by u s in g p rism w aveleng th s e le c t io n , was id e n t i f i e d in th e same way, u s in g th e c lo se 6l2.7iun. l a s e r l i n e as a r e f e r e n c e , g iv in g 6 0 6 .9 Î O.lnm ( cT = 0.04um* on s e t s o f 11 m easurem en ts).-1 5 7 -This io n ise d io d in e l i n e a t 6o6.893nm (6s - 6p i s a l in e onwhich l a s e r a c t io n might be expected s in ce i t has bo th upper and low er le v e ls common to o th e r l a s e r l i n e s , and i t would only o s c i l l a t e when the very s tro n g l a s e r l i n e (6s - 6p a t 376 .Onm was suppressed*(b) H yperfine component id e n t i f i c a t i o nA Fabry P e ro t é ta lo n was cro ssed w ith th e D l86 C onstant D ev ia tion sp ec tro m ete r, w ith th e é ta lo n p laced in the p a r a l l e l l i g h t p o s i t io n between th e c o llim a to r le n s and d isp e rs in g p rism , (Tolansky ( l 8 ) ) .T yp ica l ranges o f th e h y p erfin e components in th e io n ise d io d in e spectrum a re 0 - 0.6cm To avo id overlapp ing o f the componentso f one o rd e r w ith th o se o f an o th e r , th e s e p a ra tio n between o rd e rs o f th e é ta lo n , ( l / 2 t cm , where t i s the s e p a ra tio n o f th e é ta lo n p la te s , must be g re a te r than th e sp read o f components. Having e s ta b l is h e d th e r e la t io n s h ip s o f th e components, an é ta lo n w ith a l a rg e r sp acer can be used fo r more ac c u ra te h y p erfin e s e p a ra tio n m easurem ents.In th e work d escrib ed h e re , an é ta lo n having a fused s i l i c a sp ace r o f 10.12mm, w ith o rd er s e p a ra tio n o f 0.494cm ^ was used , a s w e ll as one having ruby b a l l b earin g sp ace rs of 0 .123 in , (acc u ra te to 0.0001 i n ) , g iv in g approx im ately 1 .3 cm"^ between o rd e rs .-158 -The f in e s s e o f th e 10.12mm. é ta lo n , w hich had a lu m in ise d f l a t sand a s p e c i f ie d r e f l e c t i v i t y o f 90 p e r c e n t , was c a lc u la te d asb e in g 36O. The in s tru m e n ta l h a l f -w id th f o r t h i s f in e s s e , g iv enby B = 1 / ( 1 + F s i n S / 2 ) , i s equal to I /3 0 o f an o rd e r (19)» 2where B; i s th e t r a n s m it te d i n t e n s i t y , and F i s th e f i n e s s e .T his was checked u s in g a 6 3 2 . 8nm helium -neon l a s e r , i n s in g le mode o p e ra t io n , in w hich th e l i n e w id th i s l e s s th a n th e in s tru m e n ta l w id th . Under th e se c o n d itio n s th e m easured h a lf -w id th g iv e s d i r e c t l y th e in s tru m e n ta l h a l f -w id th . I t was found to be l e s s th a n l / lO o f an o rd e r , so t h a t s u f f i c i e n t r e s o lu t io n was o b ta in a b le to en ab le s e p a ra t io n o f h y p e rf in e components o f g r e a te r th a n 0 .0 5 cm and D oppler l i n e w id th s o f t h i s o rd e r , to be d e te rm in ed .No o p t ic s were u sed betw een th e s l i t o f th e s p e c tro m e te r / sp e c tro g ra p h and th e l a s e r , and th e l a s e r beam shone d i r e c t l y on th e s l i t . T y p ica l exposure tim es were 10 - 120 s e c , (u s in g I l f o r d R40 p l a t e s ) , w ith F a b ry -P e ro t f r in g e s in o n -a x is and o f f - a x is p o s i t i o n s .Scans were made o f th e F a b ry -P e ro t f r in g e s shown in P la te s 7»3 and 7 . 4 , on a Joyce - Loebl m ic ro d e n s ito m e te r, and measurem ents made to w ith in - 0.02mm u s in g a m icrom eter e y e -p ie c e .The h y p e rf in e components s e p a ra t io n was c a lc u la te d u s in g ;(a ) l i n e a r i n t e r p o la t io n on th e o f f - a x is f r in g e s(b) th e J? a r r a y method o f M eissner (20), on o n -a x is f r in g e s , (s e e Appendix 7 .1 ) .“159 -A ty p ic a l c a lc u la t io n i s g iv en in th e Appendix 7 .2 , f o r th e th re e component i d e n t i f i c a t i o n on th e 6 5 8 , 5nm l i n e .By com parison w ith t r a n s i t i o n diagram s g iven by Murakawa ( 2 l ) - ( 2 3 ) and T olansky ( 2 4 , 2 5 ) , and c a lc u la te d t r a n s i t i o n diagram s to g iv e agreem ent w ith T olansky ex p e rim en ta l r e s u l t s , th e h y p e rf in e components were i d e n t i f i e d .7 . 2 .5 L ine w id th d e te rm in a tio n o f l a s i n g com ponents.M easurements w ere made on th e m ic ro d en s ito m e te r scans o f th e Fabry - P e ro t f r in g e s to y ie ld th e l i n e w id th o f th e in d iv id u a l l a s in g com ponents.- l6 o -7 .3 T reatm ent o f R e s u lts and C o n c lu sio n s .7 . 3 .1 L aser O s c i l l a t io nL ase r o s c i l l a t i o n i n io n is e d io d in e was found to o ccu r d u r in g th e e x c i ta t io n p u ls e h u t m ain ly in th e a f te rg lo w p e r io d , h av in g a d u ra tio n o f up to ap p ro x im a te ly 250/A sec. The d u ra tio n depends on th e vapour p re s s u re o f io d in e and on th e p re s s u re o f he lium , w ith o s c i l l a t i o n o c c u rr in g o v er th e he liu m p re s s u re range o f 3 - 60 t o r r . Maximum l a s e ro u tp u t o ccu rs a t 6 t o r r o f h e lium , a t <—'0 .2 t o r r vapour p re s s u re o fio d in e .P ig . 7 .5 shows th e tim e r e la t io n s h ip o f th e 5 7 6 .Onm l a s e r o u tp u t (u p p er t r a c e ) and th e c u r re n t p u ls e (lo w er t r a c e ) , a t a r e p e t i t i o n freq u en cy o f 1 K c /s . (The ap p a re n t s t r u c t u r e on th e waveforms i s due to u se o f 'C hop ' on th e T e k tro n ix o s c il lo s c o p e u s e d .) A s im i la r tim e b eh av io u r i s e x h ib ite d by th e 6 5 8 . 5 , 6 1 2 .7 , 540*7 and 5 2 1 . 6nm l in e s exam ined. F ig 7*6 shows th a t maximum spontaneous em issio n occu rs on th e5 7 6 . Onm l i n e , when th e c u r re n t p u ls e i s p r a c t i c a l l y z e ro , a t abou t 3 /^ seca f t e r th e commencement o f th e p u ls e . A s im i la r tim e dependence i s shown by th e n o n - la s e r io n is e d io d in e 533.8%# l i n e , n e u t r a l io d in e and he liu m l i n e s , - F ig s . 7*7 - 7*9.As th e vapour p re s s u re o f io d in e d e c re a se s on ru n n in g th e d is c h a rg e , a r e d u c t io n i n l a s e r o u tp u t o c c u rs , and f i n a l l y o s c i l l a t i o n c e a se s .P r io r to c e s s a t io n o f o s c i l l a t i o n , and a f t e r , th e spontaneous em issio n o f l a s e r l i n e s has a d i f f e r e n t tim e b eh av io u r from th a t when l a s in g s t ro n g ly .- l 6 l -There i s an i n i t i a l r a p id in c re a s e i n i n t e n s i t y on commencement of th e c u r re n t p u ls e , th e n a l e s s r a p id in c re a s e to a maximum a t abou t 20/A se c , and f i n a l l y a slow decay , (P ig . ? .1 0 ) .Io n is e d io d in e l i n e s on w hich o s c i l l a t i o n was n o t o b se rv ed , n e u t r a l helium and io d in e l i n e s e x h ib i t th e same in t e n s i t y b eh av io u r. F ig s . 7 .11 and 7 . 1 2 .7 . 3 .2 L ase r t r a n s i t i o n sThe h ig h e r term s o f I ^ a re o f th r e e ty p e s acc o rd in g to w hether th e p a re n t term in I^'*’ i s 5p^ - ^D°, o ri . e . 5s^ 5p^ ( V )" (" ( ^ P ° ) , in d ic a te d byI IInx , nx and nx r e s p e c t iv e ly ( 4 ) .A ll th e l a s e r t r a n s i t i o n s fexcept one) observed by Jenson and Fow les, and th e 7 1 3 . 9 , 6 8 2 .5 , 65I . 6 and 606.9nm l i n e s a re t r a n s i t i o n s from th r e e t r i p l e t 6p le v e l s c lo se to and w ith in th e range 0 .2 3 - O .5I eV below th e helium io n ground s t a t e .A ll th e l a s e r t r a n s i t i o n s have TJ7 te rm in a l t r a n s i t i o n s to th e t r i p l e t io d in e io n ground s t a t e , o r th e t r i p l e t 5p^ ^P^ le v e l c lo se to th e io n ground s t a t e . The in te rc o m b in a tio n l a s e r t r a n s i t i o n 651.6nm (5p^ ^P^ ^ - 6p ^Fg) has an in te rc o m b in a tio n U7 te rm in a l t r a n s i t i o n to t h i s l e v e l .F ig . 7 . 1 , a p a r t i a l energy le v e l diagram f o r io n is e d io d in e ,sh o w s th e l a s e r t r a n s i t i o n s o b se rv ed , and th e n o n - la s e r t r a n s i t i o n ( 6 s ' ^Dg - 6p ^F^) a t 5 3 3 . 8%#'-162 -7*3.3 Comparison o f io n is e d io d in e and io n is e d brom ine l a s e r t r a n s i t i o n s .G ro tia n diagram s F ig s . 7*13 and 7*14, show t h a t l a s e r t r a n s i t i o n s in io n is e d io d in e d i f f e r s i g n i f i c a n t l y from th o se o b served in p u re bromine by K eefe and Graham ( 5 ) , where th e s e le c t iv e e x c i ta t io n i s by e le c t r o n im pact e x c i t a t io n from th e io n ground s t a t e . A ll th e t r a n s i t i o n s in io n is e d io d in e ex cep t th e u s u a l p - s ( 6p - 6s ) io n l a s e r t r a n s i t i o n a t 5 6 2 . 5nm in th e co re and th e 5 2 1 . 6nm np - ( n - l ) d t r a n s i t i o n d i f f e r .No l a s e r t r a n s i t i o n s from s in g l e t le v e ls have been o b se rv ed , even though s im i la r energy le v e l s in io d in e a re a t a low er energy above th e io n ground s t a t e th a n in brom ine, from w hich o s c i l l a t i o n o c c u rs .F ig . 7*15 and |* ig . 7*13 show th a t io d in e p ro v id es th e m a jo r i ty o fI Ith e np - ns l a s e r t r a n s i t i o n s in th e io n is e d h a lo g en s .7 . 3 .4 Comparison o f l a s e r t r a n s i t i o n s in io n is e d io d in e , and doubly- io n is e d Argon, K rypton and Xenon.F ig . 7*16 shows t h a t l a s e r t r a n s i t i o n s in io n is e d io d in e obey th es e le c t io n r u l e s ;A s = 0 , A L « A j , j u s t as th o se o f doubly io n is e d a rg o n , k ry p to n and xenon, ex cep t t h a t more l a s e r t r a n s i t i o n s occu r in io d in e f o r A J = 0 and AL = +1 ; andA J = —1 and A II » +1 .F ig . 7 .1 7 g iv e s th e c a lc u la te d l i n e s t r e n g th s o f t r a n s i t i o n s indoubly io n is e d nob le gas atoms and w ith F ig , 7*13 shows th a t o s c i l l a t i o non th e (^D°) co re t r a n s i t i o n s in io n is e d io d in e does n o t o ccu r on th o se t r a n s i t i o n s analogous to th o se w ith la rg e r e l a t i v e l i n e s t r e n g th in th e doubly io n is e d nob le g a se s , o r on th o se in Xe I I I on w hich o s c i l l a t i o n has been o b serv ed .-l63 -7 . 3 .5 L ase r o s c i l l a t i o n on h y p e rf in e componentsIo d in e i s p a r t i c u l a r l y advantageous f o r a s tu d y o f o s c i l l a t i o n on h y p e rf in e components because i t i s m ono iso top ic w ith a n u c le a r s p in of 5/ 2 , and has wide h y p e rf in e s t r u c t u r e to i t s spectrum l i n e s , (2 5 ) . I t has s t r o n g ly p e r tu rb e d energy l e v e l s , t h e e x ac t cause o f w hich has y e t to be d e te rm in ed , ( (2 1 ) - ( 2 5 ) , ( 2 7 ) ) .S im ultaneous l a s e r o s c i l l a t i o n on h y p e rf in e components has been o bserved f o r th e f i r s t tim e on th e fo llo w in g io n is e d io d in e l i n e s ; in nm 7 0 3 . 3 » 6 5 8 . 5 ; 6 12 .7 ; 6 0 6 , 9 ; 5 7 6 .0 and 5 6 7 . 8 , as shown i n P la t e s 7-3and 7 . 4 .P re v io u s ly , l a s e r o s c i l l a t i o n had been r e p o r te d by Fowles and Jenson ( 1 6 ) as on ly o c c u rr in g on a s in g le h y p e rf in e component o f a g iv en l i n e , even though adeq u a te g a in f o r o s c i l l a t i o n to o ccu r on o th e r components was r e a l i s e d i n t h e i r ex p erim en ts .O s c i l l a t i o n in t h i s work was o b serv ed , w aveleng ths in nm, a t* -(1 ) 705.5 on two components I 65 - 50 x 10 ^ cm ^ a p a r t( 2 ) 6 5 8 .5 on th r e e com ponents, w ith s e p a ra t io n from th e s t r o n g e s t o f(i) 150 ±50 X 10"3 cm"^(ii) 210 Î38 X 10"5 cm"^( 3 ) 6 1 2 .7 on two com ponents, I 90 -5 0 x 10 ^ cm ^ a p a r t( 4 ) 6 0 6 .9 on two com ponents, 180 -55 x 10 ^ cm a p a r t(5) 5 7 6 .0 on two com ponents, IO4 -20 x 10” ^ cm“^ a p a r t , and a t(6 ) 5 6 7 .8 on two com ponents, 524 -1 5 x 10“ ^ cm“^ s e p a r a t io n .— 164 —As a r e s u l t o f th e n u c leu s p o s se s s in g a m echan ical moment I , co u p lin g i s ex p e rien ced betw een th e t o t a l m echan ical moment J o f th e e x tra n u c le a r e l e c t r o n s , g iv in g a r e s u l t a n t moment o f th e n u c leu s F , The h y p e rf in e s p l i t t i n g s o f h y p e rf in e s t r u c tu r e due to n u c le a r s p in a re sm a ll compared w ith m u l t ip le t s p l i t t i n g s on w hich LS: co u p lin g i s b ased , so t h a t th e i n t e r a c t io n o f I i s much s m a lle r th a n th a t betw een L and S, o r any o th e r in t e r a c t io n s t h a t a re in v o lv e d , (Kuhn (28) and W hite ( 2 9 ) ) .In th e case o f io d in e , th e r u le s a p p l ic a b le to m u l t ip le t a n a ly s is w i l l ap p ly to h y p e rf in e d e te rm in a tio n , L, S and J b e in g re p la c e d by J , I and F r e s p e c t iv e ly , where F has th e v a lu es*F * * J + I , J + I —1 , . « . « « # « . . | j — l |(a ) The in t e r v a l s betw een two components F and F + 1 w i l l be g iv e n by th e Lande I n te r v a l Ruler . A( p + 1 ) ( 7 . 4 )where A i s an i n t e r v a l f a c t o r depending on th e le v e ls conce rned . I r r e g u l a r i t i e s in i n t e r v a l s , and d e v ia t io n s from th e I n te r v a l Rule o ccu r due to p e r tu r b a t io n s .(b) The r e l a t i v e i n t e n s i t i e s o f h y p e rf in e components w i l l be g iv en by th e F - sum Rule o f L orgelo ( 5 0 ) , G rn s te in and B urger ( j l ) , i n which th e r a t i o o f th e sums o f th e i n t e n s i t i e s o f a l l t r a n s i t i o n s from two quantum le v e ls F and F* a re in th e r a t i o o f t h e i r s t a t i s t i c a l w eigh ts(2F + 1 ) / ( 2 f ‘ + 1 ) ,A ll th e la s i n g components have been i d e n t i f i e d and a re shown in F ig s . 7 .18 - 7 . 2 3 .-16$ -In t r a n s i t i o n s from th e 6 s ' and 6p ' l e v e l s , o s c i l l a t i o n i s observed in o rd e r o f th e c a lc u la te d r e l a t i v e i n t e n s i t i e s , c a lc u la te d u s in g th e F - sum R ule .In t r a n s i t i o n s from th e 6p ^Fg le v e l ($ 6 7 .8 and 606.9nm ), o s c i l l a t i o n o ccu rs on th e s t r o n g e s t c a lc u la te d component, from th e F 9 /g l e v e l , and th e n from th e F $ /g l e v e l , even though th e re a re s t r o n g e r c a lc u la te d components from th e F 7 /2 l e v e l , (se e F u r th e r w ork).U sing th e observed l a s e r components in nm f o r th e l i n e s 6$8.5 ( 6 s ' - 6p* ^D^), 6 0 6 .9 (6s* - 6p ' ^Fg) and 576.07 ( 6 s ' -6p ) , th e in t e r v a l s f o r th e 6s and 6p le v e ls have beene s ta b l i s h e d so t h a t agreem ent o f observed and c a lc u la te d components i s now o b ta in e d . W ith th e 5 6 7 . 8nm l i n e (6a ^Dg - 6p ^Pg) agreem ent o f l a s e r components has been shown w ith th e components observed by Murakawa ( 2 2 ) w hich d is a g re e d w ith th o se r e p o r te d by Tolansky ( 2 3 ) .I t has been e s ta b l is h e d from th e measurem ents made, and th ei d e n t i f i c a t i o n of th e l a s i n g components on th e 6 5 8 . 5 , 6 1 2 . 7 , 606,9 and5 7 6 . Onm l i n e s , t h a t th e u pper l a s e r l e v e l 6p and th e low er l a s e rI 3l e v e l 6s a re v e ry s tro n g ly p e r tu rb e d .The h y p e rf in e in t e r v a l s f o r th e s e two le v e ls do n o t obey th e in t e r v a l r u l e , s in c e th e y a re n o t p ro p o r t io n a l to th e s t a t i s t i c a l w eigh ts o f th e h y p e rf in e l e v e l s . S ince th e i n t e r v a l r u le i s n o t obeyed, th i s p e r tu r b a t io n co u ld be a t t r i b u t e d to a S ch u le r - Jones ( 3 2 ) p e r tu r b a t io n , due to th e c lo se approach o f two te rm s .-166 -From measurements o f th e two la s in g components o f th e (5d -6p ^Fg), 7 0 3 .3nm l in e th e two la s in g components have been id e n t i f i e d as thetwo s tro n g e s t c a lc u la te d h y p erfin e components, i f the in te r v a l f a c to r i staken a s 69 f o r th e term . Tolansky (24) g iv es th e in te r v a l f a c to ra s being e i th e r 69 o r - 89 fo r t h i s term . This value o f A i s much la rg e rthan i s expected fo r th i s 5s 5p^ 5d term and in d ic a te s th a t th i s term i sa ls o s tro n g ly p e r tu rb e d , supposedly by a Shenstone - R u sse ll p e r tu rb a tio n(3 2 ) due to te rm (s) as much a s a few thousand cm ^ away. In t h i s type ofp e r tu rb a tio n th e in t e r v a l r u le i s obeyed, b u t th e in te r v a l fa c to r i s la rg e .5 1^ * 3The in te rco m b in a tio n l a s e r t r a n s i t io n a t 651.6nm (5p - 6p F^)may occur a s a r e s u l t o f p e r tu rb a tio n , s in c e th e in te r v a l f a c to r fo r the5p^ terra i s the la r g e s t (80 - 90) fo r any term o f J = 1 , and a lso(acco rd in g to Murakawa (27)) does no t obey th e in te r v a l r u le .The term s re sp o n s ib le fo r th e assumed S chuler - Jones p e r tu rb a tio n have n o t been determ ined . No c l a s s i f i e d energy le v e ls a re c lo se to e i th e r th e 6p* o r th e 6s le v e ls so such te rm (s) can only be h y p o th e tic a l .The r a t i o o f th e quadrupole coup ling c o n s ta n t, B/A, can be ob ta ined from th e form ula o f Casim ir (2 8 ) , where the in t e r v a l , ( A v a b ), between two h y p e rfin e le v e ls , in the absence o f term in te r a c t io n , i s g iven b y :-A ^ a t, = 1/2A(C^ - C^) + 3/8B + D - C^(C^ + 1) ] /1(21 - 1) J (2J - 1)..................... .... (7 .5 )where C = F(F + l ) - 1(1 + l ) - J ( J + 1 ) .-167 -Assiuning th a t th e p e r tu rb a tio n , o f th e 6p* and 6s* term s i s n o t due to term in t e r a c t io n , h u t i s due to an e l e c t r i c quadrupo le moment,B/A i s “3*2, and -3 .1 2 f o r each te rm r e s p e c t iv e ly .These v a lu e s a r e l a r g e r th a n th a t o f B/A o f + 1 .385 f o r from2th e l e v e l g iv en by Kuhn, and a re o f a d i f f e r e n t s ig n .As th e r a t i o f o r b o th term s i s n e g a tiv e th e p e r tu r b a t io n cannot be due to a common te rm , ( th e term s a ls o do n o t obey th e i n t e r v a l ru le ,) and f u r t h e r i t seems u n l ik e ly t h a t p e r tu r b a t io n due to two term s would g iv e th e same v a lu e s f o r B/A.I f th e wave fu n c tio n s of th e (^D) 6p and (^D) 6s term s can be c a lc u la te d , w ith th e v a lu e o f B, a v a lu e f o r th e quadrupo le moment (Q ( l^ ^ ^ ) ) sh o u ld be c a lc u la b le and be equ a l to ap p ro x im ate ly - 0 .6 b a m ( 2 7 ) . F a i l in g ag reem en t, some o th e r p e r tu r b a t io n i s e v id e n t.I 37 . 3 .6 Non e q u il ib r iu m p o p u la tio n o f h y p e rf in e s u b le v e ls o f th e 6s o f io n is e d io d in e d u rin g l a s e r o s c i l l a t i o n .I f two term s have n e a r ly th e same energy and d i f f e r on ly in v a lu e s o f J o r F , t h e i r p o p u la tio n i n a gas d isc h a rg e co rresponds v e ry c lo s e ly to s t a t i s t i c a l e q u i lib r iu m . The r a t i o o f i n t e n s i t i e s o f t r a n s i t i o n s from th e se te rm s , i n th e absence o f s e l f a b s o rp tio n , i s equal to th e r a t i o o f th e s t a t i s t i c a l w eig h ts tim es th e t r a n s i t i o n p r o b a b i l i t i e s . This form s th e b a s is o f th e i n t e n s i t y sum r u l e s .- l68 -P rev io u s s tu d ie s by Turkin in 1959 (34) o f th a lliu m show th a t non- eq u ilib riu m p o p u la tio n co n d itio n s a re ob ta ined in h y p erfin e su b - le v e ls of t h i s elem ent in a h igh c u r re n t hollow cathode d isc h a rg e . S im ila r s tu d ie s by Schrammen (35) and S chu ler (3 6 ) o f cadmium showed th a t the r e l a t iv e i n t e n s i t i e s of h y p erfin e components depended on th e e x c i ta t io n c o n d itio n s . Turkin (37) re p o rte d th a t th e anomalous in te n s i ty changes re p o rte d p re v io u s ly in cadmium depended on th e c u r re n t d e n s ity o r th e p re ssu re of th e cadmium vapour.The expected r a t i o o f th e th e o re t ic a l in t e n s i t i e s was observed only a t low c u r re n t d e n s i t ie s in both hollow cathode and p o s i t iv e column d isc h a rg e s . In a l l th e l i n e s s tu d ie d th e in te n s i ty o f th e long w avelength component in c re a se d w ith in c re a se in c u r re n t d e n s ity w h ils t the in te n s i ty o f the sh o r t wavelength component decreased .S im ila r anomalous in te n s i ty changes, r e s u l t in g in a change o ft 3o s c i l l a t in g components o f th e io n ise d io d in e l i n e 6 3 8 .3nm (6s -6s ^D^) a re re p o rte d in th i s t h e s i s .O s c i l la t io n on th e 6 3 8 . 3nm l in e has been ob ta ined on th re e h y p erfin e components s im u ltaneously w ith two components o f the 3 7 6 . Onm l i n e , as shown in F ig . 7*24. With a h igh vapour p re ssu re o f io d in e , o s c i l l a t io n occurred on th e F 7 /2 - 3 /2 t r a n s i t io n o f the 6 3 8 . 3nm l in e a lo n e . This component has a low er th e o re t ic a l r e l a t iv e in te n s i ty than th a t o f the F 7 /2 - 7 /2 h y p erfin e t r a n s i t io n a t a s l i g h t ly lon g er w avelength.As th e vapour p re ssu re o f io d in e decreased due to c lean up in the d isc h a rg e , th e in t e n s i ty o f th e o s c i l l a t in g component decreased , b u t-169 -o s c i l l a t io n commenced on th e lo n g e r wavelength F 7 /2 - 7 /2 t r a n s i t io n . F u rth e r re d u c tio n in th e vapour p re ssu re o f io d in e r e s u l te d in o s c i l l a t io n a lone on t h i s lo n g e r w avelength component.S ince th e two o s c i l l a t in g components have a common upper h y p erfin ele v e l (F 7 /2 ) , th e change o f th e o s c i l l a t in g components cannot bea t t r ib u t e d to changes in p o p u la tio n o f th e s u b -le v e ls o f the upper l a s e r le v e l 6p and can only be a t t r ib u te d to n o n -equ ilib rium in th ep o p u la tio n s o f the s u b - le v e ls o f th e low er 6s l e v e l .H yperfine t r a n s i t io n s o f th e 376 .Onm l in e in to th e F 7 /2 o r 3 /2le v e ls p o p u la tin g th e se le v e ls ,c a n n o t account fo r th e v a r ia t io n in p o p u la tio n in v e rs io n , a s th e f in a l in te n se o s c i l l a t in g components a t low p re s su re a re long w avelength components having a common F 7 /2 su b - le v e l o f th e low er 6s le v e l .S im ila r anomalous behaviour in th e o s c i l l a t in g components i s observed when th e 6 3 8 . 3%m l in e i s o s c i l l a t in g on ly .In a l l th e o th e r l in e s on which o s c i l l a t io n on h y p erfin e components was observed , no anomalous in te n s i ty changes occurred , and th e in te n s i ty o f th e la s in g components follow ed th e observed and th e o re t ic a l i n t e n s i t i e s .From th e se changes in o s c i l l a t in g components i t i s ev id en t th a t th e reI 3i s a n o n -eq u ilib riu m p o p u la tio n o f th e 6s le v e l , as th e r a t i o o fth e p o p u la tio n s o f th e su b - le v e ls 3 /2 and 7 /2 must be lower in a d ischarge a t h ig h e r vapour p re ssu re o f io d in e than a t a low er p re s su re . Such changes could a ls o be due to c u rre n t changes determ ined by changes in th e vapour p re ssu re o f io d in e .- 170 -7*3*7 Line w idth o f o s c i l l a t in g components6 3 8 *3%# The lin e w id th o f the s tro n g e s t o s c i l l a t in g component wag found to be approx im ately 0 .07 cm 3 6 7 «8nm The lin e w id th o f th e s tro n g e s t component was 0 .11 cm”^ . Thislin e w id th o f approx im ately 4,000 Mc/s i s s im ila r to th a t observedin th e noble gas io n la s e r s (Byer e t a l . (3 8 ) ) , and i sapprox im ately 3 to 4 tim es the Doppler w idth fo r io d in e a t room te m p era tu re .7*3*8 S e le c tiv e e x c i ta t io n mechanismS trong np - ns t r a n s i t io n s a re observed in the doubly io n ise d nobleg a se s , and in s in g ly io n ise d f lu o r in e , c h lo rin e and bromine in pu lsedd is c h a rg e s . These t r a n s i t io n s occur in io n ise d io d in e only when an r fd isch a rg e i s coupled w ith a p u lsed d ischarge , suggesting th a t th e p r in c ip a ls e le c t iv e e x c i ta t io n o f the o th e r 6p upper l a s e r le v e ls in io d in e i s byan a to m ic /io n ic c o l l i s io n a l mechanism and no t by e le c tro n im pacte x c i ta t io n from e i th e r th e n e u tra l o r io n ground s t a t e s .4 oIn io n ise d io d in e th e np le v e ls o f th e S core a re o f low er energy than th e np core le v e ls , and th e se would be expected to be e x c ite d i f , a s in the noble gas l a s e r s , the e x c i ta t io n was p u re ly e le c t ro n ic .F u r th e r , th e h ig h helium p re ssu re (up to 60 to r r ) used in th i s work would, acco rd ing to B ridges and C hester (39 ), suppress normal e le c tro n im pact e x c ite d io n l a s e r l i n e s .-171 -A ll e x c i ta t io n mechanisms ? ,1 - 7*3 a re p o s s ib le . Argon and the h e a v ie r i n e r t gases do no t have s u f f ic ie n t ly e n e rg e tic m e ta s tab le s to be a b le to e x c ite io d in e ground s t a t e io n s in a s in g le c o l l i s io n so th a t o s c i l l a t io n in a k ryp ton - io d in e m ixture (1) cannot be exp la ined by mechanisms s im ila r to 7 .2 and 7 .3 in v o lv in g helium and neon m e ta s ta b le s .In helium - io d in e m ix tures th e s e le c t iv e e x c i ta t io n appears to be most l i k e ly th a t o f asymmetric charge t r a n s f e r (10/01 ) between a helium atom ic io n and a ground s t a t e io d in e atom, w ith th e energy d iscrepancy o f 0 .2 - 0 .6 eV accounted fo r by th e p o la r i s a t io n energy invo lved in th e c o l l i s io n . In t h i s r e a c tio n Wigner sp in s e le c t io n a p p l ie s , a s in the ion-atom , re so n a n t energy t r a n s f e r e x c ite d helium -kryp ton , neon-xenon io n la s e r s o f Dana and Laures (3 )•A f u r th e r p o s s ib le r e a c tio n , in view o f 1) th e o s c i l l a t io n a t h igh (60 to r r ) helium p re s s u re s , 2) the enhancement o f c e r ta in io n ise d io d in e l i n e s in th e h ig h p re s su re , recom bination-dom inated plasma o f a hollow cathode d isch a rg e , could inv o lv e a Ee^g m olecular io n a s fo llow sHe '^'" + ^ 2He ( }s^ ) +N o te :- P ro fe s so r Murakawa has confirm ed th e a n a ly s is o f the h y p erfin e components o f th e l in e a t 6 3 8 . 3nm w ith re c e n t h igh accuracy measurements made on t h i s l i n e . See F ig . 7*23 fo r unpublished r e s u l t s k in d ly se n t by P ro fe sso r Murakawa.Table 7.1 "P a r t i a l l i s t o f t r a n s i t io n s having upper le v e ls w ith in approxim ately0 .3 eV o f io n l im i t o f E e*(0 ).Wavelength Lower Upper A e Enhanced in L aser- (nm) le v e l le v e l eV helium (E) A ction (L)480.64 3d 6p '^ 2 0 .3 4 (E) -488.48 6s* 6p -0 .0 1 - -4 9 8 .6 9 3d 6p 0 .2 3 (E) (L)304.64 6s* 6p ^p0 - 0 . 0 1 - -3 2 1 .6 3 3d - 6p ^^2 0 .3 4 (E) (L)3 2 6 .3 2 6s* 6p 0.02 - -3 3 3 .8 2 6s* 6p* 0 .2 - -3 4 0 .7 416s 6p* 0.23 (E) (L)3 3 9 .3 1 3d 6p 0 .3 1 (E) -3 3 9 .8 2 3P^ 6p 0.02 - -3 6 7 .8 1 6s* 6p* % 0 .3 4 (L)3 7 1 .0 3 6s* 6p* s —0.01 -3 7 6 .0 7 6s* 6p 0 .2 3 (L)3 8 3 .3 3t3d 6p s 0.01 -6 0 6 .8 916s 6p* % 0 .3 4 (L)6 0 7 .3 0 6s* 6p* s -0 .0 1 —612.73 6s* 6p 0 .3 1 (L)6 1 6 .2 2 3p^ 6p* 0 .2 3 -6 2 0 .4 9 6s* 6p 0.02 -173 -Wavelength. - (nm)Lower Upper A ele v e l le v e l eV6 2 3 .3 3f3d % 6p S - 0 .0 16 3 4 .0 0 6s ' S 6p ' % 0 .26 3 1 .6 2 6p 0 .3 46 3 1 .8 9 (5d ' 6p 0 .26 3 8 .3 2t6s 6p 0 .3 1662.24 3d ' 6p ' 0 .2 36 7 6 .4 2 3d ' 6p s - 0 .0 16 8 2 .3 2 6 s ' s 6p' 0 .3 46 9 0 .4 8 6 s ' 6p ' 0 .2 37 0 1 .8 9 3 d ’ s 6p S 0 .2 07 0 3 .3 0 3d ' 6p' % 0 .3 47 1 1 .3 1 3P^ 6p 0 .3 17 1 3 .9 0 3d ' 6p 0 .2 37 6 1 .8 3 3d ' s 6p' % 0 .3 47 7 3 .3 8 3d’ 6p 0 .3 18 1 7 .0 1 3d ' s 6p ' 0 .2 38 2 3 .3 8 3d '06p ' 0 .3 1L aser A ction (1)(L)(L)(L)(L)(L)(L)(L)i bt_ 6s S*% _He Ilaser transit ions (in nm) non laser transitionPartial energy level diagram showing laser transitions in ionised iodine.Fig. 7.1 .- 175 --xftsiwin\JJUUU—IELU003OCLUCOooT O0)oQCo3ÛoLUCO33CLLLO3ËUÜ00F ig . 7 .2 .— 1V6 —Spring loaded tension screwsM icrom eter ) screw th read4 0 T P IM irror held by com pressedRingF ig . 7 .3 . Laser m irro r mounts.- 177 -A.219 cm— “1mPLASMA LENGTH 197cm -2 mB.235cm“2mMirrors: Peaked reflectivity M «^2 6328nmMg 5500 nmARRANGEMENT FOR PRISM WAVELENGTH SELECTION.Fig. 7.4.— 17Ô —-F ig . 7.5R elationsh ip of the 576,Onm la s e r outpu t (upper trace) and the cu rren t pu lse (lower tr a c e ) , a t a r e p i t i t io n ra te of 1 Kc/s.F ig . 7.6R elationship of the 576.Onm spontaneous emission (upper trace) and the cu rren t pulse under lasin g conditions. Scan: 20^ sec/cm.F ig . 7 .7R e la tio n sh ip o f th e in te n s i ty of the n o n -la se r io n ised iodine l i n e a t 533.4nm fupper tra c e ) and th e cu rren t pu lse under la s in g co n d itio n s . Scan: 20 yusec''em.F i g . 7 .8R ela tio n sh ip o f th e in te n s i ty o f th e n e u tra l iodine l in e a t 589.4jun (upper tra c e ) and the cu rren t pulse under la s in g c o n d itio n s . Scan: 20yUsec/cm.— l . S i jF ig . 7 .9R e la tio n sh ip o f th e in te n s i ty o f th e n e u tra l helium l in e a t 388.9nm (upper tra c e ) and th e cu rre n t p u lse . Vapour p ressu re of io d in e lower than optimum fo r l a s e r o s c i l l a t io n . Scan 20^sec /cm ,F ig . 7.10R ela tio n sh ip o f th e spontaneous em ission o f the 576.Onm la s e r l in e (non la s in g ) a t a low vapour p ressu re of iodine(upper tra c e ) and th e c u rre n t p u lse . Scan: 20yUsec/cm.-181F ig . 7.11R e la tio n sh ip of th e in te n s i ty of th e n o n -la se r io n ised iod ine l in e a t 533.8nm (upper t r a c e ) , and th e cu rren t p u lse a t a low vapour p re ssu re o f io d in e . Scan: 20 yusec/cm.F ig . 7.12R ela tio n sh ip o f the in te n s i ty of th e n e u tra l helium l in e a t 388,9mn (upper tra c e ) and th e cu rren t pu lse a t a low vapour p re ssu re of io d in e . Scan: 20yusec/cm.1822p°coreD corecore__ 3mWi01LU§ lOODIS, **DDDOBU lONIdKU AWaUl, tRIFTOH, AMD iUMUi.08 3^*11.0R" 3.a5^Neon 3Argon kkrypton 5lanon 6n« ' ^d“ ( n - l ) d ' ^D°3 2 1 3 2 1>0 0 1 .33 0 0 ■>.50 .75 .25 0 7 .9 2 .61 .^ .25 .02 i w 2 .6 .20 .75 2.-' 0 2 .6 7 .9.8 3 .5 .8 2 .7 12.2 2 .66 .2 .8 0 ^ 8 2 .7 0J » J 6 .2 .01 .22 1 .2.8 6 .2 0 .22 1.8 09*0 0 0 2 .6 0 00Fige 7el7.(«non m l« s « r o a o iU a tio n » «hown by n n d a rlln ln f '.Calculated relative line strengths of transitions in doubly ionised noble gas atom s.- 185 -%%I%Vs%3/2VsL ase r Transitions172153123911806p' \es' %236123F ig . 7.13,T ItT t t L lLT I ^ CM .Wave num ber in 10” ^ cm~^NObserved"componentsTransition d iag ram for th e ionised iodine lin e a t 5 6 7 8nm2^7/25/25/2I/27/25/2%2L aserT ransitions> I41_30 6p' ^Dj 20 121326 s ' ^D,120Calculatedc o m p o n e n tsW ave n u m b er in 10"5 cm'^ Transition diagram for th e io n ised iodine line at 5 76-0nmF ig . 7 .1 9 .136F9^27/25/z3/21/27/22^LaserTransitions172153123Tl6p'1326s'120F ig . 7.20.1 1 T A. i; g g 15 oWave number in 10‘3 cmCalculatedObservedTransition diagram for the ionised Iodine line at 6 0 6 9nm%%%%S eS g34■78236236180“l236s%2F ig . 7 .2 1 .T T T T L _ l j ; ObservedH § 8 : 3Wave number in10~^cm^Transition diagram for the ionised iodine line at 612-7nmin components- 187 -F7/2^2-3/27/2%21Laser Transitions1 1 1 . I,34786p'1326 s '120' CalculatedJ _______L O bservedWave num ber in 10"3cm“ ^Transition diagram forttte ionised iodine line at 6585nmF ig . 7.22,9/27/25/23/21/29/27/25/23/21/2L ase rTransitionsI 1172153"l23"913102(0170Sd'^ D^103s * £ s § BE*Wave number in 10'^ cm"1y j I Calculated com ponentsF ig . 7 .2 3 .Transition diagram for the ionised Iodine line at 703-3nm-r 1 ‘CNO c fm m' q . 'h(D (DT3BB3Ü3(0•s(0c0)IN B N O dH O OW 3X V 3MsivniosoO i iN 3 N O d H 0 3isvi QNv isyys ginQ.(DiN 3 N 0 J W 0 3i S 3 X V 3 MlN 3 N 0 d W 0 3i S ^ I d3 1 > m i 0 S 0 0 1 lN 3 N 0 d W 0 0iS V l QNV 0 N 0 0 3 S ^I8CMtn mCO3O(0Eoccdg -0j=(01 0) c o a E80)cEImCO)cI 5i(O0s:Ic.2«ooco% 1 1 (O % 5F ig . 7.24,189-4 0 0 IS* / ± J%xés-g^(0t 3 *2 iO em' 6 . > 7 - He on lyCd 441 .6 „ Cascade la s in g S tro n g , He on lyZn 775.8 6 s 0 .2 5 1 .0 He on lyCd 840.0 7s » 1 .5 2 .3 —Eg 1 0 5 8 .3 8s ” 0 .6 loO He* (^S)Zn 7 5 8 .8 5P 2 2 .5 He on lyCd 7 5 0 .0 6p " 3 4 . -Eg 6 1 5 .0 7p " 0 .1 7 0 .1 5 V.V. S tro n gZn 4 9 2 .4 4 f - 0 .0 2 0 .6 3 V. S tro n g He on lyCd 5 3 7 .8 4 f 4 ^ / 2 1 .2 3 .3 He and NeEg 5 6 7 .7 5 f " - 0 .5 1 .3 He (^S) V .S trong- 202APPENDIX 4.2P artial l i s t of energy le v e ls for Zinc, Z = 301st I .P . 75,767 or 9.391 VL evelC o n fig u ra tio n D esign, JLevel ( in cm3d^° (^S) 4s^ ' s 0 0Znl 4s 4p 4p ^P° 0 3 2 ,3 0 01 3 2 ,5 0 12 3 2 ,8 9 0Z n ll 3d^° (^S) 4s 4s ^8 1 /2 04p ^P° 1 /2 48,480(^8) 3 /2 4 9 ,3 5 43d^ 4s^ 4s^ ^D 3 /2 6 2 ,7 2 23 /2 65,4413d^° (^8) 5s ^8 1 /2 8 8 ,4 3 7»» 4d ^D 3 /2 96,9093 /2 9 6 ,9 6 0" 5p V 1 /2 1 0 1 ,3 6 53 /2 10 1 ,6 1 1" 6s ^8 1 /2 114,499tt 4 f 4 f 7 /2 1 1 7 ,2 6 33 /2 1 1 7 ,2 6 3" 5d ^D 3 /2 1 1 7 ,9 6 93 /2 1 1 7 ,9 9 3“ 203 - APPENDIX 4 .3P artia l l i s t of energy le v e ls for Cadmium^ Z = 481st I .P . 72,539 cra" ,^ or 8.991 VL evelC o n fig u ra tio nD esign. J L evel ( in cmCdl 4d^^ (^S) 5s^ " s 0 03s 3p 3pO 01230,1143 0 ,6 5 63 1 ,8 2 7C d ll 4d^° (^S) 5s 2s 1 /2 03p2p O 1 /23 /24 4 ,1 3 646,6194d9 5s^ 3 /23 /26 9 ,2 5 97 4 ,8 9 44d^° (^S) 6s 1 /2 8 2 ,9 9 1ft 3d 3 /23 /28 9 ,6 8 989,844ft 6p V 1 /23 /29 4 ,7 1 09 3 ,3 8 3ti 7s 1 /2 1 0 7 ,3 0 1tt 4 f V 3 /27 /2108,4191 0 8 ,4 3 2tt 6d 3 /23 /21 1 0 ,1 7 4110,248— 20/ —APPENDIX 4 .4P a r t i a l l i s t o f energy le v e ls f o r M ercury Z = 80 1 s t I .P . 84 ,184 cm"^, o r 10.43 VL evelC o n fig u ra tio n D esign JL evel, • —1 \ ( in ora ;Hgl Os) 6s^ E 0 06s Os) 6p - V 0 37,6431 39,4122 44,043H gll 5d^0 (^S) 6s 2g 1/2 05d^ Gs^ ^D 3 /2 33 ,3143 /2 30,3323d^0 Os) 6p 2p° 1 /2 31,4833 /2 6o,6o8" 7s 2g 1 /2 93,714" 6d 2d 3 /2 1 0 4 ,9 8 33 /2 1 0 3 ,3 4 3" 7p 2pO 1 /2 1 0 8 ,2 9 83 /2 1 1 1 ,9 7 0” 8s 2g 1 /2 121,416.1 3 f 7 /2 1 2 3 ,1 3 23 /2 1 2 3 ,4 0 9" 7d ^D 3 /2 123,3243 /2 1 2 3 ,3 7 8— 20'5 — APPENDIX 4 .3P artia l l i s t of energy le v e ls for Helium, Z = 2-11 s t I .P = 1 9 8 ,3 0 3 cm , o r 2 4 .3 8 VC onfign . D esign J L evel1s^ ' s 0 0 - 13I s 2s 2s ^S 1 1 3 9 ,8 3 0I s 2s 2s ^S 0 1 6 6 ,2 7 2E le c tro n and S h e ll d i s p o s i t io n o f Z inc , Cadmium and MercuryK L M N 0 Ps s p s p d f s p d f s p d f s p d fZn 2 2 6 2 6 10- 2Cd 2 2 6 2 6 10- 2 6 10- 2Hg 2 2 6 2 6 10- 2 6 1 0 1 4 2 6 10- 2— 2c 6 —APPENDIX 3 .1P o s s ib le re so n a n t energy com binations o f th e i n e r t g ases and M ercury, w ith s u i ta b le ca thode m a te r ia l e lem en ts .C o in c id en ces g iv in g p o s s ib le t r a n s i t i o n s , ( >s in nm) w ith ;He (2^8) m e ta s ta b le l e v e l ,1 . Aluminium A E = 0 . l 6 eV, a t 1 ,4 0 0 .2 . Magnesium A E = 0 .1 eV, a t 2 ,400 th en 800, l i f e t im e r a t i o 27.He io n l e v e l .1 . S i lv e r2.3 .4 .3 .AluminiumA rsen icA E = 0 .003 eV, t r i p l e t l e v e l e x c ite d g iv in g 900 th en240 t r a n s i t i o n s .AE = 0 .16 eV, many l i n e s a t 800.AE = 0 .0 4 eV, t r a n s i t i o n a t 3 ^ 0 «A E = 0 .0 4 eV, number o f t r a n s i t i o n s , one a t 820.CopperPhosphorous A E = 0 .1 3 eV, t r a n s i t i o n a t 784.3 which hasis o e le c t r o n ic l a s e r t r a n s i t i o n in Argon IV.A E = 0 .6 3 eV, g iv in g most p e r s i s t e n t l a s e r t r a n s i t i o n P I I a t 6 0 4 . 3 .A E = 0 .2 3 eV, t r a n s i t i o n a t 309*7 in Se I I w hich has la s e d .6 . Selenium7 . Zinc A E = 0 .3 eV, g iv in g l a s e r t r a n s i t i o n a t 6 IO.O? 2 (3d D^y^ - 3P ^3/ 2 ^ ’ t h i s l a s e r l i n e wasp re d ic te d d u rin g t h i s work.2A E = 0 .2 3 eV, g iv in g t r a n s i t i o n from 6p P^y^ l e v e l a t 1 ,210 which has analogous l a s e r t r a n s i t i o n 8 6 2 .2 i n H g ll, t h i s h as y e t to be r e p o r te d ."207 -Ne ( I s ) m e ta s ta b le s1 . Manganese A e = 0 .0 4 eV, t r a n s i t i o n a t 320.2 . Zirconium T ra n s i t io n a t 4 8 0 .Ne io n le v e l1 . Aluminium A s = 0 .022 eV, t r a n s i t i o n a t 247.3A E = 0 .003 eV, " " 331*3(D uffendack and Thomson, (2 .1 9 ))*Argon - no co in c id e n c e s foundMercury ( Hg 6 ^P^, 6 ^P^) m e ta s ta b le s .1 . Aluminium A e 0 .003 eV, e x c i ta t io n o f 3s 1 /2 l e v e lt r a n s i t i o n a t 2000.2 . Bismuth A e = 30 cm e x c i ta t io n o f 6d le v e l .3 . Calcium A e = - 100 cm e x c i ta t io n 4d le v e l iA e = 100 cm " 7sII4 . G allium A e 70 cm“^ " 6s IIA e 100 cra“^ p o s s ib leII 4 ft r a n s i t i o n a t 6 0 0 .II3 . Barium A e - 30 cm ^1 -%e x c i ta t io n 6d P II6 . Molybdenum A e- -160 cm »» Z ^ II7 . Niobium A e 100 cmT^ »» t IIA E =-1- 20 cm " V ^ II8 . Osmium A e = - 30 cm ^ »' z 7p 119 . P la tinum A e = 30 cm"^ '» 6 s M- 2üB -10 . Ruthenium A E = 30 cm e x c i ta t io ny \A e = 30 cm t t 2 ° ( 5 6 ° )1 1 . S tro n tiu m A E 1 cm t t 11s1 2 . Technetium A E 30 cm"1 t t^® 7/2A E 20 cm t t1 3 . Tungsten A E - 30 cm ^ t t ZA E 20 cm t t- 209 - APPENDIX 7 .1Measurement o f sm all w ave-leng th d if f e r e n c e s o r i n t e r v a l s (M eissner, (7 * 2 0 )) .L e t us suppose t h a t two v ery c lo se l i n e s \ ^ and \ ^ ( u ^ and Àproduce o v e rla p p in g in te r f e r e n c e p a t t e r n s , th e co rresp o n d in g d ia m e te rsb e in g D , D „ , D _ . . . D ,^ , D, _ , D,_ . . . The c e n t r a l o rd e r number o fa l a2 ap bO b l b2A and \ , may be denoted by P and P. , Then the fundam ental form ulaa b a bP = 2 t/X y ie ld s P = p + ^= 2t'v/ , and P = p + f , - 2 t À, .a a ^ a a b b ^ b bThus, fo r th e i n t e r v a l o f th e two l i n e s we g e t :a - = (Pa - P b ) / 2 t + (( : a " ^The f r a c t i o n a l p a r t s ^ ^ and ( ^ have to be c a lc u la te d . The v a lu e o f (p^ - p^) w i l l be a sm all in t e g e r o r z e ro , b u t i t rem ains undeterm ined u n t i la n o th e r pho tograph o f th e f r in g e system , tak en w ith a d i f f e r e n t gap i s a thand . From th e change o f th e p a t te r n s th e n i t i s easy to determ ine w hetherth e s e p a ra t io n o f th e l i n e s i s sm a lle r th a n th e ran g e o f d is p e r s io n , o r th eo rd e rs o v e r la p . In th e f i r s t c a s e , (p^ - p^) i s z e ro , in th e second 1 o r 2 , o r even more. U sing th e e x p re s s io n sP = 2 f ^ c o s 0 = 2 t i i (1 - R ^ /2 f^ ) = 2 t \ ) (1 - D ^/8f^)and (d ^^ - D^^) / (k - i ) = 4A 0 / t = A D^,fo r two co n secu tiv e r in g s o f th e a and b system we can w r ite'-A -^ 'b = (Pa - P b ^ /2 t + ( D ^ D 2 )/2 t,- 210-As the lines are very close A D ^ and A D, ^ will be the samea b(= A d^ = 4f^/A t) • Therefore,^ a - ^ b = ( P a - P b ) / 2 t + ( B ^ - D ^ ^ ) / 2 t A D ?2 2From this relation we see that not only the values (D ^v - D , )a(k - 1; ak2and (D^ ^ in the single ring systems are constant but also the2 2corresponding values - D between two or more different systems.Thus, it is possible to arrange the observations of the jO in a rectangularscheme with constant horizontal (A H) and vertical ( A V) differences.Sometimes, such a scheme will show, by discrepancies in the horizontal or2vertical differences, that the one or the other D value is far off and may be discarded.The general scheme of an interference pattern, produced by three closecomponents a, b, and c, can be arranged as follows:A N A h A H A h a hD' L 4 4 4’bO 4 "b3 “ bif "b5®C2 ^o4 “ c5AASupposing that the orders do not overlap, the intervals A - 'Aand \) ^ ” given as-^ b ^ = (“L - A d2 = /2 t < À i) ,C^b 0^ “ K k - A d^ = < A v ^ 2 t < A h> ,where the sign<^ \ indicates the average values of the horizontal andvertical differences*b-211 -APPENDIX 7.2Component identification on the 638.3nm line to illustrate jQ array method.A.ComponentA h A H A h A H A HDoa 46.2 74.8 121 73 196 73 269 69 338 78 4l6Dob 67. 72. 139 74 213 76 289 72 361 77 43820.8 18 17 20 23 22< A v p = 2 0 . 1 3 , < A h 7 = 7 4 . 1("Ô -Ù. ) = 20.13/(7 4 .1 X 2.024)cm"^ = 134 X 10"^ cm"^a D —B.ComponentA H A H A H A h A H2Doa 46.2 7 4 .8 121 73 196 73 269 69 338 78 4i62Doc 77.3 76 .3 154 71 223 73 300 76 376 74 450AV2 3 1 .3 33 29 31 28 34< A v,> = 5 1 .0 5 , < A h > = 7 4 . i(Ti - Ù ) = 31.05/(74.1 X 2.024) cm”"^ = 207 x 10 ^ cm"a c-1 .“3 —1Note:- All frequency differences are measured from the ' strongest component.-212 -References - Chapter 11. 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W ., Photometry, (Constable) (1938)(2 9 ) Daughters, G. T., Fairchild Application Rept. APR - 4?(3 0 ) de Vos, J. C., Physica (Haag), 690, (1934)(31) Philips Electrical Ltd., "Tungsten Ribbon lamps for Opticalmeasurements".(3 2 ) Robertson, J . K. JOSA, 7 , 9&3, (1923)(33) Helmick, P. S. Phys. Rev, 37, 8, 133, (1921)(34) Mitchell, K. B. JOSA, 8, 846 ( I 96I )(33) Sawyer, R. A., Experimental Spectroscopy, Chapman Hall, (1931)(3 6 ) Harrison, G. R., Lord, R. C, and Loofbourow, J. R.Practical Spectroscopy, Prentice-Hall, N, J. (1939)(37) Bennett Jr., W. R., AD 426961(3 8 ) Schawlow, A. L. and Townes, C. H., Phys. Rev. 112, 6, 1940, (1938)(3 9 ) Cobine, J, D ., Gaseous Conductors, McGraw-Hill, (1941 ).(40) Mandelstam, S. L., and Nedler, V. V. Opt. Spectr. 3^, 196, (I96I)(41) Chebotayev, V. P., Rad. Engy. Electr. Phys (U.S.A.) 10, 2, 314, (I963)(42) Zarowin, C. B., Schiff M. and White G. R. Brooklyn Symp. Opt.Masers, 423, (1963)- 2 2 4 -(45) Smith, J. J. Appl. Phys. 53, 5, 723, March (1964)(44) Znarnenskii, V. B., Buinov, G. N. and Bursakov, E, S.Opt. Spectr., 20, 3, 292, (I966)(43) Sturgess, D. J. and Oskam, H. J., J. Appl, Phys. 10, 2887, (1964)(46) von Engel, A., Ionised Gases, Clarendon Press, (I963).(47) Parker, P., Electronics, Arnold, (1933)(48) Chebotayev, V. P., Opt. Spectr. 1, 10, (I966)-225 ”References - Chapter 6.(1) Jav an , A ., B en n e tt W. R. J r . , and H e r r io t t D. R.Phys. Rev. L e t te r s 6 , 106, (19^1)•(2) W hite A. D ., and R igden J .D . , P ro c . l .R .E . 7 , (1962)(3) Basov N .G ., and K rokhin 0 . N ., A ppl. O pt. 1, 213, (1 9 6 2 ).(4) Javan A ., P hys. Rev. L e t te r s 3 , 8?, (1939 ).(5) V. A. F a b r ik a n t , Sov. Phys. JETP 3A, 373, (1962 ).(6) Ward R .C ., A erospace Tech. R ep t. TDR-930 (2230-20) TN-1.(7) B en n e tt W. R. J r . , A ppl, O pt. Supplem ent 1, 33, (1 9 6 2 ).(8) B ennett W. R. J r . , B u ll . Am. Phys. Soc. 7 , 13, (1 9 6 2 ).(9) P a t e l , C. K. N. , B ennett V/. R. J r . , F au s t W. C. and M cFarlane R.A.P hys. Rev. L e t te r s 9 , 102, (1962 ).(10) R igden J . D. and White A. D ., P ro c . I .E .E .E . 6 943, (1 9 6 3 ).(11) B ennett W. R. J r . , AD 426961, I 9 6 3 .( 1 2 ) Engel A. v o n .. Io n is e d G ases, (C larendon P re s s , Oxford)2nd E d i t . Ch.” 8 I 9 6 3 .( 1 3 ) I b id , Ch. 3 , p . 6 3 .(14) Toyoda K. and Yamanaka C ., Japan J . A ppl. P hys. 4 , 226 ( I 9 6 3 ) .( 1 3 ) Wada J . W. and H e il H ., I .E .E .E . J . Quantum E l e c t r . 8 , 327, (1 9 6 3 ).( 1 6 ) Engel A. von and S teenbeck M., E le c tr i s c h e G asen tladungen , Band 2 ,(S p r in g e r-V e r la g , B e r l in 8 5 , (1932)•( 1 7 ) D orgelo H. B . , A ltin g H and Boers J . P h y sica Haag 2 , 939, (1 9 3 3 ).( 1 8 ) Young R. T . , J . A ppl. P hys. 7 , 2324, ( I 9 6 3 ) .( 1 9 ) Labunda E. F . , and Gordon E . I . , J .A p p l. P hys. 33 , 1647 (1 9 6 4 ).(2 0 ) K oro lev , F . A ., O din tsov A. I . and M its a i V. N.O pt. S p e c try . 19, 1, 36 , ( I 9 6 3 ) .— 226 -(21) M ielenz K. D. and N e ffle n K. A ppl, O pt. 4 , 3&3, (1963 ).(22) W hite A. D. and Gordon E. I . , A ppl. Phys. L e t te r s 3 , 199 (1963).(2 3 ) Bochkova 0 . P . and Razumovskaya L. P . O pt. S p e c try , 17* (1964).(24) Bochkova 0 . P . , Razumovskaya L . P . and F r is c h S . E . O pt. S p e c try 11,3 7 6 , (1 9 6 1 ) .(2 3 ) Bochkova 0 . P . and Razumovskaya L. P . , O pt. S p e c try , I 8 , 5 , 438, ( I 9 6 5 ) .(2 6 ) Bochkova 0 . P . and Razumovskaya L. P . , O pt. S p e c try , 1 8 , 3 , 389, (1963 ).-227 -References - Chapter 7(1(2 (3 (4(3(6(7(8(9(10(11(12(13(14(13(1 6(17(18(19(20(21Jenson R. C ,, and Fowles G. R. P ro c . I .E .E .E . , 32 , 1330 (1964) Fowles G. R. and Jenson R .C ., A ppl. O p tic s 3 , 10, 1191, (1964). Dana L . , and L aures P . , P ro c . I .E .E .E . 7 8 , ( I 9 6 3 ) .M artin W. C. and C o r l i s s , C. H ., J . R es. N at. Bur. S td s , 64A, 6,4 3 3 , Nov. - D ec ., ( i 9 6 0 ) .Keefe W. M. and Graham W. J . , Phys. L e t te r s 6, 643, ( I 9 6 6 ) .B ridges W. B . , and C h este r A. N ., Nerem R ecord, IO6 , (1964)B erez in I . A. and Yanovskaya G. N ., O pt. S p e c tr . lA , 1 , 11 , ( I 9 6 3 )M cFarlane R. A ., A ppl. O p tic s ._ 3 , 1196, (1964) .P r in c ip le s o f R adar, M .I .T ., McGraw H i l l , 2nd E d it io n (1964)W hitteman W. J . , Q .E .C ., 'P h o e n ix ', A p r il 1966, (M.S. 3063 r e p o r t )W hite, A. D ., A ppl. O p tic s , 3 , 3 , 431, (1964)T a y lo r, IT. S . Amer. Phys. S o c ., r e p o r te d by E. P a in e .Conf. L a se rs and A p p lic a tio n s , I .E .E . London S e p t. ( I 9 6 3 ) .Ahmed, S. A. and F a i th T ., Qu. E le c t r . Conf. Phoenix A r iz . A p r il12 - 1 3 , IB - 2 , (1 9 6 6 ) .H eavens, 0 . S . , and W i l le t t , C. S . O pt. A cta , 13, 3 , 271, (1966) W i l l e t t C. S . and Heavens O .S ., O pt. A cta , ] J , ',4,i ( I 9 6 6 )Fowles G. R. and Jenson R. C ., Phys. Rev. L e t t e r s , l 4 , 10, 347 (1963) W i l le t t C. S . , I .E .E .E . , J . Q uant. E l e c t r . , J a n . ( I 9 6 7 )Tolansky S . , P h i l . Mag 229, (1944)C and ler C ., Modern In te r f e ro m e te r s , H ilg e r , (1931)M eissner, K. W., JOSA 31 , 6 , 403 (1941)Murakawa, K. Z. f . Phys. 109, 3 , 162 (1938)-228-(22) Murakawa, K. Z. f . Phys. 112, 3 , 234 (1939)(23) Murakawa, K, N ature 137, 1030 (1936)(24) T olansky , S . , P ro c . Phys. Soc. 49, (1936)(23) T olansky , S . , and F o r r e s te r G. 0 . , P ro c . Roy. Soc. A l68, ?8 , (1938)(26) Kopfermann H ., N uclear Moments. Academic P re s s I n c . , N .Y ., (1 9 3 8 ).(27) Murakawa, K. J . , Phys. Soc. (Japan) 3^ , 9, 1339, (1964)(28) Kuhn, H. G ., Atomic S p e c tra , Longmans, (1962)(29) W hite, H. E . , I n t r o , to Atomic S p e c tra , McGraw-Hill N. York, (1934).(3 0 ) D orgelo , H. B ., Z. Phys, 2 6 , 7 3 6 , (1923)(3 1 ) O rs te in , L . S . , and B urger, H. C. Z. Phys. 403, (1926)(3 2 ) S c h u le r , H. and Jo n e s , E . , Z. P hys. 77, 8OI, (1932)(3 3 ) Shenstone A. G. and R u s s e ll H. N ., Phys. Rev. 39 , 413, (1932)(34) T u rk in , Y. I . , O pt. S p e c try , 7 , 5 , (1939)(3 3 ) Schrammen A ., Ann. P hys. §7^ 6 3 8 , (1928)(3 6 ) S c h u le r , H ., and K eyston, J . E . Z. Phys. 67 ,433 , (1931)(3 7 ) T u rk in , Y. I . , O pt. S p e c try 20, 4 , 4 l l , ( I 9 6 6 )(3 8 ) Byer R. L . , B e ll W. E . , Hodges E . , and Bloom A. L . JOSA, 12 , 1398(1 9 6 3 )(3 9 ) B rid g e s , W. B. and C h e s te r , A. N ., I .E .E .E . J . Q uant. E l e c t r . QE-1,2 , 66, (1 9 6 3 ) .229References, Appendices and Future work.Appendix 4 .11 . Fowles G. R ., and S i l f v a s t , W. T, I .E .E .E . J . Q uant.E l e c t r . , Q.E. - 1 , 3 , 1 (1965)2 . S i l v a s t , W. T ., Fowles G. R. and H opkins, B. D.A ppl. Phys. L e t t e r s , 8 , 12 , 3 l8 , (1966)3» Bloom, A. L . , B e ll W. E. and Lopez F . 0 . ,Phys. R ev ., 135, A3?8, (1964)F u tu re work1 . B e l l , W. E. and Bloom A. I .E .E .E . J . Q uant. E l e c t r . 400, D ec., ( I 9 6 3 ) .2 . A ssagoe, K ., Jap an . J . Phys. 4 , 8 5 , (1927).3* B en n e tt, W. R. and K nutsen, J . W, P ro c . I .E .E .E . 86 I , 1964.4 . D w oretsky, S . H, N ovick, R . , Sm ith W. W. and Tolk N.1 9 th Ann. Gaseous E l e c t r . Conf. O ct. 12-14 , ( I 9 6 6 ) A t la n ta .5* D eheer, F . J . , M u lle r, L. W., G eb a lle , R. P h y s ica , 1743, (1963)»6 . Froome, K. D ., P ro c . Roy. S o c ., A223, 195, (1954 ).APPENDIX A.Reprint fromJ O U R N A L OF A P P L I E D MATHIàMATICS A N D P H Y S I C S (ZAMP)Vol. 1(), Fasc. 1 (1965) m i i k - K . t e s E K v e r i . a g b a s e l Page 87Received ; Uctober 14, 19(>4P ossibility of the Use of a Hollow Cathode D ischarge as an A m plifying M ediumB y O. S. H e a v k n s , U n iversity of York, H eslington , Y ork and C. S. W i l l e t t , R oyal H ollow ay College, U n iversity of London, London, G reat B rita inThe hollow -cathode discharge has long been used as a source o f sharp, intense spectral lines. The m echanism s of th is ty p e of discharge have not been ex ten siv e ly studied , although it is know n th a t the current in the discharge is predom inantly electronic. The source of the very high electron current is th a t of photo-em ission from the cathode surface, produced b y the high in ten sity of u ltrav io let rad iation present in the discharge. Such discharges contain a large concentration of ions and atom s of the cathode m aterial, produced b y sp uttering action of the carrier gas. Under som e conditions, the in ten sities of the m etallic lines is very high and m ay be com parable w ith those from the carrier gas.In recent work on lasers using ion spectra (e .g . A, Kr, X e) very h igh in ten sities and large gains have been reported. A lthough th e detailed m echanism s of these system s are not y e t fu lly understood, a contrib uting feature is the fact th a t th e low er levels of the laser transitions are connected to the ion ground sta te b y u ltrav io let transitions, of very high transition probability .In th e sy stem under consideration , use is m ade of th e fa c t th a t there ex ists a near coincidence in energy betw een an excited level of A 1 11 (3 p'^ ^D) and one of the m etastab le (or quasi-m etastab le) levels of the Is ( P a s c h e n ) group of neon. B y a sim ilar m echanism to th a t used in th e H e-N e laser, ex c ita tio n to th e leve l of A1 II by collision w ith m eta stab les appears prom ising. From the leve l under consideration , return to the ground sta te o f th e ion occurs b y rad iative transitions v ia th e 3 p level. C alculation o f the transition probabilities, using a central field approxim ation, suggests th a t population inversion is possib le. L im ited experim ental work so far has established th e conditions under w hich the A 1 II (3 p^ ^ 3 p ^P) line (at 3900 A) is obtained. T he present system has an un cooled cathode and cannot be operated continuou sly a t a high enough pow er to give a su ffic ien tly high d en sity of A1 II for laser action to be possible. A w ater-cooled sy stem is now under construction .OPTICA ACTA, 1966, VOL. 13, NO, 3, 271-274 APPENDIX B.CorrespondenceLaser transition at 651*6 nm in ionized iodineC. s. W IL L E T and O. S. H EA V EN SD epartm ent of Physics, University of York, England{Received 20 M ay 1966)Laser action has been observed on an intercom bination line of the spectrum of singly ionized iodine, in a pulsed, positive column discharge in a m ixture of helium and iodine.Oscillation was observed on the six lines of wavelength 703-299, 658-521, 612-749, 576 072, 567-808 and 540-736 nm, which have been previously observed by Fowles and Jenson [1, 2]. In addition laser action was obtained at a wavelength of 651-6 nm. T his line corresponds to the intercom bination transition 6p'^p2-5p®^Pj, in the notation of M artin and Corless [3].The discharge tube incorporating internal electrodes was of 7 mm bore and the plasma length 88 cm. It was excited by means of a pulse modulator, discharging a 0-0033 /U.F capacitor through a 4 :1 pulse transform er at an input voltage of approximately 2-4 kv, at 1 kc/s. T he laser output pulse was insensitive to the value of the discharge capacitor and was of approximately 20/isec duration. T he gain on the 576 0 nm line was greater than 5 per cent per metre. T he optical resonator consisted of dielectric-coated concave mirrors, one of which had a radius of 2 m and with a reflecting coating centred at a wavelength of 540 nm, whilst the other was of radius Im and centred on A = 632-8nm.T h e vapour pressure of the iodine was similar to that used by Jenson and Fowles [1 ,2], although the optim um helium pressure was found to be approximately 10 torr. As shown in the figure the newly reported line, in common with the other iodine laser transitions, has a term inal ultra-violet transition (5p®^Pi-5p^^P2) to the iodine ion ground state.As is.seen from the figure laser action is observed in the (6p' ^Dg-bs' ^Dg) line at 540-7 nm in the I I I spectrum but not in the 533-8 nm line (6p'^Fg-bs'^Dg). This is surprising in view of the fact that the bp'^Fg level lies closer to the He+ ground state level than does the 6p'^Dg level. M oreover, the corresponding transition in ionized chlorine at A = 490-47 nm (4 p '® F g -4 s '®Dg) is known to show laser action under comparable conditions [4]. Also, the intensities of the 533-8 nm and the 540-7 nm lines are observed to be of the same order in the experim ents of Berezin and Yanovskaya [5].T he 533-8 nm line would appear to be similar to the 606-89 nm line (bp'^Fg-bs'^D j), which has an upper level from which' laser transitions are observed (567-8, 703-2, 651-6 nm ), and a lower level common to the other laser transitions (576-0 and 658-5 nm), and yet does not show laser action.Both 533-8 nm and 606-9 nm lines have been shdwn by Tolansky [6] to possess two almost equal intensity components in their wide hyperfine structure. Jenson and Fowles [7] have reported laser action on only the strongest component272 Correspondenceoî the many hyperfine com ponents of the lines they have examined (540-7, 567-8, 576-0, 612-7, 658-5 and 703-2 nm ).Other I I I levels close to the He+ ground state level possess direct (ultra-violet) transitions to the iodine ion ground state, so large populations would not be expected in these levels.Although it is probable that the helium ion plays a part in populating the upper levels of ionized iodine, ju s t as it does in the helium -m ercury ion laser [8], it is likely that direct electronic excitation plays an Im portant role. T hus laser action occurs at 562-5 nm (bp^Pg-bs^S^) and at 690-4 nm [9} in neon-iodine mixtures.He ^ l SUHIUlKtlU113965113656112619112179111298110007------- 11 -/ - Gp' =0;1026139 7 0 8 39 6 8 2 5936918 6 8 6 2(86186)- 6p \P2— A////P G r o u n d s t a t e( 0 ) — I s * — — — ISo 5p*Laser t r a n s i t i o n s------------- N o n - l a s e r t r a n s i t i o n sP a r t i a l e n e r g y l e v e l d i a g r a m s h o w i n g l a s e r t r a n s i t i o n s i n i o n i z e d i o d i n e .T he 562-5 nm line of ionized iodine corresponds to the isoelectronic laser transition in X e l l l at 378-0 nm, and to analogous laser transitions in argon, fluorine and brom ine together with the 521-8 nm (4p^Pg-4s^Si) transition (cw) in ionized chlorine.T he regularity with which these p -s transitions occur arises from the high probability of electron collision excitation from the ground state to p-levels, from which direct transitions to the ion ground state are forbidden, and the low probability of direct excitation to the s-levels.Work is cuuently in progress to determ ine the large cross section occurring in the helium ion-atorn resonant collision process, in which energy discrepancies of m ore than the accepted few AT are involved [10].Correspondence 273R e f e r e n c e s[ 1 ] F o w le s , G . R . , a n d J e n s o n , R . C . , 1 9 6 4 , Appl.-Optics, 3 , 1 0 , 1 1 9 1 ,[2] F o w le s , G . R ., a n d J e n s o n , R , C ., 1 9 6 4 , Proc. IEEE, 5 2 , 8 5 1 .[ 3 ] M a r t i n , W . C ., a n d C o r l i s s , C . H., 1960, J . Res. nat. Bur. Stand. A , 5 4 , 4 4 3 .[ 4 ] M c F a r l a n e , R . A ., 1 9 6 4 , OAhcs, 3 , 1 1 9 6 . \[5] B e re z in , I . A ., a n d Y a n o v sk a y a , G . N ., 1 9 6 3 , Opt. Spectrosc., 1 4 , 1 , 1 1 ,[ 6 ] T o la n s k y , S., 1 9 3 5 , Proc. roy. Soc. A, 1 4 9 , 2 6 9 .[7] F o w le s , G . R . , a n d J e n s o n , R . C ., 1 9 6 5 , Phys. Rev. Letters, 1 4 , 1 0 , 3 4 7 ,[8] D y s o n , D , J . , 1 9 6 5 , Nature, Land., 2 0 7 , 3 6 1 .[ 9 ] J e n s o n , R . C ., a n d F o w le s , G . R . , 1 9 6 4 , Proc. IEEE, 5 2 , 1 3 5 0 ,[ 1 0 ] M o t t , N . R . , a n d M a sse y , H . S . W . , 1 9 5 2 , The Theory of Atomic Collisions ( O x f o r d University Press),APPENDIX G. O PTIC A ACTA, 1966, VOL. 13, N O . 3, 0-000Laser oscillation on hyperfine transitions in ionized iodideC. s. W IL L E T T and O. S. HEA V ENSDepartm ent of Physics, University of York, York, England{Received 4 Ju ly 1966)In previous studies of laser action in ionized iodine [1], oscillation has been observed on only one component of a set of hyperfine transitions. In the experim ents reported here, simultaneous oscillation on several hyperfine components was obtained. T he following transitions were examined :Tr ansi t ion 5d3Di-6p'3Fg 6s'3D2-6p'^D2 6s'3D2-6p'3F2 6s' 3Di-6p'3D2 6s'3Di-6p'3p2 6s'»D2-6p'^Di 5p"Pi-6p'^Fg 6s'3Di-6p'3DiWavelength (nm )521-6 540-7 567-8 576-0 606-9 ' 612-7 651-6 658-57nm—'tp ,...'..JLmAFigure 1. F abry-Perot fringes on dispersed part-spectrum of ionized iodine laser lines.Etalon spacer 10-12 mnj.Laser oscillation was observed in a cavity supporting m ulti-wavelength oscillation on the two strongest hyperfine components, classified by Murakawa[2] and Tolansky [3], of the lines 567-8, 576-0, 612-7 nm, whilst oscillation occurred on three hyperfine transitions at 658-5 nm.T he laser was operated in a near confocal arrangement using pairs of dielectric- coated m irrors of Im radius of curvature, (reflectivity peaked at 632-8 jind 500-0 nm), at a separation of 140 cm.002 CorrespondenceT he active plasma length was approximately 1 m, and the bore of the discharge tube was 2 cm. T he discharge was excited by means of a pulse m odulator discharging a 0-0033/.tF capacitor through a 4 :1 pulse transform er, at an input voltage of about 3 kv as previously described [4]. T he initial vapour pressure of iodine, prior to close-off of the discharge tube was 0-1-0-2 torr, with helium added to about 5 torr. To obtain laser action on the hyperfine components it was found necessary to m aintain the initial iodine vapour pressure, and to maintain a discharge having a part molecular spectrum appearance.7/2I■/2%inziifsi in aj lo«*> m ^S t (S3 (J3Wave num ber in lO '^ c m T ’O StO n CM CMjT1, 6 p'6s'I ObservedI T r I , 1 T T componentsFigure 2. Tnmyilion diagram for! lire iodine line at 612-7 nm.in d ic a te s its re la tiv e iiiteh s ity .T he height of a lineLaser oscillation was obserVcd on tlid following hyperfine components ofthe lines :(1) 567-8 nm(2) 576-0 nm(3) 612-7nmI I(i) F 9/2-9/2 strongest coniponcnt.(ii) F S/2-3/2, with a Reparation of 324 x IQ-^cm On two components having a separation of 100 x 10~® (i) F 7 /2-9/2 strongest component.(ii) F S/2-7/2, with a separation of 200 x 10“® cm, - i-1cm -1(4) 658-5 nm On three components, with, separation from the strongest, of130 X 10“®cm“b and 210 x 10“® cm -1Figure 1 shows the dispersed laser lines 658-5, 651-6 and 612-7nm crossed with F abry-Pcro t fringes. Laser oscillation on hyperfine components is made evident by the clear doubling of the ftingeè at 612-7nm ; and three components at 658-5 nm by the triple structure. Also partly shown is the doubly forbidden laser transition 651-6 nm (5p®iPi-6p'®Fa).Correspondence 005Figure 2 gives the transition diagram for the line 612-7 nm, showing the laser and non laser transitions, together with observed relative intensities [3].Figure 3 shows a similar transition diagram for the line 567-8 nm (after M urakawa [5 ^T he use of the prism wavelength selection technique for suppressing transitions competing for upper level population, [6] resulted in no further hyperfine transitions, but enabled laser oscillation at 606-9 nm (6s'®Dj-6p'^Fg), previously unreported, to be obtained. This line has both upper and lower levels common to other laser transitions.F5/27/2I9/27/29/2%L A S E RTRANSITIONS6 $ ' %- Î Î T T . 1 TObservedcom ponenlaWave num ber in 10"®cm7’Figure 3. Transition diagram for the iodine line at 567*8 nm. (after Murakawai l lI !a c k n o w l e d g m e |^t .s ï t is a pleasure to acikndwicdke, thL com petent technical assistance given by J. Clarke in this work, and the pr ovision of excellent silica/glass blowing serviccB by B. Davis and S. M oehr of this University. * MR e f e r e n c e s i ./[1] F o w l e s , G. R., and Je n s o n , R. C., 1965, F/tys. Rev. Letters, 1 4 , 10, 347.[2] M u r a k a w a , K., 1938, Z. Physik., 1 0 9 , 162.[5 ] T o la n s k y , S., 1936, Proc. phys. Soc. Loud., 48, 49 .[4] H e a v e n s , O. S., and W il l e t t , C. S., 0000, Opt. Acta (In press)-.[5] M u r a k a w a , K., 1936, Naturct Loud., Ii7, 1030.[6 ] W h it e , A . D . , 1964, p / ’h c j, 3 , ^^ 4 3 h , -C(JAPPENDIX D."N otes and L in es" ( I .E .E .E . J . Quantum E le c tro n ic s , J a n . 196?)"New l a s e r o s c i l l a t i o n s in s in g ly io n is e d io d in e "In a d d i t io n to l i n e s p re v io u s ly re p o r te d , ( 1 ) , ( 2 ) , f u r th e r o s c i l l a t i o n h as been observed a t : - 7138.97 S (5d ^D^ - 6p ^D^) and a t 6825.23 S.(6 s ^D^ - 6p in s in g ly io n is e d io d in e in a p u lse d h e liu m -io d in em ix tu re u s in g p rism w avelength s e l e c t io n .L ase r o s c i l l a t i o n on h y p e rf in e t r a n s i t i o n s a t 7033*0 S (5d ^D^ -6p and 6068.9 S (6 s ^D^ - 6p ^F^) in a d d i t io n to th o se re p o r te de a r l i e r (2) have now a l s o been o b served .E x c i ta t io n was by means o f a p u lse m odulator d isc h a rg in g a 0 .0 0 6 6 ^ F c a p a c i to r th ro u g h a 4 :1 p u ls e tra n s fo rm e r g iv in g p u ls e s up to l6 KV a t a few am peres. The optimum p re s s u re s in a plasm a tube o f 1 .8 - 2 .0 cm bore w ere: he lium , 5 - 6 t o r r ; vapour p re s s u re o f io d in e ap p ro x im ate ly 0 .2 - 0 .5 t o r r . H igher o u tp u t was found to r e s u l t from u s in g wide bore plasm a tu b e s and s h o r t , low c u r re n t p u ls e s a s in th e helium -m ercury l a s e r ( 3 ) , g iv in g g a in s on th e s tro n g l a s e r l i n e s o f th e o rd e r o f 10 p e r c e n t /m e tre .C. S . W i l le t t D epartm ent o f P h y s ic s , U n iv e rs ity o f York, England,(1 ) R. C. Jen so n and G. R. Fow les; "New L ase r T ra n s i t io n s i n io d in e- I n e r t Gas M ix tu re s" , P ro c . I .E .E .E . , V ol. 52 , p . 1330» (1964 ).(2) G. S . W i l le t t and 0 . S . H eavens; "L ase r o s c i l l a t i o n on h y p e rf in et r a n s i t i o n s in io n is e d io d in e " , O p tica A cta , V ol. 13» No. 3»( in P r e s s ) , (1 9 6 6 ).(3) D. J . Dyson; "Mechanism o f p o p u la tio n in v e rs io n a t 6 l49 8 in th e m ercury io n l a s e r " N a tu re , V ol. 207» p p . 361-363» 24 J u ly , (1965)*

Laser heating of ionized helium

Abstract

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Laser heating of ionized helium

Abstract

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