Intrinsic luminescence of alkali halides at 10°K

Thesis (Ph.D.)--University of Rochester. College of Engineering and Applied Science. Institute of Optics, 1967.Studies of luminescence of alkali halides so far have been carried out either with ultraviolet radiation at liquid nitrogen temperatures or with x-rays. Luminescence studies with ultraviolet irradiation in conjunction with the absorption and photoconductivity data give an insight into the electronic transitions in these materials. In the present work the results of measurements of the emission and excitation spectra of KI, RbI, KBr, RbBr, KCl, NaCl, CsI, CsCl, RbI:Tl at 10°K and KCl:Mn at room temperature are presented. It is shown that impurities greatly affect the luminescence of alkali halides. Foreign ions luminesce during irradiation not only in their own absorption bands but also in the intrinsic absorption region of the host. Intrinsic emission is differentiated from this host sensitized impurity luminescence on the basis of a study of samples from various sources and by an examination of the excitation spectra of the luminescence. The intrinsic emission spectra of KI, KBr, CsI and CsCl consist of two bands. These are at 3.31, 4.15; 2.29, 4.43; 3.62, 4.27 and 2.00, 3.81 eV respectively. Those of RbBr, KCl, NaCl consist of one band at 4.13, 2.27 and 3.49 eV respectively. RbI shows three bands at 2.23, 3.15 and 3.95 eV. A fourth band appears strongly at 2.64 eV for excitation in the region of the first exciton peak. This band appears also for stimulation in the C band of RbI:Tl and its excitation spectrum is similar to that for 2.82 eV emission of RbI:Tl. It is not possible to establish the intrinsic or extrinsic nature of the 2.64 eV band on the basis of the data presented in this work. A comparison of the excitation spectra of the intrinsic bands of KI, RbI, KBr and RbEr with the absorption spectra shows that the low energy band is preferentially stimulated in the region of the low energy exciton peak and the high energy band in the region of a higher energy exciton. Some of these spectra show peaks in excitation which lie below the threshold for photoconductivity. On the basis of these results and those of previous workers it is concluded that trapped excitons are responsible for these emission bands. The excitation spectra show minima at the position of known exciton peaks. They also show additional minima which can be correlated with exciton lines not clearly resolved in absorption spectra. On this basis the third component of the Wannier exciton series first observed by Fisher and Hilsch is identified in KI and RbI. The band gaps of these materials are recalculated using the energies of the second and third lines of the series