Optically detected NMR and ENDOR [electron nuclear double resonance] with F center pairs in KCI

F centers in KCl crystals, paired by bleaching near room temp. and excited by F-band light, experience an exchange interaction favoring a nonradiative deexcitation. In a magnetic field at 10°K, the luminescent intensity showed EPR, ENDOR, and NMR, from changes of the spin populations induced by microwave and radio-frequency fields. When excited with F light at 10°K, these crystals showed a luminescence which decreased with magnetic field. With applied microwaves in the X band, the EPR in the ground state was detected optically as an increase of the luminescence with a g-factor of 2.001 and a full width at half-max. of 64 G, and a long relaxation time, of 10-3-10-1 sec, depending upon the incident light intensity. ENDOR yielded optical signals of the same sign and order of magnitude as the EPR signal. When the microwave power was turned off, the luminescence displayed as a function of the radio frequency the same spectrum (pure NMR)