Low-Temperature Optical Bleaching ofFCenters in KCl

Optical bleaching of $F$ centers in x-rayed and additively colored KCl has been studied at 77°K with pulse irradiation methods. Constant "$F$ light" incident on the colored crystal establishes a photostationary equilibrium between $F$ and $F^{\prime }$ centers. The application of a high-intensity light flash of appropriate spectrum displaces the system from equilibrium to either direction. The return to equilibrium is exponential in time, with a rate controlled by the intensity and spectrum of the constant light, but not depending on the $F$-center concentration or the method of coloration. The results are in agreement with a kinetic model involving only $F$-to-$F^{\prime }$ center interconversions. Prior room-temperature optical bleaching inhibits the extent and rate of low-temperature bleaching, probably due to the growth of other bands under the $F$ band. The saturation of the extent of bleaching at high pulse-light intensity and a lower bleaching efficiency for the additively colored crystal indicate that the distribution of $F$ centers in the crystal is significant in low-temperature bleaching.