Dependence of the Optical Bleaching Rate of X-Irradiated KCl Crystals on Light Intensity

Measurements have been made on the changes in the bleaching rate of the $F$ band as the result of varying the intensity of $F$ light. Bleaching was done on KCl crystals which had been irradiated at room temperature with hard x rays into either the first or second stage of coloration. In the first stage irradiated crystals the total bleaching of the $F$ centers during the first ten seconds of illumination varied approximately linearly with light intensity at low intensities, but becomes saturated at intensities greater than 5×${10}^{14}$ photons/sec ${\mathrm{cm}}^2$. The bleaching process is thought to occur primarily through the capture of photoelectrons at pre-existing electron traps. Based on this model it is possible to set up the kinetic equations for the bleaching process. Using reasonable values for photoelectron capture coefficients forces one to conclude that the number of photoelectrons is always very small throughout the bleaching process. The resulting simplification in the analysis of the kinetic equations leads to a prediction of the experimentally observed variation of the initial bleaching rate with light intensity. The analysis shows that the ratio of the capture coefficient for photoelectrons of the electron trapping centers to that of negative-ion vacancies is about unity, which strongly suggests that the trapping centers are either traps with net positive charge or are neutral traps located close to the original $F$ centers.