Production and Bleaching of Color Centers in X-Rayed Alkali Halide Crystals

The x-ray production and optical and thermal bleaching of color centers in NaCl and LiF have been studied quantitatively in regions of substantially uniform concentration, by making optical absorption measurements in a direction perpendicular to the axis along which the crystals were x-rayed. At room temperature $F$ center growth rate for short x-ray exposures is found to be proportional to the rate of absorption of x-rays. For long exposures the rate of growth of the $F$ band increases greatly near the x-rayed surface. Large $M$ band growth accompanies this increased $F$ band growth. The rate of bleaching of $F$ centers by $F$ light in NaCl has been studied at room temperature for low concentrations where the light absorption is nearly uniform along the light path. Differential equations describing the rates of excitation of $F$ center electrons to the conduction band and the trapping of conduction electrons by negative ion vacancies and holes have been integrated under special conditions. The results are found to be consistent with the experimental data. The rate of thermal bleaching of $F$ centers in NaCl has been studied at temperatures up to 150°C. It has not been found possible to analyze the data in terms of a mechanism involving trapping of conduction electrons by negative ion vacancies and holes. The results of several qualitative experiments to investigate the possible formation of vacancies during x-raying are presented.