Photochemically Produced Color Centers in KCl and KBr

Abstract

KCl and KBr crystals were exposed to unfiltered mercury arc radiation at 15°K. If an "OH" band was present in the crystals before irradiation, the ultraviolet irradiation produced an optical absorption spectrum similar to that produced by x rays at 15°K. The optical absorption spectrum and the changes in the absorption produced by annealing at higher temperatures were measured and compared with the spectra observed during similar annealing of x-irradiated crystals. In KCl the 335-mμ band, formed by the uv irradiation, bleaches thermally at 56°K as does the 335-mμ $H$ band of x-irradiated KCl. In KBr the 381-mμ band bleaches in steps at 35°, 46°, 56°, and 80°K as compared with the 381-mμ $H$ band in x-irradiated KBr which bleached at 30°, 46°, 56°, and 80°K. In both KCl and KBr the $V_1$ band appears with the disappearance of the $H$ band. Illumination in the $V_1$ band causes regeneration of the $H$ band as occurs in x-irradiated KCl and KBr. In both KCl and KBr the photoproduced $H$ band grows by about 10% at approximately 25°K. It is concluded that the $H$ centers bleach thermally by diffusing to and combining with other color centers. Recombination with $F$ and $\alpha$ centers annihilates the $H$ centers and leads to the formation of $V_K$ centers, while recombination with a third center (possibly a positive-ion vacancy) results in the formation of $V_1$ centers.