Photochemically Produced Color Centers in KCl and KBr

Abstract

KCl and KBr crystals were exposed to unfiltered mercury arc radiation at 15\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}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-m\ensuremath{\mu} band, formed by the uv irradiation, bleaches thermally at 56\ifmmode^\circ\else\textdegree\fi{}K as does the 335-m\ensuremath{\mu} $H$ band of x-irradiated KCl. In KBr the 381-m\ensuremath{\mu} band bleaches in steps at 35\ifmmode^\circ\else\textdegree\fi{}, 46\ifmmode^\circ\else\textdegree\fi{}, 56\ifmmode^\circ\else\textdegree\fi{}, and 80\ifmmode^\circ\else\textdegree\fi{}K as compared with the 381-m\ensuremath{\mu} $H$ band in x-irradiated KBr which bleached at 30\ifmmode^\circ\else\textdegree\fi{}, 46\ifmmode^\circ\else\textdegree\fi{}, 56\ifmmode^\circ\else\textdegree\fi{}, and 80\ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}K. It is concluded that the $H$ centers bleach thermally by diffusing to and combining with other color centers. Recombination with $F$ and $\ensuremath{\alpha}$ 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.$U$, ${U}_{1}$, ${U}_{2}$, ${O}^{\ensuremath{-}}$, and $\ensuremath{\alpha}$ centers are produced by the uv irradiation as well as $F$ and $H$ centers. The photoproduced $F$ band may be bleached optically with negligible effect on the $H$ band.