Optical Bleaching ofV3Centers in KCl

Bleaching of $V_3$ centers by uv light absorbed in the $V_3$ band was studied in Harshaw KCl crystals which had been x-ray irradiated at room temperature. The bleaching measurements were made in the temperature range from 75 to 110°C, where the uv bleaching rate was appreciable but purely thermal bleaching (in the dark) was not significant. The $V_3$ band decay was approximately exponential in time. Furthermore, the decay constant was approximately an exponential function of inverse temperature, with an activation energy of 0.6 eV. The calculated quantum efficiency for the bleaching was small throughout this temperature range, being given by $\eta \approx {10}^6\exp (-0.6\mathrm{} \mathrm{eV}/kT)$. Some interaction between $V_3$ centers and $F$ centers was observed, since bleaching of either band by light absorbed in the band led to partial bleaching of the other band. The results are interpreted in terms of a model for the $V_3$ center consisting of a ${\mathrm{Cl}}_3^{-}$ molecular ion at a cation vacancy. The $V_3$-band bleaching differs from $U$-band bleaching in its strong temperature dependence and the lack of any accompanying $F$-band production.