Production ofV3Centers in KCl by X Rays

The growth of the $V_3$ band has been studied in x-ray-irradiated KCl at room temperature, by measuring the optical absorption as a function of x-ray dose. In the region in which the $F$ band grows linearly with time (after the initial "fast coloration"), the $V_3$ band height increases approximately as the square root of the time. The $V_3$ band growth rate varies with x-ray intensity and depth in the crystal in the same way as that of the $F$ band, however. Although the $F$ band can be optically bleached without affecting the $V_3$ band, if the $F$ band is optically bleached during the x-ray irradiation, the $V_3$ band likewise fails to grow. These facts are thought to be consistent with a model of the $V_3$ center in which a ${\mathrm{Cl}}_3^{-}$ molecular ion is located at a cation vacancy. The center could be formed by the x-ray production of an anion vacancy-interstitial pair and the diffusion of a cation vacancy to the interstitial.