Color Centers Produced in KCl and KBr by Prolonged X Irradiations at Low Temperatures

The principal purposes of this study were to observe the effect of an x irradiation of extended duration on the production of color centers in KCl and KBr at low temperatures and to examine the implications with regard to possible color-center models. Growth curves were determined at 5^∘{}K for the F, K, M, and H bands in KCl and KBr and for a new band at 245 m{μ} in KCl and at 278 m{μ} in KBr, called the H^' band. Growth curves were determined at 80^∘{}K for the F, K, and M bands in KCl and KBr. The growth of the V₄ band in KBr and of a band having its absorption maximum at 240 m{μ} in KCl, designated as the $V(240)$ band, was also followed at 80^∘{}K. The ratio of the absorptions at the peak of the F and K bands was found to remain constant indicating that both of these bands arise from transitions in the same center. The concentration of M centers was found to depend quadratically on the F-center concentration. This result gives conclusive support to the model proposed by van Doorn and Haven that the M center consists of a pair of associated F centers. The F-to-H ratio was observed to be constant for short x-ray exposures, supporting the viewpoint that the two centers are produced as complementary defects; however, for prolonged exposures, this ratio is no longer constant. The number of F centers is in constant ratio to the combined concentrations of H and H^' centers provided that the oscillator strength of the H^' band is suitably chosen. The linear dependence of the generation of both V₄ centers in KBr and $V(240)$ centers in KCl on F-center concentration suggests that the V₄ and $V(240)$ centers, rather than V₁ centers, are fundamentally related to the mechanism of formation of F centers near liquid-nitrogen temperature for a prolonged irradiation.