A 893where e= ~iE«(5u-Sn)T; S n and Su, are the elastic compliance constants, T is the applied stress and E w is the shear-deformation-potential constant of the conduction-band minima. Also, 6A C = chemical splitting of the ground state. The ls(E) and ls(Zi) states are assumed to be degenerate. The solutions to the secular equation are e-A c (3 roots), -2e-A c , andThe variation of the ground states with strain is shown in Fig. 15. It is seen that the two states described by the first two types of roots vary with strain in a manner which is identical to that of the p states.The following linear combinations of the effectivemass wave functions permit the selection rules of Fig. 12 to be deducedThe principal purposes of this study were to observe the effect of an x irradiation of extended duration on the production of color centers in KC1 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 KC1 and KBr and for a new band at 245 m/i in KC1 and at 278 m/j in KBr, called the W band. Growth curves were determined at 80°K for the F, K, and M bands in KC1 and KBr. The growth of the F 4 band in KBr and of a band having its absorption maximum at 240 nut in KC1, designated as the 7(240) band, was also followed at 80 C 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 Doom and Haven that the M center consists of a pair of associated F centers. The F-to-TI 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 E' band is suitably chosen. The linear dependence of the generation of both Vi centers in KBr and 7(240) centers in KC1 on F-center concentration suggests that the F 4 and 7(240) centers, rather than V\ centers, are fundamentally related to the mechanism of formation of F centers near liquid-nitrogen temperature for a prolonged irradiation.

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Recent results of van Doom 1 on the thermal equilibrium of F andM centers in KC1 show that the concentration of M centers varies quadratically with the concentration of F centers. This result supports the proposal of van Doom and Haven 2 and Pick 3 that the model of the M center is a pair of associated F centers (F 2 model) rather than a vacancy pair plus a single F center as suggested earlier by Seitz 4 and modified by Knox. 5 This Letter presents new results on the relative concentration of F and M centers induced by irradiation, and the implication of these results on the M-center model.Thin plates of KBr and KC1, approximately 0.09-0.20 mm thick, have been irradiated at liquid helium and liquid nitrogen temperatures with x rays from a 50-kv dc generator for periods up to sixty hours. Sensitive optical absorption measurements show that a weak M band appears with the generation of F centers in KBr, KBr doped with Ca, KC1, and KC1 doped with Sr. The data for KBr (Fig. 1) show that the concentration of M centers produced by these low-temperature irradiations is a quadratic function of the F -center concentration. A 12 en Q_

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