Effect of Applied Uniaxial Stresses on the Optical Absorption ofFCenters in Alkali Halides

Abstract

The method of moments is used to analyze the effect of applied uniaxial stresses on the optical absorption of alkali halides containing $F$ centers. By measuring the first-moment change of the $F$ band due to uniaxial stresses applied in $〈100〉$ and $〈110〉$ directions, the coupling of the $F$-center excited state to lattice distortions of tetragonal and trigonal symmetry is evaluated. By measuring the third-moment change due to the same applied stresses, the separate contributions to the second moment of the $F$ band of dynamic lattice distortions of tetragonal, trigonal, and breathing symmetries are evaluated. Measurements of the zerothmoment change due to applied stresses allow evaluation of the validity of the Condon approximation. Calculations based on three simple models of the $K$ band are compared with the results of a measurement of the change in zeroth and first moments of the $K$ band produced by applied uniaxial stresses. The temperature dependence of the zeroth and first moments of the $K$ band is calculated for the same three models. The results suggest that the $K$ band is not due to transitions to a state of even parity.