Impurity-induced Raman scattering in CsBr and CsI

Measurements of the first-order impurity-induced Raman scattering in ${\mathrm{C}\mathrm{s}\mathrm{B}\mathrm{r}:\mathrm{I}}^{-}$ and ${\mathrm{C}\mathrm{s}\mathrm{I}:\mathrm{B}\mathrm{r}}^{-}$ are reported. Comparison is made with a corresponding phonon Green's-function calculation. Using known perfect-crystal shell-model parameters, only one additional parameter, the change in the longitudinal nearest-neighbor force constant, is needed to describe the dynamical properties of the defect and to fit the shape of the three independent Raman spectra with $A_{1g}$, $E_g$, and $T_{2g}$ symmetry. Assumptions about the form of the terms giving rise to the Raman scattering permit the fitting of the relative intensities of these three spectra with one parameter.