Theoretical interpretation of the second-order Raman spectra of caesium halides

Abstract

The results of calculation of the second-order Raman spectra of CsCl. CsBr, and CsI are presented. For the last two crystals the results can be compared with observed spectra. It is found that the spectra derived by allowing for the polarizability and overlap deformation of the ions are in much better agreement with the experimental results than those derived by treating the ions as rigid point charges. In the first case the overall agreement is remarkably close, even though no allowance has been made for the wave-vector dependence of the Raman polarizability tensor. The probable reason for this is that the main peaks in the theoretical spectra arise from super-positions of critical points in the two-phonon dispersion curves which occur along symmetry axes. The present results tend to support the view that the observed Raman spectra provide a reasonably close mapping of the two-phonon density of states and can be used both to test and refine theoretical calculation of the dynamical matrix. Also, it appears that low-temperature, high-resolution spectra should reveal considerably more detail if the computed spectra are reliable