Selection rules for second order infrared and raman processes. I. Caesium chloride structure and interpretation of the second order Raman spectra of CsBr and Csi

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Abstract
Two phonon selection rules for Raman and infrared processes are given for crystals having the caesium chloride structure. The selection rules are quite similar to that of the rocksalt structure, favourable for second order Raman scattering processes and quite severe for the second order infrared processes. With the help of the theoretical dispersion curves it is shown that the second order Raman spectra of CsBr and CsI can be accounted for in terms of phonon pairs at symmetry points in the s. c. Brillouin zone. Each of the observed peaks is found to arise from several phonon pairs at the different symmetry points. The many contributions to the observed peaks together with the high polarizability of the caesium and halogen ions account for the relatively high intensity of the spectrum. The limited structure in the infrared absorption spectrum of CsBr can be accounted for in terms of phonon pairs at A and Σ symmetry points