Selection Rules for Second-Order Infrared and Raman Processes in the Rocksalt Structure and Interpretation of the Raman Spectra of NaCl, KBr, and NaI

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Abstract
Group-theoretical selection rules are given for two-phonon infrared absorption and Raman scattering processes in crystals having the rocksalt structure. In common with the diamond structure, two-phonon overtones are infrared inactive. The selection rules for second-order infrared absorption processes in the rocksalt structure are much less favorable than those in the diamond and zinc-blende structures. The selection rules for second-order Raman scattering processes are less restrictive and there are many more Raman-active phonon pairs than infrared-active pairs. With the help of the selection rules, the second-order Raman spectra of NaCl, KBr, and NaI are interpreted in terms of phonon pairs at specific symmetry points in the Brillouin zone, using the theoretical phonon dispersion curves calculated by Hardy and Karo for NaCl and the experimental phonon dispersion curves determined by Woods, Brockhouse, Cowley, and Cochran by neutron spectroscopy for KBr and NaI.