Shell model description of hyper‐Raman scattering from alkali metal halides

Hyper‐Raman scattering by the zone‐centre optical phonons of alkali metal halides is described in terms of an anharmonic shell model. In order to reproduce the measured relative values of the hyper‐Raman tensor elements, three types of fourth‐order coupling parameters, representing intra‐ionic, inter‐ionic and Coulomb anharmonicities have to be taken into account. Deducing the non‐linear force constants of the short‐range inter‐ionic interactions from a Born‐Mayer potential and supplementing them by exact expressions of their Coulomb Counterparts, it is possible to determine all parameters contributing to hyper‐Raman scattering. The model also predicts the third‐harmonic generation coefficients in satisfactory agreement with experimental data.