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.