MICROSCOPIC MODEL FOR THE 2-PHONON RAMAN-SPECTRA OF ALKALI-HALIDES

The second-order Raman-scattering (SORS) cross section of alkali halid es has been evaluated by using the Buckingham expansion for the induce d polarizability. This ab initio approach leads to a microscopic expre ssion of the SORS cross section without free parameters. The first two nonzero contributions of the expansion, namely, the dipole-induced di pole (DID) coming from the first-order dressed polarizability and the BTQ (second-dipole hyperpolarizability X dipole propagator X effective charge) coming from the second-dipole-quadrupole polarizability, have been considered. The short-range contribution to the induced polariza bility coming from the electronic overlap has not been included in the present work. The SORS spectra are expressed in terms of the dynamica l quantities of the crystal (eigenvectors and eigenfrequencies) and in terms of some bare ionic susceptibilities, namely, the ions' bare pol arizabilities alpha+ and alpha- and the second-dipole-quadrupole bare polarizabilities B+ and B-. As an example the results are shown in the case of KBr. Considerations on the long-range nature of the DID spect rum and on the short-range nature of the BTQ spectrum are made. The re lative weights of the two contributions to the SORS in the whole class of alkali halides are also discussed.