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

The second-order Raman-scattering (SORS) cross section of alkali halides has been evaluated by using the Buckingham expansion for the induced polarizability. This ab initio approach leads to a microscopic expression of the SORS cross section without free parameters. The first two nonzero contributions of the expansion, namely, the dipole-induced dipole (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 polarizability coming from the electronic overlap has not been included in the present work. The SORS spectra are expressed in terms of the dynamical quantities of the crystal (eigenvectors and eigenfrequencies) and in terms of some bare ionic susceptibilities, namely, the ions' bare polarizabilities 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 spectrum and on the short-range nature of the BTQ spectrum are made. The relative weights of the two contributions to the SORS in the whole class of alkali halides are also discussed.