Raman intensities of lattice vibrations in molecular crystals

An exact expression within the point-dipole rigid-molecule approximation is derived for the intensity of polarized Raman scattering from lattice modes in molecular crystals. By treating the local electric field properly, the method gives the contribution to the intensity from translational motions as well as rotational ones. The intensity varies as the fourth power of the local field correction for both types of motion. It also depends on the polarization vectors of the lattice modes and the effective molecular polarizability. Methods for obtaining these quantities are discussed. Numerical calculations for the pure rotational modes in hexachlorobenzene and naphthalene crystals show that the intensifies are very sensitive to the input data, making it difficult to draw conclusions from a comparison with measured intensifies.