THEORY OF DEPOLARIZED RAYLEIGH-BRILLOUIN SPECTRA AND ROTATION-TRANSLATION COUPLING IN MOLECULAR LIQUIDS AND SOLIDS

The dynamics of translations and rotations of molecular solids and liquids has been studied by using a microscopic theory. The effects of translation-rotation coupling on the depolarized Rayleigh-Brillouin spectrum are investigated in detail. The theory takes into account anharmonicity and thus includes both the static and the dynamic couplings. In the limit of the harmonic approximation, the present theory reduces to the earlier published result for solids. The earlier published result for ideal liquids can also be derived from the present theory. This calculation is based on linear response theory and thus provides a statistical mechanical deviation of the phenomenological theory of the depolarized Rayleigh scattering spectrum of condensed matter. The relations between the transport coefficients as well as the potential and coupling constants in the macroscopic theory and the kinetic coefficients in the microscopic theory are established. The correlation function for the lattice displacement derived from solving the macroscopic equation is shown to be in agreement with the microscopic theory. It is shown, however, that the phenomenological theory does not provide the same polarizability correlation function for solids as that obtained from microscopic theory, despite the fact that the two theories agree in the case of liquids. The effects of relaxation and resonance behavior of the depolarized Rayleigh-Brillouin spectral density are also discussed in detail. © 1979 American Institute of Physics.