Light Scattering and the Coupling of Molecular Reorientation and Hydrodynamic Modes

Abstract

A statistical theory, based on linear response theory, is presented to describe the coupling between molecular reorientation and both longitudinal and shear hydrodynamic modes in liquids, and the effect of this coupling on the depolarized light scattering spectrum. The theory is valid if molecular reorientation takes place by rotational diffusion. The “doublet” features of the observed spectra are explained and both the $\mathrm{VH}$ and $\mathrm{HH}$ depolarized spectra can be understood in terms of one nonhydrodynamic parameter which depends upon the anisotropic intermolecular potential. Although the longitudinal modes contain traveling waves which couple to the molecular reorientation and give rise to depolarized Brillouin doublets, the doublet due to the coupling to the shear modes arises from purely dissipative motion and not from shear waves.