Relaxation Equations for Depolarized Rayleigh and Brillouin Scattering in Liquids

Abstract

The spectral distributions of light scattered under various polarization conditions in liquids composed of optically anisotropic molecules are analyzed. Macroscopic equations are derived using analogous electric circuits which simulate the viscoelastic behavior due to shear stress, reorientations, and coupling with internal degrees of freedom. The equations describe the long-wavelength thermal fluctuations in the liquid and form the basis of a unified calculation of depolarized and Brillouin scattering. Under the condition that shear relaxation takes place more rapidly than reorientational processes, the sharp component and broad background in the depolarized spectrum are shown to depend primarily on the fluctuations in orientations and shear stress, respectively. Our results also provide a simple interpretation of the recently observed doublet structure. The Brillouin spectrum, on the other hand, shows the well-known effects of thermal relaxation as well as the effects of coupling with shear stress and reorientations. Numerical results on quinoline and nitrobenzene are presented, and the computed depolarized spectra are in good agreement with the experimental data of Stegeman and Stoicheff.