The depolarized Rayleigh scattering from fluids of spherical molecules

Abstract

A calculation of the depolarized Rayleigh scattering from a liquid of isotropically polarizable molecules is described. The spectrum reflects cooperative, many-particle, structural relaxation which is represented by a generalized Langevin equation for the bilinear number density fluctuations. The theory is applied to the particular case of liquid argon at the triple point and is shown to give a very close quantitative agreement with the experimental lineshape, using data from neutron-scattering experiments on liquid argon and no adjustable parameters. The various features of the observed spectra are related to the behaviour of the correlation functions of different Fourier components of the bilinear number density fluctuations which are interpreted in different limits of the molecular motion, diffusion, gas-like, etc. The sensitivity of the lineshape to modifications of the induced polarizability functional from DID form is investigated. Within the range of physically reasonable modifications little change in the lineshape is found, although the calculations suggest that the intensity, in the liquid, is more sensitive than in the gas phase. The theory is compared with Stephen's theory of collision-induced light scattering which uses a different approximation to the molecular dynamics. This theory gives a better overall description of the argon spectrum than had previously been realized but fails qualitatively to predict several features of the observed spectrum.