Density and temperature dependence of spectral moments in depolarized light scattering by rare gases

Abstract

We present an extensive study of the first three even moments M _2n of depolarized light scattering (DLS) spectra of rare gases argon and xenon. Most of our results were obtained by molecular dynamics and Monte Carlo computer simulation methods, which were used to calculate the moments for pairwise-additive intermolecular pair potential and interaction-induced polarizability models over a wide density range and at two temperatures, one above and one below the critical point. Simulation was used to determine the dependence of the moments on the pair polarizability anisotropy β(r) by comparing the results for the first order dipole-induced dipole (DID) model with those obtained for a semiempirical model of β(r), recently proposed by Meinander, Tabisz and Zoppi (MTZ), which includes electron overlap, dispersion and second order DID interaction contributions. Comparison with experiment indicates that the MTZ model for β(r) is clearly superior. Moments of increasing order are found to be increasingly more sensitive to the functional form of β(r), pointing to the importance of short range interactions for the spectral line shape. By contrast, the cancellation effects due to short range ordering at high densities become less pronounced as the order of the moment increases. In the case of argon, simulation results were obtained for Lennard-Jones and Aziz-Slaman pair potential models. Their comparison revealed a weak dependence of the moments on the details of the intermolecular potential. The Kirkwood superposition approximation approach to estimating the contributions of 3 and 4 molecule correlations to M ₀ and M ₂ was generalized to a non-DID model of β(r). The limits of accuracy of this approximation and of the lattice-gas model estimates of multiparticle correlation contributions to M _2n are critically examined.