Experimental and molecular dynamics study of depolarized Rayleigh scattering by O2

Abstract

Collision-induced effects on depolarized Rayleigh scattering (DRS) in gaseous oxygen at 300 K in the pressure range from 10 to 3000 bar have been studied experimentally. DRS spectra for four thermodynamic states at this temperature and in the pressure range from 130 to 3000 bar have also been obtained from molecular dynamics (MD) simulation. Molecular trajectories were calculated using the Lennard-Jones atom–atom potential and the system polarizability was modeled using the first order dipole–induced dipole approximation. The frequency dependence of DRS band shapes is well reproduced by the MD results, which predict that the orientational contribution dominates the overall band shape. The agreement between experimental and calculated second, fourth, and sixth spectral moments is very good. The observed density dependence of the spectral moments is analyzed in terms of contributions from the collision-induced (CI) polarizability component and orientational pair correlations. The second spectral moment, M2, is found to decrease with increasing density at low densities and to increase at higher densities. The MD results indicate that the initial decrease in M2 is due primarily to slow relaxation of the CI polarizability at low densities. The increase in M2 at higher densities is found to be mainly due to the contribution from orientational-CI cross correlations. MD results as well as analytical calculations, carried out for the low density limit, predict that the CI contribution to the total DRS intensity remains relatively small (≤4%) throughout the entire density range. The observed density dependence of the intensity is not consistent with these calculations. Possible sources of this disagreement are discussed.