Depolarized Interaction-Induced Light Scattering from Rare Gas Systems at High Pressure.

Depolarized interaction-induced light scattering (DILS) experiments were carried out on several rare gases and rare gas mixtures at room temperature and at pressures to 10 kbar. The gases Kr, Ar, He and mixtures 10% He/90% Kr, 20% He/80% Kr, 90% He/10% Kr, 95% He/5% Kr and 10% Ar/90% Kr were studied. Pure rare gases have been studied extensively in the past while similar studies have not been carried out for binary mixtures. The theory for DILS from binary mixtures is developed according to a previously developed formalism. This theory is cast into the form of a lattice gas model using established techniques. Expressions are derived for the density dependence of the two lowest order even spectral moments. Observed line shapes for pure substances exhibit a definite evolution with increasing density, from a gas with interactions which are principally binary to a dense fluid with characteristics attributable to solid-like behavior. Pure Ar and Kr also demonstrate low frequency scattering attributable to diffusive modes while He does not. Kr rich mixtures exhibit the same type of lineshape evolution with increasing density as pure Kr, with differing intensities, curvatures, and slopes in various frequency regions. The sample 90% He/10% Kr surprisingly demonstrates a lineshape which is attributable to collective excitations in the frequency range characteristic of pure Kr, persisting to the lowest densities studied. The sample 95% He/5% Kr does not exhibit such behavior at any density. The first three even spectral moments are measured for all samples studied and computations for the first two are compared with experiment. An enhancement in integrated intensity is observed for He-Kr mixtures with maximum occurring at about 10% He concentration. This enhancement is in qualitative agreement with the predictions of the lattice gas model and is attributable to an interference with many -body Kr interactions due to the presence of He. A much slighter enhancement is predicted for the second moment for He-Kr mixtures at low He concentration but this is not observed. Calculations for other binary mixtures are presented and indicate that the amount of enhancement observed might be directly related to the degree to which the components are disparate in their physical characteristics. Suggestions for further work are put forth.