Light-scattering studies of moderately dense gas mixtures: Hydrodynamic regime

Measurements of the Rayleigh-Brillouin spectra of light scattered by two classes of gaseous mixture; helium-xenon, and helium-hydrogen and helium-deuterium, have been performed over a range of density, concentration, and wave vectors. For He-Xe mixtures, under certain experimental conditions the data are accurately accounted for by a complete two-component hydrodynamic theory and light scattering has been shown to be a good method to deduce the thermal diffusion coefficient. We have thus determined in a quantitative way the range of applicability of the hydrodynamic theory. On the other hand, when the partial desnity of helium is not too small, systematic deviations between experimental and the calculated spectra are observed. These deviations can be understood in terms of kinetic effects to be discussed in a subsequent paper. For He-${\mathrm{H}}_2$ and He-${\mathrm{D}}_2$ mixtures, the hydrodynamic theory is in good agreement with our experimental data in the full range of densities and concentrations. We discuss the reason for this different behavior for the two classes of mixtures.