RAYLEIGH-BRILLOUIN LIGHT-SCATTERING FROM NOBLE-GAS MIXTURES .1. THE LANDAU-PLACZEK RATIO

We present the Rayleigh-Brillouin light scattering data for He + Xe mixtures of various compositions and pressures between 2 and 10 MPa. We compare our experimental data to calculations for a mixture that behaves according to the van der Waals equation of state. Our analysis shows that the contribution of concentration fluctuations to the Landau-Placzek ratio is very important. Furthermore, we demonstrate experimentally the relation between the Landau-Placzek ratio, the adiabatic velocity of sound, and the reduced second moment of the dynamic light scattering spectrum. We show that the van der Waals equation of state can be used to predict the landau-Placzek ratio in these mixtures quantitatively for mixtures with compositions x(Xe) < 0.9. For mixtures with higher xenon concentrations, experimental results agreed quantitatively up to 5 MPa with calculations.