Brillouin scattering in the rare gas liquids and some mixtures of the rare gases.

We have observed the Brillouin scattering of light from the pure monatomic liquids Argon and Krypton and the binary monatomic gas mixtures Argon-Neon .and Krypton-Helium.

The spectrum of light scattered from a fluid for a scattering process with momentum transfer K and frequency to is proportional to the power spectrum, S (K, to), of fluctuations in the thermodynamic properties of the fluid. For a one-component system this reduces to the power spectrum of fluctuations in the density of the fluid; while for a binary mixture S(K, to) can be expressed as the total power spectrum of fluctuations in a set of independent thermodynamic variables.

When the fluid is dense the mean free path of the atoms (or molecules) is much smaller than the wavelength» A, associated with the scattering process (A = 2tt/K) and the scattering process is dominated by interatomic collisions. In this case S(K, to) can be calculated from the equations of hydrodynamics.

The spectrum of light scattered from liquid Argon and liquid Krypton along their saturated vapour pressure curves has been measured. The experiments were performed over a range of scattering angles for both liquids. The velocities of hypersound in each liquid, calculated from the Brillouin shifts of the scattered light, show no dispersion over the frequency range measured, i.e.^300-3000 MHz.X V i

These results also agree with sound velocities measured by ultrasonic techniques within the experimental errors. We have also measured the Brillouin linewidths from these liquids, and from these calculated the intrinsic bulk viscosities of these liquids.

The spectrum of light scattered from the binary monatomic gas mixtures Argon-Neon and Krypton-Helium as a function of Neon and Helium concentrations respectively has been measured. From the shifts and widths of the Brillouin lines we have investigated the propagation and attenuation of sound waves in these mixtures. In the case of the Krytpon-Helium mixture measurements were also made at different scattering angles to investigate the dispersion of sound in this mixture.

From the measurements of the attenuation coefficients in the binary monatomic gas mixtures we have estimated the mutual diffusion coefficients, D12/ in these mixtures. The results obtained agree with theoretical calculations for atoms with the Lennard-Jones potential.