Inelastic light scattering from density fluctuations in dilute gases. II. Nonhydrodynamic behavior of a binary gas mixture

Inelastic light scattering has been used to study nonhydrodynamic behavior in the helium-xenon binary gas mixture with xenon dilute. Because of the large atomic mass and light-scattering cross section of xenon relative to helium, the contribution of xenon may be separated from the total scattered light spectrum. The spectrum of light scattered by xenon is studied as a function of helium pressure. At low helium pressure, xenon atoms are in free flight over the length $\frac{2\pi }{K}$, where $K$ is the scattering vector. The resulting scattered light spectrum is Gaussian. At high helium pressure, xenon atoms diffuse over the length $\frac{2\pi }{K}$ and the spectrum is Lorentzian. Quantitative study of the Gaussian-Lorentzian transition shows the Fokker-Planck model to be appropriate to describe the xenon-helium collision. Brillouin splitting and linewidth measurements are used to demonstrate the existence of nonhydrodynamic relaxation processes associated with momentum and temperature equilibration of the two species in a disparate mass mixture where the heavy component is dilute.