Coupling Between Rayleigh and Brillouin Scattering in a Disparate-Mass Gas Mixture

Brillouin and Rayleigh scattering have been studied in a gas mixture of S${\mathrm{F}}_6$ and He at various concentrations. A strong coupling exists between the concentration and pressure fluctuations in the mixture that produces a heavy damping of the thermal sound-wave excitations and prevents them from propagating adiabatically. Consequently, a fully coupled solution of the hydrodynamic equations is necessary to describe the spectrum accurately.