Light scattering in gas mixtures: Evidence of fast and slow sound modes

Campa and Cohen [Phys. Rev. A 39, 4909 (1989)] have predicted that dilute, binary mixtures of gases with disparate masses should exhibit a (fast) sound mode whose velocity is considerably greater than expected on the basis of conventional hydrodynamic theory, and which should be observable via light-scattering experiments. Effects that are consistent with this prediction were observed in the Brillouin spectra of the ${\mathrm{H}}_2$+Ar system, but were not detected for the case of ${\mathrm{CH}}_4$+${\mathrm{SF}}_6$. Results for the ${\mathrm{SF}}_6$+${\mathrm{H}}_2$ mixture demonstrate the existence of an analogous slow-mode contribution to the spectrum.