Observation of fast sound in disparate-mass gas mixtures by light scattering

We performed light-scattering experiments on a mixture of hydrogen and argon. By varying the density of the sample, we can probe the range of reduced wave vectors in which Campa and Cohen [Phys. Rev. A 39, 4909 (1989)] predicted, in dilute disparate-mass gas mixtures, the onset of a mode supported by the light particles: the fast sound mode. The presence of the additional sound mode can be established most conveniently by analyzing ω²I(k,{ω}) rather than I(k,{ω}). Our results for the shift of fast and slow sound match the theoretical predictions very well.