FAST SOUND IN A DENSE HELIUM-ARGON GAS-MIXTURE

By means of inelastic neutron scattering we measure the dynamic struct ure factors S(k, omega w) for a dense He-4(75) Ar-40(25) gas mixture ( total number density n = 13 nm(-3), p = 370 bar) and for pure He (n = 12.1 nm(-3), p = 350 bar) at T = 160 K for Wavenumbers 2 less than or equal to k less than or equal to 5 nm(-1). For both samples C(k, omega )= omega(2)S(k, omega) shows clear maxima at the same frequency omega = c(He) k with c(He) = 970 m/s the sound velocity in pure He, much lar ger than the sound velocity c(mix) = 635 m/s of He-4(75) Ar-40(75), th us directly confirming the existence of fast sound oscillations in the mixture, due to the light He particles alone. Molecular dynamics simu lations and kinetic theory calculations for corresponding Lennard-Jone s and hard sphere mixtures, respectively, indicate that the experiment al C(k, omega) of He-4(75) Ar-40(25) is almost completely due to fast sound.