Dynamic Structure Factor of a Helium-Neon Dense Gas Mixture: Crossover from Hydrodynamics to the Microscopic Regime

The dynamic structure factor for a dense gas He/Ne mixture is calculated by molecular dynamics. At length scales within the realm of hydrodynamics, k≤0.03^-1, the spectrum shows a well-resolved Rayleigh-Brillouin triplet, evidencing the presence of sound-wave excitations. For intermediate wave vectors, the partial spectra apart from showing a broadening of the inelastic peaks unveil the presence of two additional modes with phase velocities below and above hydrodynamic sound. The present results cover a broad range of wave vectors, which cannot, at present, be reached by experimental techniques, and where present theoretical approaches are not well established.