Is there any evidence of a positive sound dispersion in the high frequency dynamics of noble gases?

Abstract

The dynamic structure factor, S( Q, ω), of neon has been studied by Inelastic X-ray Scattering and by Molecular Dynamic simulations in the momentum transfer region Q=1–25 nm −1 at T=295 K and P=3 kbar. At this density, comparable to that of the liquid at ambient pressure, the shape of S( Q, ω) evolves from a Brillouion triplet towards a complex single lineshape, which precurs the shape expected for single particle behavior. The data, analyzed using the three modes model of the molecular hydrodynamics, show the absence of positive dispersion in the sound velocity and a minimum in the dispersion curve. Analogous measurements have been performed on a sample of supercritical 4He at a density double of the liquid one. The inelastic shifts obtained from the fits coincide with the 0-frequency values predicted by the viscoelastic theory. The analysis of the whole set of measurements leads to the conclusion that no relaxation processes as those expected in the viscoelastic behavior are observable for these systems in the explored THz frequency range.