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

The dynamic structure factor, S(Q, omega), of neon has been studied by Inel astic 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 densi ty, comparable to that of the liquid at ambient pressure, the shape of S(Q, omega) evolves from a Brillouion triplet towards a complex single lineshap e, 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 t he dispersion curve. Analogous measurements have been performed on a sample of supercritical He-4 at a density double of the liquid one. The inelastic shifts obtained from the fits coincide with the 0-frequency values predict ed by the viscoelastic theory. The analysis of the whole set of measurement s 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. (C) 2000 Elsevier Science Ltd. All rights reserved.