Dynamics of Dense Supercritical Neon at the Transition from Hydrodynamical to Single-Particle Regimes

The dynamic structure factor, $S(Q,E)$, of neon has been studied by inelastic x-ray scattering and by molecular dynamic simulation in the momentum transfer region $Q=1\mathrm{}–25{\mathrm{nm}}^{-1\mathrm{}}$ at $T=295\mathrm{}\mathrm{K}$ and $P=3\mathrm{}\mathrm{kbar}$. At this density, comparable to that of the liquid at ambient pressure, the shape of $S(Q,E)$ evolves from a Brillouin triplet towards a complex single line shape, which precurs the shape expected for single-particle behavior. The data, analyzed using the three mode model of the molecular hydrodynamics, show the absence of positive dispersion in the sound velocity and a minimum in the dispersion curve.