Density fluctuations in the quasi-hydrodynamic regime: neutron scattering and molecular dynamics simulations

The onset of deviations from hydrodynamic behaviour in the dynamic structure factor S(k, omega ) of gaseous 36Ar has been recently observed in small-angle inelastic neutron Brillouin scattering experiments. The interpretation of S(k, omega ) in terms of three extended hydrodynamic modes, already applied in other works to several experimental and simulation results obtained at much larger k-values and densities, was shown to be valid also for k approximately=1 nm-1 at densities well below the critical one. In order to investigate in more detail the onset of the non-hydrodynamic regime we have calculated the intermediate scattering function F(k,t) of a Lennard-Jones system by means of molecular-dynamics simulations at the same densities as in the experiment and at wave vectors small enough for the Brillouin triplet to be visible. We present here some preliminary results where F(k,t) is analysed in terms of extended hydrodynamic modes.