We report X-ray and neutron scattering results in vitreous silica. As in earlier data, clear evidence is found for sound wave modes at the energies of the boson peak. We have observed by X-ray experiments a clear dispersion of Brillouin peaks and we have detected experimental evidence of inelastic scattering in the second Brillouin zone. By neutron scattering experiments we have found coherence eOE ects in S…Q; E †. These eOE ects are consistent with average acoustic phonon dispersion curves, which are similar in character to the dispersion curves found in crystals but, obviously, broadened by the topological disorder.

We have performed an inelastic X-ray scattering experiment on normal liquid H 2 at a temperature of 31.5 K and a density of 21.5 nm 3 on the ID16 beam line at the European Synchrotron Radiation Facility. We compare three independent methods to determine experimentally its static structure factor S…Q †: method (i) is from a diOE raction measurement; method (ii) is through an ! integration of the dynamic structure factor S…Q; ! †; method (iii) is from a best-®t procedure of the spectral line shape with a viscoelastic model. The measurements have then been compared with both classical and quantum path integral Monte Carlo simulations. The comparison between experimental and computed S…Q † indicates that quantum eOE ects must be explicitly taken into account in the simulation in order to obtain good agreement with the data. } 1. Introduction Despite great interest from both a theoretical and a practical point of view, liquid H 2 has only recently become the subject of intensive experimental studies and computer simulations. In fact, a quantitative understanding of its microscopic behaviour at a level similar to that achieved for liquid He is still far from being reached. While on the computational side this gap is being ®lled quite rapidly (Kinugawa 1998) since quantum exchange eOE ects are rather small (Poll and Miller 1971) and liquid H 2 behaves almost like a`Boltzmann quantum¯uid', on the experimental side there are several practical di culties still to be overcome.As far as quantum eOE ects are concerned, these become observable on time and length scales comparable with the quantum coherence length and time, roughly given by ¶ 1 B and c ¶ 1 B ( ¶ B being the de Broglie wavelength and c the speed of light in vacuum). This occurs in the dynamic range accessible to both neutron and X-ray