Study of the high frequency dynamics in glass-forming systems

The dynamic structure factors of three glass-formers, o-terphenyl, glycerol and vitreous silica, have been measured at different temperatures T and ex changed momenta Q using inelastic x-ray scattering. We find propagating exc itations at wavelengths approaching the interparticle separation region. We will show that these excitations are strongly related to the excess of vib rational states in glasses. Moreover, taking as example the measurements pe rformed on o-terphenyl, we demonstrate that the temperature dependence of t he dispersion curves allows the determination of the infinite frequency sou nd velocity C-infinity(T). The derived values of C-infinity (T) in the liqu id phase can be used to discriminate among different phenomenological model s previously proposed.