Study of the Longitudinal Dynamics of Glass-Forming Systems in the Mesoscopic Energy-Momentum Region

The dynamic structure factors for two glass formers, o-terphenyl and glycerol, have been measured at different temperatures T and exchanged momenta Q using inelastic X-ray scattering. We find propagating excitations in both liquid and glass phases in the 1-6nm(-1) e range. The temperature dependence of the energy Omega(Q,T) of these excitations shows a cusp at a temperature T-x which is discussed in terms of a microscopic arrest temperature. Using the Q --> 0 limit of the dispersion curves we develop a scheme to extract the infinite-frequency sound velocity C-infinity(T) and use this scheme for the case of glycerol. The derived values of C-infinity(T) are in disagreement with previous determinations obtained by extrapolating low-frequency data.