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 g lycerol, have been measured at different temperatures T and exchanged momenta Q using inelastic X-ray scattering. We find propagating excita tions in both liquid and glass phases in the 1-6nm(-1) e range. The te mperature dependence of the energy Omega(Q,T) of these excitations sho ws a cusp at a temperature T-x which is discussed in terms of a micros copic arrest temperature. Using the Q --> 0 limit of the dispersion cu rves we develop a scheme to extract the infinite-frequency sound veloc ity C-infinity(T) and use this scheme for the case of glycerol. The de rived values of C-infinity(T) are in disagreement with previous determ inations obtained by extrapolating low-frequency data.