High frequency transverse-like excitations in glassy glicerol

The dynamic structure factor of glassy and liquid glycerol has been measured by inelastic X-ray scattering in the exchanged momentum Q region Q = 2-23 nm(-1) and in the temperature range 80-570 K. Besides the propagating longitudinal mode, at low temperatures the spectra show a second non-Q-dispersing peak at hOhm(T) approximate to 8.5 meV. This peak is the signature of the transverse dynamics which, in topologically disordered systems, acquire a small Q-dependent longitudinal symmetry component. This assignment is substantiated by a study of the temperature dependence of the intensity of the second peak. This study showed that, in the liquid, the second peak intensity vanishes when the structural relaxation time tau(alpha) approaches Ohm(T)(-1), a behaviour consistent with the condition tau(alpha)Ohm(T) much greater than1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.