High-frequency transverse-like excitations in glassy glycerol

Abstract

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 ≈ 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 τ _α approaches , a behaviour consistent with the condition τ_α Ω_T ≫1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.

Acknowledgements

We thank C. Henriquet and R. Verbeni for help in handling the samples.

Notes

† The fitting procedure always gives χ² values within three (about 200 fitted points and seven (low-Q) or ten (high-Q) fitting parameters; at high temperatures a further parameter is needed for the stretching of the Cole–Davidson function). In all cases the relevant fitting parameters (peaks positions) were found to be only slightly correlated with the other (maximum of the covariance matrix, about 0.7).