Density fluctuations in the intermediate glass-former glycerol: a Brillouin light scattering study

Brillouin scattering has been used to measure the dynamic structure factor of glycerol as a function of temperature from the high temperature liquid to the glassy state. Our investigation aims at understanding the number and the nature of the relaxation processes active in this prototype glass forming system in the high frequency region. The associated character of glycerol is reflected by a rather simple relaxations pattern, while the contributions coming from intra-molecular channels are negligible in the GHz frequency region. The temperature behavior of the characteristic frequency and lifetime of the longitudinal acoustic modes is analyzed, suggesting that a phenomenological model which only includes the structural ($\alpha$) process and the unrelaxed viscosity is able to catch the leading contributions to the dynamics of the density fluctuations. This ansatz is also supported by a combined analysis of light and inelastic x-ray scattering spectra. The temperature dependence of the characteristic time of the $\alpha$-process, $\tau_\alpha$, obtained by a full-spectrum analysis conforms to the $\alpha$-scale universality, i.e. the values $\tau_\alpha$ revealed by different experimental techniques are proportional the ones to the others. The non-erogodicity parameter smoothly decreases on increasing the temperature, and no signature of the cusp-like behavior predicted by the idealized mode coupling theory and observed in other glass-formers is found in glycerol.