Light scattering by transverse waves in supercooled glycerol

Evidence of transverse acoustic waves in glycerol is given by Brillouin light scattering over a temperature interval ranging from a few kelvins below the glass transition temperature T-g = 187 K to some 30 K above T-g. The analysis of Brillouin doublets shows a linear temperature dependence of the unrelaxed shear modulus G(infinity) above T-g, which is close to that extrapolated from ultrasonic data. This behaviour seems to be rather unusual when compared with the linear dependence of G(infinity)(-1)(T) previously documented in a number of glass-forming systems. By decreasing the temperature, Ginfinity(T) shows a bend at the glass transition and, for lower temperatures, a weaker temperature dependence. In this region, a value of about 0.5 has been found for the ratio of the characteristic frequencies of transverse acoustic to longitudinal acoustic excitations, consistent with the value recently found for the ratio of the energies of the collective modes revealed by inelastic X-ray scattering in the terahertz region.