Universal and non-universal features of the dynamic susceptibility of supercooled liquids

We discuss the dynamics of the glass-forming liquids glycerol, propylene carbonate, and benzophenone, as revealed in their dielectric behaviour and in depolarized light scattering (DLS) data. Above the melting point, the liquids exhibit two-step stretched relaxation behaviour typical of 'glassy dynamics' at all attainable temperatures greater than or similar to 2T(g). There is no sign of a transition to exponential relaxation; rather, the stretching depends only weakly on the temperature. The spectral change, in first approximation, amounts to shifting the alpha-relaxation peak with the temperature. This behaviour is in contrast with the temperature-dependent spectral shapes observed in the low-temperature state close to T-g. Analysing corresponding dielectric spectroscopic (DS) data for comparisons, similar tendencies are observed, except that the stretching parameters are different. Below a certain temperature in glycerol, the tail of the excess wing, which is clearly pronounced in the DS data, appears also in the DLS spectra, with the same ratio of the wing exponents in DLS versus DS as of the stretching exponents at higher T. In propylene carbonate, the presence of a wing in the DLS data is not immediately obvious. However, its low-temperature DLS spectra are fully compatible with an empirical model for the temperature-dependent spectral shape that adequately describes the DS data of both materials and includes the wing. We were however unable to reconcile the DLS results of glycerol with this model. This, together with the pronounced difference in DLS and DS spectral widths of glycerol, cast doubts over the general validity of universal scaling procedures for different techniques.