Depolarized light scattering spectra of molecular liquids: Described in terms of mode coupling theory

摘要

Depolarized light scattering spectra of eight molecular liquids as obtained from applying tandem-Fabry-Pérot interferometry and double monochromator are analyzed in the frame work of the mode coupling theory (MCT). The susceptibility spectra are fitted to the numerical solution of the schematic F12 model of MCT and the validity of the asymptotic laws is discussed. The model is able to quantitatively describe the spectra up to the boiling point, where the main (structural) relaxation and the contribution of the microscopic (vibrational) dynamics essentially merge, and down to the moderately super-cooled liquid where glassy dynamics establishes. The changes of the spectra with temperature are mapped to only two control parameters, which show a smooth variation with temperature. Strong correlation between experimental stretching parameters and extrapolated values from the model is found. The numerical solutions are extrapolated down to T_c, where the asymptotic scaling laws can be applied. Although the spectra apparently follow scaling relations, the application of the asymptotic laws usually overestimates T_c by up to 12 K. In all the cases, the experimental spectra are outside the applicability regime of the asymptotic laws. This is explained by more or less strong vibrational contributions. Within a phenomenological approach which extends the spectral analysis down to T_g and which allows for separating fast and slow dynamics, the strength of the fast dynamics 1 – f_(rel) is revealed. It shows the cusp-like anomaly predicted by MCT; yet, the corresponding critical temperature is significantly higher than that derived from the F_(12) model. In addition, we demonstrate that close to T_g, the susceptibility minimum is controlled by the interplay of the excess wing and the fast dynamics contribution.