Reorientational dynamics in molecular liquids as revealed by dynamic light scattering: From boiling point to glass transition temperature

We determine the reorientational correlation time tau of a series of molecular liquids by performing depolarized light scattering experiments (double monochromator, Fabry-Perot interferometry, and photon correlation spectroscopy). Correlation times in the range 10(-12) s-100 s are compiled, i.e., the full temperature interval between the boiling point and the glass transition temperature T-g is covered. We focus on low-T-g liquids for which the high-temperature limit tau congruent to 10(-12) s is easily accessed by standard spectroscopic equipment (up to 440 K). Regarding the temperature dependence three interpolation formulae of tau (T) with three parameters each are tested: (i) Vogel-Fulcher-Tammann equation, (ii) the approach recently discussed by Mauro et al. [Proc. Natl. Acad. Sci. U. S. A. 106, 19780 (2009)], and (iii) our approach decomposing the activation energy E(T) in a constant high temperature value E-infinity and a cooperative part E-coop(T) depending exponentially on temperature [Schmidtke et al., Phys. Rev. E 86, 041507 (2012)]. On the basis of the present data, approaches (i) and (ii) are insufficient as they do not provide the correct crossover to the high-temperature Arrhenius law clearly identified in the experimental data while approach (iii) reproduces the salient features of tau(T). It allows to discuss the temperature dependence of the liquid's dynamics in terms of a E-coop(T)/E-infinity vs. T/E-infinity plot and suggests that E-infinity controls the energy scale of the glass transition phenomenon. (C) 2013 AIP Publishing LLC.