Relaxation Stretching, Fast Dynamics, and Activation Energy: A Comparison of Molecular and Ionic Liquids as Revealed by Depolarized Light Scattering

Depolarized light scattering (DLS) spectra of a series of 16 molecular and 6 room temperature ionic liquids are investigated by applying tandem-Fabry-Perot interferometry, double monochromator, and photon correlation spectroscopy. Temperatures up to well above the melting point, in some cases, even up to the boiling point, are covered, and all liquids can be supercooled. The accessed time constants are between 1 ps and 10 ns; in some cases, even longer times are reached. The susceptibility spectra and likewise the corresponding reorientational correlation functions are characterized by stretching parameter beta(CD) (0.32-0.80) for the long-time decay (alpha-process), strength of fast dynamics 1 - f, and time scale at shortest times expressed by k(B)T/I* with the apparent quantity l* reflecting essentially inertia effects. An additional (intermediate) power-law regime (or excess wing in the frequency domain) between fast dynamics and the alpha-process has to be taken into account. For a given system the spectral parameters are virtually temperature independent up to the boiling point, i.e., frequency-temperature superposition applies for the alpha-process. Among the liquids, the quantity I* correlates with molecular mass, and the larger 1 - f, the smaller the inertial quantity 1*. No correlation among 1 - f and beta(CD) is revealed. Testing for correlation of beta(CD) or 1 - f with parameters describing the temperature dependence of the correlation time tau(omega) namely, high-temperature activation energy E-infinity, fragility in, or glass transition temperature T-g, no significant correlation is found. Regarding molecular vs ionic liquids, no relevant difference in the evolution of their DLS spectra is observed.