From Single-Particle to Collective Dynamics in Supercooled Liquids

It has been recognized recently that the considerable difference between photon-correlation (PCS) and dielectric (BDS) susceptibility spectra arises from their respective association with single-particle and collective dynamics. This work presents a model that captures the narrower width and shifted peak position of collective dynamics (BDS), given the single-particle susceptibility derived from PCS studies. Only one adjustable parameter is required to connect the spectra of collective and single-particle dynamics. This constant accounts for cross-correlations between molecular angular velocities and the ratio of the first-rank and second-rank single-particle relaxation times. The model is tested for three supercooled liquids, glycerol, propylene glycol, and tributyl phosphate, and is shown to provide a good account of the difference between BDS and PCS spectra. Since PCS spectra appear to be rather universal across a range of supercooled liquids, this model provides a first step toward rationalizing the more material specific dielectric loss profiles.