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- 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. Because 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.