From Single-Particle to Collective Dynamics in Supercooled Liquids

It has been recognized recently thatthe considerabledifferencebetween photon correlation (PCS) and dielectric (BDS) susceptibilityspectra arises from their respective association with single-particleand collective dynamics. This work presents a model that capturesthe 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 ofcollective and single-particle dynamics. This constant accounts forcross-correlations between molecular angular velocities and the ratioof the first- and second-rank single-particle relaxation times. Themodel is tested for three supercooled liquids, glycerol, propyleneglycol, and tributyl phosphate, and is shown to provide a good accountof the difference between BDS and PCS spectra. Because PCS spectraappear to be rather universal across a range of supercooled liquids,this model provides a first step toward rationalizing the more material-specificdielectric loss profiles.