Non-exponential relaxation without dynamic heterogeneity in van der Waals liquids above the melting point

We investigate the origin of relaxation stretching in van der Waals liquids above the melting point by means of depolarized dynamic light scattering. To this end, we study optically anisotropic probe molecules both in the bulk and when diluted in an optically isotropic solvent. Strikingly, the relaxation shape of the probe molecules in dilution is indistinguishable from that of the pure liquid composed of the probe molecules. By contrast, when explicit dynamic heterogeneity is introduced through a distribution of probe molecule sizes, the relaxation shape becomes sensitive to the solvent concentration. These findings indicate that dynamic heterogeneity has a negligible influence on the diverse relaxation shapes observed above the melting point, which seem to arise purely from intrinsic properties of the molecules such as anisotropic rotation and internal degrees of freedom. The observed behavior differs greatly from what has been reported close to the glass transition temperature, where both heterogeneous dynamics and a generic spectral shape of structural relaxation indicate a subordinate role of intrinsic molecular properties.