Correlation of Positron Annihilation and Other Dynamic Properties in Small Molecule Glass-Forming Substances

Positron annihilation spectroscopy is used to characterize the ortho-positronium lifetime, ${\tau }_3$, in a broad range of small molecule organic glasses over a wide temperature range. The magnitude and thermal variations of ${\tau }_3$ reflect changes in the dielectric $\alpha$-relaxation time ${\tau }_{\alpha }$ and its non-Arrhenius or “fragile” characteristics. ${\tau }_3$ also displays striking similarities with the fast relaxations reflected in the hydrogen-weighted mean square atomic displacements $〈u^2〉$. The parallel temperature variations of $〈u^2\left({T/T}_g\right)〉$, ${\tau }_3\left({T/T}_g\right)$, and ${\tau }_{\alpha }\left({T/T}_g\right)$, and their similar patterns of change between the different glasses, are discussed in terms of anharmonicity in the intermolecular caging potentials and the subnanometer density heterogeneities.