Positronium Annihilation in Molecular Substances

The behavior of the Ps atom in molecular substances, particularly liquids, is investigated. The pickoff rates of o-Ps in various liquid compounds are found to have a simple empirical relationship to the values of the surface tension of the liquids. The relationship is found to have a theoretical foundation. The Ps atom is highly localized in a cavity created by the balance of various molecular forces inside the liquid. From the above relationship, other simple relationships between the pickoff rates of o-Ps and the various properties of the medium, e.g., polarizability, cohesive energy density, etc., and the temperature or pressure changes can be derived and explained. The diffusion of o-Ps is discussed. A similar approach can also be used for molecular solids.