Dependence of positron lifetimes on the temperature and phase of some organic media

Abstract

Measurements of the value τ₂ and the intensity I₂ of the long component in the life-time spectra of positrons annihilating in some simple unassociated, polar, and associated organic liquids are reported as a function of temperature. A semi-empirical free volume model for the formation and quenching of ortho-positronium atoms is developed for simple molecular materials to explain the correlation between viscosity η, density ϱ, and τ₂ observed in unassociated and polar liquids. Such a correlation is not observed for associated liquids in which the presence of hydrogen bonds between molecules apparently influences the annihilation process. The data of positron lifetimes during the solid-liquid phase transition in some organic systems are discussed in terms of the free volume model.