Dependence of Positron Lifetimes on the Temperature and Phase of Some Organic Media

Abstract

Abstract Measurements of the value τ 2 and the intensity I 2 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 τ 2 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.