Positron studies of condensed matter

Abstract

This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. A preliminary chapter outlines the theory of the annihilation process, establishes the importance of the two photon mode of decay and describes how the observable results depend on the initial positron and electron states of the system. A brief account of the principal experimental techniques is included. An adequate description of the initial particle states can demand consideration of the chemical, electronic and structural properties of the material. A brief discussion of the complex nature of positron behaviour in molecular substances suggests the particular value of investigations directed at the two photon pick-off mode of decay of the orthopositronium atoms that are formed in these materials. The possibility of positronium formation in ionic and metallic materials is also considered. The important many electron aspects of positron annihilation in metals are dealt with at length. The latter and larger part of this article is concerned with applications. The independent particle approach to the analysis of two photon angular distributions is discussed and illustrated. A survey of electronic structure investigations includes studies of polycrystalline and single crystal specimens. The relevance of angular correlation measurements in investigations of the Fermi surfaces of metals and alloys is discussed. A final chapter deals with more recently developed applications to the study of defected and disordered systems. A preliminary account of the interpretation of multicomponent lifetime spectra in terms of varying numbers of distinguishable positron states provides the basis for a discussion of studies of positron trapping by defects, voids, and surfaces in ionic and metallic solids, liquids and powders.