Positron Annihilation in Real Metals. II. Calculation of Core Enhancement Factors

Abstract

A first-principle calculation of core-annihilation enhancement factors in sodium and aluminum is presented. The computations are based on a previously developed theory whose numerical results had formerly been only crudely estimated. The core electrons are treated in the tight-binding approximation using atomic wave functions appropriate to the metallic state. The conduction-band states are taken to be single orthogonalized plane waves. The central core-conduction matrix element of the theory is evaluated with one computational approximation: a simplifying angular average. The enhancement factors obtained have a weak momentum dependence, so the shape of the core contribution calculated without correlations is unaffected. The enhanced contribution of the orthogonalization parts of conduction-electron wave functions remains to be evaluated, so that comparison of our results with experiment is not completely unambiguous. With this reservation, however, the agreement on the whole is satisfactory for both distributions and total lifetimes.