Positron Annihilation in Liquid and Solid Mercury

Abstract

Results are presented of a study of the angular correlation of photons from the annihilation of positrons with electrons in solid and liquid mercury. The angular distribution of photon coincidences can be separated into contributions arising from annihilations with the ionic-core electrons and with the conduction electrons. The angular variation of the former does not appear to change at the solid-liquid transition, but the distribution for the conduction electrons is considerably modified. The plot of the number of coincidences against angle for the conduction electrons in the solid can be fitted very well by a parabola corresponding to two free electrons per atom, which indicates that the Fermi surface in extended $k$ space does not depart very significantly from a sphere. The relative number of annihilations from the conduction electrons in the liquid is considerably greater and the distribution departs from the free electron parabola at large angles. These effects are interpreted in terms of the distortion of the wave functions and the broadening of the electronic energy levels by the disorder in the liquid. It is concluded that the uncertainty in the wave vector of an electron at the Fermi surface in the liquid is about 20% of the Fermi wave vector.