Calculations of the angular correlation of positron annihilation at ideal and defected aluminium surfaces

Abstract

Previous calculations of the angular correlation of annihilation radiation (ACAR) spectra for positron surface states on aluminium have resulted in varying degrees of anisotropy with respect to momentum components parallel and perpendicular to the surface. However, all experiments to date have yielded almost perfect isotropy. The present calculations show that the ACAR spectrum of a positron localised in a surface monovacancy is nearly isotropic, providing some evidence that this defect-trapping state may be responsible for the observations.