Two-dimensional angular correlation of annihilation radiation study of positron interactions with surfaces of aluminum

Studies were made of the interactions of positrons with three low index surfaces of aluminum [(100), (110), and (111)] at room temperature with the aluminum surface at elevated temperatures and with oxygen adsorption on the aluminum surface. Our measurements using the improved two-dimensional angular correlation of annihilation radiation technique on surfaces support three established processes: spontaneous positronium (Ps) formation and emission, positrons bound in a surface state, and surface-state positrons thermally desorbed as Ps atoms. A method has been developed to accurately separate these components. The positron surface-state annihilation spectra are nearly isotropic for all three Al surfaces, which indicates a lateral localization and is inconsistent with either the ideal image-potential-induced bound state or the physisorbed Ps state. The thermal Ps momentum distributions agree well with the thermostatistical emission mechanism. The momentum distributions of the directly emitted Ps atoms are found to be very sensitive to the surface conditions, and to reflect the electron density of states near the surfaces, thus suggesting a new surface spectroscopy, angle-resolved Ps spectroscopy.