Physical processes in positron scattering at metal surfaces

Results of a theoretical study of low energy positron diffraction (LEPD) from metal surfaces is presented. The use of LEPD as a complement to LEED for surface structure analysis is suggested. Elastic scattering is analyzed for jellium, jellium plus single core layer, and jellium plus fully periodic core lattice models for the substrate. The role of the electrostatic dipole barrier is particularly brought out and a possible experimental technique for the direct determination of this quantity via LEPD is proposed. This would require an improvement of positron beam angular resolution by roughly an order of magnitude over what is now available. Positron-conduction electron inelastic scattering using the Green's function approach has also been calculated. Plasmon and particle-hole pair excitation were considered and useful results for the mean free path of a positron moving in an electron gas throughout the entire positron energy range were presented. In addition a new general approximate formalism for computing the range of a particle incident upon a homogeneous many-body system is presented and applied to the case of a positron entering an electron gas. Finally, the theory of the recently observed high effciency (approx. 50%) of positronium formation at metal surfaces (pick-up) occurring when a medium energy positron beam (approx. 1 KeV) is made incident upon a metal is investigated. This process is formulated via phenomenological Boltzmann equation approach. Positron diffusion in a metal is studied and it is found that positron diffusion from within the characteristic stopping thickness to the metal surface plays a major role in determining the pick-up fraction. It is noted that the first principles calculations agree with recent experimental data