The interaction of positrons and positronium with small atoms

Abstract

This is a report on recent progress in the theory of small atomic systems including a positron. I have selected four main topics that seem interesting, instead of trying to cover the entire extensive subject. First, I consider the two-channel region in e+–H scattering which lies between 6.8 and 10.2 eV, and have reported the results of positronium-formation and resonant-scattering calculations. Second, I introduce the concept of "iso-leptonic families" with fixed electron and positron numbers and discuss in detail the two-electron, one-positron family. This family includes several interesting bound and resonant states. Next, I derive a new method of calculating positron annihilation rates and cross-sections from approximate wave functions. Although more difficult to apply than the direct method, this technique is more accurate. Finally, I show the applicability of many positron calculations to currently interesting astrophysical problems, especially the recent observations of annihilation radiation coming from the direction of the galactic center.