Estimating positronium formation for plasma applications

Abstract

The basic physics of ${e}^{+},{e}^{\ensuremath{-}}$ creation and annihilation is overviewed. It is shown that for atomic hydrogen targets electron capture by a free positron to form positronium is vastly more probable than in-flight annihilation. Cross sections are presented using the classical trajectory Monte Carlo (CTMC) approach for the charge-exchange process:${e}^{+}+{A}^{q+}\ensuremath{\rightarrow}Ps+{A}^{(q+1)+},$where ${A}^{q+}$ is some target ion of charge $q$. Charge-exchange cross sections for hydrogenic ion targets are presented. It is found that while the CTMC gives adequate results for positronium formation for ${e}^{+}$-hydrogen and ${e}^{+}$-cesium collisions, its high-energy behavior for hydrogenic ions is not in agreement with quantum mechanical predictions. Since we are interested in situations where many multicharged ions will be present we have looked for an alternative approach. Scaling rules are proposed and used to estimate the charge-exchange cross sections for both neutral atoms and multicharged ions.