Inverse photoionization studied via radiative electron capture into highly charged ions

Abstract

Differential aspects of the photoelectric effect for high-Z hydrogen-like and few-electron ions are studied by the time-reversed process in ion-atom collisions, i.e. by radiative capture of a quasi-free target electron. In this time-reversed situation the capture of an electron into a bound state of the ion is accompanied by the simultaneous emission of a photon, allowing for a direct and unambiguous experimental access to this process. Therefore, radiative capture provides a unique tool to investigate the details of the photoionization process even in the case of high-Z highly charged ions where such investigations are currently not accessible in the direct channel. At high-Z relativistic effects become important for which photon angular distributions turn out to be a very sensitive probe. This was demonstrated very recently, by an angular distribution study performed for electron capture into the 1s ground state of U92+. This particular experiment allowed us to identify spin-flip contributions to the photoionization process at large backward angles. Here, we review the shell and subshell differential photon-angular distribution studies for radiative capture into highly-charged uranium ions. The experimental data are compared with exact relativistic calculations giving detailed insight into the fundamental electron-photon interaction process involved.