State-selective x-ray studies of radiative recombination into bare and H-like uranium at threshold energies

A state-selective investigation of radiative recombination (RR) at threshold energies has been performed by x-ray detection at the electron cooler of the experimental storage ring (ESR) at Gesellschaft f\"ur Schwerionenforschung (GSI) for decelerated bare and hydrogenlike uranium ions. The application of the deceleration mode enabled us to observe state-selective RR into the $K$ and $L$ shells of bare and H-like uranium. The observed relative RR intensities into these low-lying bound states exhibit a good agreement with rigorous relativistic predictions, whereas the nonrelativistic approach fails to reproduce the data. Furthermore, the scaled relativistic one-electron treatment seems to be insufficient for the state-selective description of RR into H-like species at extremely low energies, suggesting the importance of electron-electron interactions for the RR process at the threshold.