Low-energy electron capture by C⁴⁺ ions from atomic hydrogen

The total-electron-capture cross section for collisions of C⁴⁺ with ground-state hydrogen (deuterium) is measured in the energy range 6–1000 eV/u using the merged-beam technique. The fraction of C⁴⁺ metastable ions present in the ion beam is measured to be 5%, which results in a correction to the cross section of only a few percent. The independently absolute measurements are generally in good agreement with previous measurements for total electron capture; however, the reduced uncertainties of these measurements allow a more detailed comparison with theory. Our observations show that existing fully quantal molecular-orbital calculations that include rotational coupling overestimate the cross section by approximately 25%. Better agreement is found with a recent semiclassical impact-parameter coupled-channel calculation. While all available theoretical data predict that the cross section varies smoothly with energy around the maximum, a sharp structure is observed around 400 eV/u.