Radiative recombination with highly charged Si⁶⁺and Si¹¹⁺ions

The rate coefficient ( nu sigma ) for radiative recombination between free electrons and highly charged Si 6+ and Si 11+ ions has been measured in the energy region from 0 to 1 eV. The experimental results were obtained with a merged-beams technique. Knowledge about the relative velocity distribution allowed a direct comparison with theory. It was found that for Li-like Si 11+ , the effective charge to be used in a calculation developed for bare ions is the total ion charge, Z eff approximately=11. This is in agreement with earlier findings for H-like ions and other Li-like ions. For O-like Si 6+ , the effective charge is found to be Z eff approximately=9. For Si 6+ , the measured rate coefficient is more than a factor of two larger than calculations based on the total ion charge (Z ef f=6). This shows evidence for significant incomplete screening in the case of Si 6+ .