Single electron capture by state-prepared Ar2+ projectiles in Ar

Electron capture by state-selected Ar2+ projectiles in Ar has been studied at low collision energies (<1 keV) with the aid of the double translational spectroscopy technique. Translational spectra for ground-state ions as well as metastable excited Ar2+ ions are measured, explaining existing discrepancies of partial and total cross sections in the Ar2+/Ar collision system. Although highly excited metastable ions (5D40, 3F40) represent a minor contamination of a non-prepared Ar2+ beam ( approximately 1%), their contributions are found to dominate the capture process due to cross section values larger than 10-15 cm2.