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

Electron capture by state-selected Ar 2+ 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 Ar 2+ ions are measured, explaining existing discrepancies of partial and total cross sections in the Ar 2+ /Ar collision system. Although highly excited metastable ions ( 5 D 4 0 , 3 F 4 0 ) represent a minor contamination of a non-prepared Ar 2+ beam ( approximately 1%), their contributions are found to dominate the capture process due to cross section values larger than 10 -15 cm 2 .