State-selective one-electron capture by 4 keV ground state and metastable C²⁺ ions in collisions with H₂, N₂ and O₂

The technique of double translational energy spectroscopy (DTES) has been used in a recently developed apparatus to study (for the first time) one-electron capture by state-prepared C²⁺ ions in collisions with H₂, N₂ and O₂ molecules. These processes are relevant to astrophysical situations, planetary atmospheres and to the modelling and diagnostics of edge plasmas in current fusion devices. The main collision channels resulting from state-selective electron capture by pure beams of 4 keV ¹S ground state and ³P metastable C²⁺ ions have been separately identified and their relative importance determined without any of the ambiguities inherent in TES studies normally carried out with ion beams of unknown metastable content. At the energy considered, only non-dissociative electron capture is observed with collision channels involving capture into the C⁺ (2s2p²)²S, ²D, ⁴P and C⁺ (2s²2p)²P states together with the formation of H⁺₂, N⁺₂ and O⁺₂ in ground and electronically excited states. Our measurements indicate that some of these molecular ions have significant degrees of vibrational excitation in accord with the Franck-Condon factors for the relevant transitions. Striking differences between the collision channels for ¹S ground and ³P metastable ions are observed.