Two-electron capture by C3+ ions: Competition between single and double electron processes

State-selective translational energy spectroscopy has been used to study two-electron capture processes for 6 keV C3+ ions in collision with He, Ne and Ar atoms. The spectrum for the formation of C+ from the He target reveals only features resulting from double-collision single-electron capture processes while the spectrum for C+ from the Ne target reveals both single-collision double-electron capture and double-collision successive single-electron capture processes. The spectrum for the Ar target consists entirely of double-electron capture channels in which the target product Ar2+ ion is left in the ground state with some indication of double capture into autoionizing states of the C+ ion.