State-selective one-electron capture from He, Ne and Ar by 4 keV state-prepared C2+ ions

A new instrument to carry out double-translational energy spectroscopy (DTES) recently developed in this laboratory has been described in detail. This permits studies of state-selective electron capture by slow state-prepared multiply charged ions using translational energy spectroscopy (TES). The effectiveness and future potential of this approach has been illustrated by studies of one-electron capture in helium, neon and argon by 4 keV beams of pure ground-state C2+ (1s2 2s2) 1S and pure metastable C2+ (1s2 2s2p)3P ions. In each case the main product channels have been identified by TES and analysed quantitatively without the ambiguities associated with previous measurements which used primary ion beams containing a mixture of ground and metastable species. A simple beam attenuation technique has also been used in conjunction with our state-prepared beams to provide estimates of the ratios σ(3P)/σ(1S) of the total one-electron capture cross sections which, in each case, show that σ(3P) is considerably larger than σ(1S). These ratios have in turn been used to show that the metastable content of C2+ ions from the present ECR ion source and sources used in other laboratories can be substantial. (author)