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⁺(1s²2s²) ¹S and pure metastable C²⁺(1s²2s2p) 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 ions 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 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.