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Experimental and theoretical studies of one-electron capture in collisions of He 2+ ions with H 2 O molecules have been carried out in the range 0.025-12 keV amu −1 corresponding to typical solar wind velocities of 70-1523 km s −1 . Translational energy spectroscopy ͑TES͒, photon emission spectroscopy ͑PES͒, and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n = 1, 2, and 3 states of the He + ion were obtained using TES while PES provided cross sections for capture into the He + ͑2p͒ and He + ͑3p͒ states. Our model calculations show that He + ͑n =2͒ and He + ͑n =3͒ formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He + ͑1s͒ formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H 2 O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

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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 and pure metastable 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 of the total one-electron capture cross sections which, in each case, show that is considerably larger than . These ratios have in turn been used to show that the metastable content of ions from the present ECR ion source and sources used in other laboratories can be substantial.