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.
We have developed a novel laboratory instrument for studying gas phase, anion-neutral chemistry. To the best of our knowledge, this is the first such apparatus which uses fast merged beams to investigate anion-neutral chemical reactions. As proof-of-principle we have detected the associative detachment reaction H−+H→H2+e−. Here we describe the apparatus in detail and discuss related technical and experimental issues.
Translational energy spectroscopy has been used to study one-electron capture by O 6+ ions in collisions with H 2 O, CO 2 , and CH 4 , and C 4+ ions in collision with CH 4 within the energy range 200-1500 eV amu −1 . The relative importance of dissociative and nondissociative electron-capture mechanisms has been determined, and the main O 5+ ͑n . l͒ and C 3+ ͑n . l͒ excited product channels have been identified. In all cases, the measurements confirm the highly selective nature of the electron-capture process in spite of the large number of possible nondissociative and dissociative product channels associated with a wide spread in energy defects ⌬E. Collisions of O 6+ in H 2 O, CO 2 , and CH 4 exhibit very similar patterns of behavior with features also observed in previous studies of the simpler O 6+ u H 2 system. Electron capture for C 4+ in CH 4 , while dominated by selective nondissociative C 3+ ͑n =3͒ formation, also includes significant contributions from highly endothermic dissociative capture channels leading to C 3+ ͑n =4͒.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.