Excited state formation in electron capture by slow multiply charged ions

We have carried out measurements of one-electron capture collisions by He2+ ions in H2O, CO2, CH4, C2H4 and C2H6 and one-electron capture by O6+ ions in H2O, CO2, and CH4 at energies within the range 250 - 2000 eV amu-1. In the case of one-electron capture by He2+ ions in H2O, while non-dissociative capture leading to He+(n = 2) is dominant at the highest energies, dissociative processes involving transfer-ionization leading to He+(n = 1) ions become very substantial at the lower energies. Our measurements for He2+ ions in CO2 and CH4 exhibit a similar behaviour. A rather different pattern is observed in the case of one-electron capture by He2+ ions in C2H4. While non-dissociative capture leading to He+(n = 2) is dominant at 2000 eV/ amu, dissociative capture also leading to He+(n = 2) formation increases with decreasing energy and is dominant at our lowest energy of 215 eV/ amu. Known total one-electron capture cross sections for He2+ ions in H2O and CO2 and values measured in this work for He2+ ions in CH4, have been used to normalise our TES data and thereby derive cross sections for the main excited product channels in these three cases. Our TES studies of one-electron capture by He-like O6+ ions in collisions with the molecules H2O, CO2, CH4 at energies below 2000 eV/amu differ from the corresponding data for He2+ impact. Total capture cross sections for these cases are unavailable. It is found that non-dissociative electron capture channels are dominant with contributions from dissociative capture channels increasing in relative importance as the impact energy increases. In all cases, selective capture takes place into O5+(4d) states through non-dissociative capture and O5+(n = 3) states through dissociative capture. It is interesting to note that, in our earlier TES studies of one-electron capture by O6+ in H2 for energies up to 900 eV/amu, the only significant excitation process observed was non-dissociative capture leading to O5+(n = 4) formation