The single electron transfer reactions between 13CO22+ and 12CO2 and between 18O22+ and 16O2 have been studied, using a position-sensitive coincidence technique, to test recently proposed explanations for the preferential dissociation of the 13CO2+ ion (the capture monocation) formed following electron transfer to 13CO22+. In our studies of the carbon dioxide collision system, in agreement with previous work, the capture monocation shows a greater propensity to dissociate than the monocation formed from the neutral, 12CO2+ (the ejection monocation). The coincidence data clearly show that the dissociation pathways of the 13CO2+ and 12CO2+ ions are different and are consistent with the ejection monocation dissociating via population of the C2Σ+g state, whilst the capture ion is predominantly directly formed in dissociative quartet states. This state assignment is in accord with an expected preference for one-electron transitions in the electron transfer process. A propensity for one-electron transitions also rationalizes our observation that following dissociative single electron transfer between 18O22+ and 16O2 the ejection monocation (16O2+) preferentially dissociates; the opposite situation to that observed for carbon dioxide. The coincidence results for this reaction indicate the 16O2+ dissociation results from population of the B(2Σ−g) state. The less favoured dissociation of the capture monocation clearly involves population of a different electronic state(s) to those populated in the ejection ion. Indeed, the experimental data are consistent with the dissociation of the capture monocation via predissociated levels of the b(4Σ−g) state. Since the population of the B(2Σ−g) state from the neutral O2 molecule involves a one-electron transition, and the population of the valence dissociative states of O2+ from the dication are multi-electron processes, the preferential dissociation of the ejection monocation in this collision system can be rationalized by the same principles used to explain the electron transfer reactivity of CO22+ with CO2.
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