Bimolecular reactions of S 2+ with Ar, H 2 and N 2 : reactivity and dynamics.

The reactivity, energetics and dynamics of bimolecular reactions between S 2+ and three neutral species (Ar, H 2 and N 2 ) have been studied using a position-sensitive coincidence methodology at centre-of-mass collision energies below 6 eV. This is the first study of bimolecular reactions involving S 2+ , a species detected in planetary ionospheres, the interstellar medium, and in anthropogenic manufacturing processes. The reactant dication beam employed consists predominantly of S 2+ in the ground 3 P state, but some excited states are also present. Most of the observed reactions involve the ground state of S 2+ , but the dissociative electron transfer reactions appear to exclusively involve excited states of this atomic dication. We observe exclusively single electron-transfer between S 2+ and Ar, a process which exhibits strong forward scatting typical of the Landau-Zener style dynamics observed for other dicationic electron transfer reactions. Following collisions between S 2+ + H 2 , non-dissociative and dissociative single electron-transfer reactions were detected. The dynamics here show evidence for the formation of a long-lived collision complex, [SH 2 ] 2+ , in the dissociative single electron-transfer channel. The formation of SH + was not observed. In contrast, the collisions of S 2+ + N 2 result in the formation of SN + + N + in addition to the products of single electron-transfer reactions.