Dissociative charge-transfer reactions of acetylene part 2 Xe +/C 2H 2, Kr +/C 2H 2, and Ne +/C 2H 2

Abstract

The three dissociative charge-transfer reactions, Xe +/C 2H 2, Kr +/C 2H 2, and Ne +/C 2H 2, have been investigated over the reactant-ion energy range 100–2000 eV, using the same techniques as described previously for the Ar +/C 2H 2 reaction [1]. In contrast to the Ar +/C 2H 2 reaction, these reactions are dominated by momentum-transfer collisions, although long-range charge-transfer products were observed in the Xe +/C 2H 2, and Ne +/C 2H 2 systems. It is argued that the possibility of long-range interaction for these systems is severely restricted by Franck-Condon-like considerations, and that even if a long-range charge transfer were possible, the symmetry of the molecular energy states involved could restrict the chances of a successful long-range collision. Evidence was found for the possible involvement of autoionising levels of C 2H 2 in the long-range interaction between Xe + and C 2H 2. An attempt has been made to assess the dissociation pathways of the excited C 2H 2 + ion in the different systems by examination and comparison of translational-energy (TE) curves and delay curves. It is possible that induced polarisation of the target C 2H 2 molecule occurs and increases as the collision energy decreases towards 100 eV. This could account for some features displayed by the fragment-ion TE curves at the lower end of the energy scale. The possible excited states of C 2H 2 +, formed in the initial charge transfer prior to fragmentation, are discussed for each system. There is evidence that the C 2 + product ion in the Ne +/C 2H 2 reaction is formed from C 2H + in a consecutive mechanism occurring over a time period of ca. 1 μs.