Translational and electronic energy dependence of S + + H 2(D 2, HD) → SH + (SD +) + H(D): spin-allowed and spin-forbidden pathways

Abstract

Guided ion beam mass spectrometry is used to examine the kinetic energy dependence of the reaction of atomic sulfur ion with molecular hydrogen and its isotopic variants. By varying the conditions for S + formation, it is possible to probe the cross-sections for ground and excited state ions. For excited states of S +, the reaction is efficient and behaves as predicted for an exothermic ion—molecule reaction. For the 4 S ground state, the cross-section shows two distinct features: one which begins at the thermodynamic threshold for the reaction, and a larger one which occurs at higher energies. In reaction with HD, S + ( 4 S) forms equal amounts of SH + and SD + at low energies, but the higher energy pathway yields SD + almost exclusively. These results are interpreted via new calculations of the potential energy surfaces for this system. We suggest that the low energy feature corresponds to a spin-forbidden surface transition, while the high energy feature is a spin-allowed process with a barrier that exceeds the reaction endothermicity by 0.3 eV.