Product energy disposal in simple thermal-energy bimolecular ion–molecule reactions

Abstract

Ion cyclotron resonance ion-trap methods have been used to measure the kinetic energy of products of a number of simple thermal-energy ion–molecule reactions. Two types of reactions are reported: charge transfer and atom transfer. In non-dissociative charge-transfer reactions three possible mechanisms for the electron-transfer step are potentially important: Franck–Condon factors, energy resonance, or intermediate collision complex. Examples are selected that allow tests of each of these mechanisms. Several systems are chosen where data is available from optical probing of the product states. In these cases we demonstrate how the kinetic energy release measurements augment the optical measurements in important ways. For many systems optical studies are impractical. We have selected a number of systems where careful kinetic energy measurements alone can be used to identify product electronic-state distributions and semiquantitative vibrational-state distributions. These include both non-dissociative and dissociative charge-transfer reactions of Ar⁺(²P_3/2) with CS₂ and OCS and atom-transfer reactions of C⁺(²P) with O₂ and OCS. In all cases strongly non-statistical product distributions are obtained. Correlation diagrams are extensively used to help interpret the results.