Investigating the gas-phase chemical reactions of molecular dications

Abstract

In the gas phase, the doubly-charged ions of small molecules (molecular dications) exhibit a wide range of reactivity following bimolecular collisions with neutral species. This feature article presents an overview of what is known of this reactivity, and presents a detailed discussion of the dynamics of some of those reactions that involve making and breaking chemical bonds. The article also presents recent results from a new position-sensitive coincidence spectrometer designed to study the dynamics of dication reactions by detecting, in coincidence, the pair of monocations often formed in such processes. Detecting the pair of product ions in coincidence from an individual reactive event, using a time-of-flight mass spectrometer equipped with a position sensitive detector, allows their initial velocity vectors to be determined. Experiments studying the electron transfer reactions of atomic dications illustrate the feasibility of this coincidence technique and scattering diagrams obtained from studies of the Ne²⁺ + Ar collision system are presented. The coincidence technique is particularly suited for the study of reactions which produce three products. For such three-body reactions the coincidence experiment, unlike conventional techniques, allows the initial velocity vectors of all three products to be determined for each reactive event. Illustrative results show that the dissociative electron transfer reaction of CF₃²⁺ with Ar proceeds sequentially via the dissociation of highly excited vibrational levels of the electronic ground state of CF₃⁺. In addition, the formation of OCF⁺ following collisions of CF₂²⁺ with H₂O proceeds via a long-lived collision complex which decays via charge-separation to form a proton and HF-OCF⁺. The molecular monocation then dissociates to yield OCF⁺ and HF.