Interactions of molecular doubly charged ions with atoms, molecules and photons

Abstract

Gas-phase molecular dipositive ions (molecular dications) are highly unusual and reactive species. The majority of dicationic electronic states rapidly fragment to form a pair of singly charged ions. However, a large number of small molecular dications possess at least one electronic state which is long-lived owing to a barrier in the charge-separating pathway. Such metastable states, which are usually thermodynamically unstable with respect to the charge-separated state, live long enough to allow the study of their interaction with atoms, molecules and photons. It is the results of such experiments that are presented here. Electron-transfer reactions dominate dicationic reactivity with atomic collision partners. The considerable state-selectivity observed in these reactions can be rationalised using a Landau–Zener model of the electron-transfer process. Neutral-loss reactions are also observed following the collisions of polyatomic perfluorinated dications (e.g. SiF ₃ ²⁺ ) with the rare gases. The form of this neutral-loss reactivity provides a strong clue concerning the structure of these perfluorinated dications. Such deductions are supported by recently performed quantum chemical calculations. With molecular collision partners, bond-forming chemical reactivity is observed. In the case of diatomic neutral molecules, the reactivity appears to involve effective negative ion transfer. Recent isotopic studies are consistent with the conclusion that the prototypical negative ion transfer reaction between CF ₂ ²⁺ and H ₂ to give HCF ₂ ⁺ , proceeds via a direct mechanism. With simple polyatomic molecules, such as NH ₃ , recent experiments have shown that molecular dications exhibit complex chemical reactivity. Perfluorinated dications, such as SiF ₃ ²⁺ , readily absorb visible laser radiation and subsequently undergo neutral-loss and charge-separating relaxation. These reactions appear consistent with the models of dicationic electronic structure that have been developed to explain the reactions of these perfluorinated dications following collisional excitation.