Dissociation Lifetimes of Molecular Ions Produced by Charge Exchange

Abstract

Ions of the benzonitrile, benzene and thiophene molecules were produced in well-defined states of excitation by charge exchange with Xe⁺, Kr⁺, Ar⁺, CO⁺, and N₂⁺. Their subsequent unimolecular decomposition was followed, as a function of time, by allowing the dissociations to take place within a strong homogeneous draw-out field and measuring the kinetic energy of the product ions. The decompositions were found to be purely exponential within experimental error, and the corresponding decay rate constants k (for 2 · 10⁵ sec^-1 ≦ k ≦ 5 · 10⁸ sec^-1) proved to be monotonically increasing functions of the excitation energy. These are the first unambiguous measurements of this function k(E) for any molecule. In the case of benzene, the reactions C₆H₆⁺ → C₆H₅⁺ + H and C₆H₈⁺ → C₄H₄⁺ + C₂H₂ were found to be definitely not in competition with one another. Furthermore the dependence of the excitation energy on the impact energy was measured. A variation of the impact energy between 10 and 200 eV_c.m. only changes the internal energy E by about 0.1 eV. This value appears to be in qualitative agreement with calculations on near-resonant charge transfer by Gurnee and Magee.