Dissociation Lifetimes of Molecular Ions Produced by Charge Exchange

Abstract

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 eVc.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.