Dynamical control of 'statistical' ion-molecule reactions

Experimental and theoretical studies of two ion-molecule reactions are reviewed. The reactions of H2CO+ and C2H2+ with methane are both mediated by long-lived complexes at low collision energies. The complex lifetimes, product recoil energy and angular distributions, and product branching ratios are all in good agreement with predictions based on statistical decay of the intermediate complexes. Nonetheless, it is clear that both reactions are, in fact, controlled by dynamical effects. In particular, reactivity is strongly and mode-specifically dependent on the vibrational state of the reactants, whereas a statistical mechanism would depend only the energy content of the vibrations. The vibrational effects reflect the dynamics involved in the formation and decay of weakly bound precursor complexes, before the collisional interaction can scramble the initial state information.