Competition between electron transfer and reactive capture in ion-molecule reactions at low collision energies: isotopic and stereodynamic effects in the reactions of CHF with H, HD and D.

The bimolecular reactions between CHF and H, HD and D have been studied in the range of collision energies between ∼0 and × 30 K using a merged-beam approach. The ion-molecule reactions were investigated following photoexciting of H (HD, D) to high Rydberg states in a supersonic beam, merging the Rydberg-molecule beam with a cold supersonic beam of CHF using a surface-electrode Rydberg-Stark deflector and monitoring the CH, CHF and CHF ions generated by the reactions of H (HD, D) with CHF within the distant orbit of the Rydberg electron. In all three reaction systems, a strong increase of the rate coefficients was observed at collision energies below × 4 K. Branching ratios for the formation of CH, CHF and CHF were measured for all three reactions as a function of the collision energy. The branching ratio for the formation of CH was found to decrease with increasing deuteration of the hydrogen molecular ion and to increase at collision energies below × 4 K. The experimental results were interpreted using model calculations based on a rotationally adiabatic capture model as well as using classical trajectory simulations. The reaction products are shown to be generated in two distinct mechanisms: electron transfer leading to a dominant CHF and a weaker CHF product channel, and short-range complex formation leading predominantly to CH by F transfer, with a weaker contribution of CHF by H transfer. The model calculations highlight the role played by quantum-statistical and stereodynamical effects associated with the = 1, || = 1 ground state of -CHF and by the reduced mass of the colliding partners: the orientation of CHF molecules induced by the electric field of the ion favours the production of CH by F transfer at low collision energies and the slower approach of the reaction partners with increasing reduced mass favours electron transfer at intermediate distances.