Abstract
The bimolecular reactions between CH(3)F and H(2)(+), HD(+) and D(2)(+) have been studied in the range of collision energies between ∼0 and k(B) × 30 K using a merged-beam approach. The ion-molecule reactions were investigated following photoexciting of H(2) (HD, D(2)) to high Rydberg states in a supersonic beam, merging the Rydberg-molecule beam with a cold supersonic beam of CH(3)F using a surface-electrode Rydberg-Stark deflector and monitoring the CH(3)(+), CH(2)F(+) and CH(3)F(+) ions generated by the reactions of H(2)(+) (HD(+), D(2)(+)) with CH(3)F 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 k(B) × 4 K. Branching ratios for the formation of CH(3)(+), CH(2)F(+) and CH(3)F(+) were measured for all three reactions as a function of the collision energy. The branching ratio for the formation of CH(3)(+) was found to decrease with increasing deuteration of the hydrogen molecular ion and to increase at collision energies below k(B) × 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 CH(2)F(+) and a weaker CH(3)F(+) product channel, and short-range complex formation leading predominantly to CH(3)(+) by F(-) transfer, with a weaker contribution of CH(2)F(+) by H(-) transfer. The model calculations highlight the role played by quantum-statistical and stereodynamical effects associated with the J = 1, |K| = 1 ground state of para-CH(3)F and by the reduced mass of the colliding partners: the orientation of CH(3)F molecules induced by the electric field of the ion favours the production of CH(3)(+) 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.