Ion-Molecule Reactions below 1 K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He+ + CH3F

The reaction between He+ and CH3F forming predominantly CH2+ and CHF+ has been studied at collision energies E-coll between 0 and k(B) . 10 K in a merged-beam apparatus. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH3F and He(n) Rydberg atoms with principal quantum number n = 30 and 35 were merged and their relative velocity tuned using a Rydberg-Stark decelerator and deflector, allowing an energy resolution of 150 mK. A strong enhancement of the reaction rate was observed below E-coll/k(B) = 1 K. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar CH3F molecules by the Stark effect as they approach the He ion. The enhancement of the reaction rate at low collision energies is primarily attributed to para-CH3F molecules in the J = 1, KM = 1 high-field-seeking states, which represent about 8% of the population at the 6 K rotational temperature of the supersonic beam.