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 CH+2 and CHF+ has been studied at collision energies Ecoll between 0 and kB⋅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 Ecoll/kB=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.ISSN:0031-9007ISSN:1079-711