Ion-Molecule Reactions below 1 K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He + + CH 3 F

The reaction between ${\mathrm{He}}^{+}$ and ${\mathrm{CH}}_{3}\mathrm{F}$ forming predominantly ${\mathrm{CH}}_{2}^{+}$ and ${\mathrm{CHF}}^{+}$ has been studied at collision energies ${E}_{\mathrm{coll}}$ between 0 and ${k}_{B}\ifmmode\cdot\else\textperiodcentered\fi{}10\text{ }\text{ }\mathrm{K}$ 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 ${\mathrm{CH}}_{3}\mathrm{F}$ and $\mathrm{He}(n)$ 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 ${E}_{\mathrm{coll}}/{k}_{B}=1\text{ }\text{ }\mathrm{K}$. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar ${\mathrm{CH}}_{3}\mathrm{F}$ molecules by the Stark effect as they approach the ${\mathrm{He}}^{+}$ ion. The enhancement of the reaction rate at low collision energies is primarily attributed to para-${\mathrm{CH}}_{3}\mathrm{F}$ 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.