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The reaction between He$^+$ and CH$_3$F forming predominantly CH$_2^+$ andCHF$^+$ has been studied at collision energies $E_{\rm coll}$ between 0 and$k_{\rm B}\cdot 10$~K in a merged-beam apparatus. To avoid heating of the ionsby stray electric fields, the reaction was observed within the orbit of ahighly excited Rydberg electron. Supersonic beams of CH$_3$F and He($n$)Rydberg atoms with principal quantum number $n= 30$ and 35 were merged andtheir relative velocity tuned using a Rydberg-Stark decelerator and deflector,allowing an energy resolution of 150 mK. A strong enhancement of the reactionrate was observed below $E_{\rm coll}/k_{\rm B} = 1$~K. The experimentalresults are interpreted with an adiabatic capture model that accounts for thestate-dependent orientation of the polar CH$_3$F molecules by the Stark effectas they approach the He$^+$ ion. The enhancement of the reaction rate at lowcollision energies is primarily attributed to para-CH$_3$F molecules in the$J=1,\,KM=1$ high-field-seeking states, which represent about 8\% of thepopulation at the 6~K rotational temperature of the supersonic beam.

Ion-molecule reactions below 1~K: Observation of a strong enhancement of the reaction rate of the ion-dipole reaction He$^+$+ CH$_3$F