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

TL;DR

This study investigates ion-molecule reactions at very low temperatures, revealing a significant increase in reaction rates below 1 K due to molecular orientation effects, using advanced merged-beam and Rydberg atom techniques.

Contribution

It provides the first detailed experimental observation of reaction rate enhancement below 1 K for He$^+$ + CH$_3$F, explained by an adiabatic capture model considering molecular Stark effects.

Findings

Reaction rate increases sharply below 1 K collision energy.

Para-CH$_3$F$ in$ J=1, KM=1$ states dominate low-energy reactions.

Experimental data agree with the adiabatic capture model.

Abstract

The reaction between He$^+$ and CH$_3$F forming predominantly CH$_2^+$ and CHF$^+$ 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 ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH$_3$F 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 $E_{\rm coll}/k_{\rm B} = 1$~K. The experimental results are interpreted with an adiabatic capture model that accounts for the state-dependent orientation of the polar CH$_3$F 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-CH$_3$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.