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

Abstract

The reaction between He${}^{+}$ and CH${}_3$F forming predominantly CH${}_2^{+}$ and CHF${}^{+}$ has been studied at collision energies $E_{\mathrm{coll}}$ between 0 and $k_{\mathrm{B}}\cdot 10$~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 $E_{\mathrm{coll}}/k_{\mathrm{B}}=1$~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.

References in corpus (5)

• Observation of Quantum Effects in sub Kelvin Cold Reactions
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Cited by in corpus (3)

• Quantum state resolved molecular dipolar collisions over four decades of energy
• Multipole-moment effects in ion-molecule reactions at low temperatures: part I -- Ion-dipole enhancement of the rate coefficients of the He${}^{+}$ + NH${}_3$ and He${}^{+}$ + ND${}_3$ reactions at collision energies near $0$ K
• Observation of quantum capture in an ion-molecule reaction