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

• URL: http://arxiv.org/abs/2112.12056v1
• Date: Wed, 22 Dec 2021 17:32:43 GMT
• Title: Ion-molecule reactions below 1~K: Observation of a strong enhancement of the reaction rate of the ion-dipole reaction He$^+$+ CH$_3$F
• Authors: Valentina Zhelyazkova, Fernanda B. V. Martins, Josef A. Agner, Hansj\"urg Schmutz, and Fr\'ed\'eric Merkt
• Abstract summary: Reaction between He$+$ and CH$_3$F forming predominantly CH$+$ and CHF$+$. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron.
• Score: 55.41644538483948
• License: http://creativecommons.org/licenses/by/4.0/
• 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.

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