Reaction of an Ion and a Free Radical near 0 K: He+ + NO → He + N+ + O
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2023-02-16
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Journal Article
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Abstract
The reactions between ions and free radicals are among the fastest chemical reactions. They are predicted to proceed with large rates, even near 0 K, but so far, this prediction has not been verified experimentally. We report on measurements of the rate coefficient of the reaction between the ion He+ and the free radical NO at collision energies in the range between 0 and ∼ kB·10 K. To avoid heating of the ions by stray electric fields, the reaction is observed within the large orbit of a Rydberg electron of principal quantum number n ≥ 30, which shields the ion from external electric fields without affecting the reaction. Low collision energies are reached by merging a supersonic beam of He Rydberg atoms with a supersonic beam of NO molecules and adjusting their relative velocity using a chip-based Rydberg-Stark decelerator and deflector. We observe a strong enhancement of the reaction rate at collision energies below ∼kB·2 K. This enhancement is interpreted on the basis of adiabatic-channel capture-rate calculations as arising from the near-degenerate rotational levels of opposite parity resulting from the Λ-doubling in the X 2Π1/2 ground state of NO. With these new results, we examine the reliability of broadly used approximate analytic expressions for the thermal rate constants of ion-molecule reactions at low temperatures.
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127 (6)
Pages / Article No.
1458 - 1468
Publisher
American Chemical Society
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03449 - Merkt, Frédéric / Merkt, Frédéric
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200478 - Precision measurements with cold molecules: Rydberg states, ions and photoionization (SNF)
743121 - Cold Ion Chemistry - Experiments within a Rydberg Orbit (EC)
743121 - Cold Ion Chemistry - Experiments within a Rydberg Orbit (EC)