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The ion-molecule reactions He+ + CO → He + C+ + O and He+ + NO → He + N+ + O have been measured at collision energies between 0 and kB · 10 K. Strong variations of the rate coefficients are observed below kB · 5 K. The rate of the He+ + CO reaction decreases by ~30% whereas that of the He+ + NO reaction increases by a factor of ~1.5. These observations are interpreted in the realm of an adiabatic-channel capture model as arising from interactions between the ion charge and the dipole and quadrupole moments of CO and NO. We show that the different low-energy behavior of these reactions originates from the closed- vs. open-shell electronic structures of CO and NO., (Copyright 2023 Fernanda B. V. Martins, Valentina Zhelyazkova, Andreas Osterwalder, Frédéric Merkt. License: This work is licensed under a Creative Commons Attribution 4.0 International License.)

We review the results of our recent experimental and theoretical studies of gas-phase ion-molecule reactions involving the He+ ion at low collision energies (Ecoll), in the kB⋅(0 - 40) K range. To avoid heating of the ions by stray electric fields, the reactions are studied within the orbit of a Rydberg electron. We reach collision energies down to ~0 K by employing a merged-beam setup. In the case of a molecule with a dipole moment (e.g., ammonia), we observe a strong enhancement of the measured reaction yield with decreasing Ecoll. This enhancement is attributed to rotational states which experience linear negative Stark shifts in the electric field of the ion. When the molecule has no dipole moment but a negative quadrupole moment (e.g., N2), we observe a suppression of the total reaction yield at the lowest collision energy. Our results are interpreted with the aid of an adiabatic-channel model., (Copyright 2022 Valentina Zhelyazkova. License: This work is licensed under a Creative Commons Attribution 4.0 International License.)