Cold Ion-Molecule Chemistry: The Very Different Reactions of He⁺ with CO and NO

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.