The Effect of the Molecular Dipole and Quadrupole moments on Ion-Molecule Reaction Rates near 0 K

Abstract

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 (E_coll), in the k_B⋅(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 E_coll. 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.