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

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.