Rotational Quenching in Ionic Systems at Ultracold Temperatures

The behavior of the rotational quenching for a molecular ion in collision with closed-shell neutral gases is investigated. We confirm that Wigner's threshold law for inelastic scattering holds in the presence of a long-range interaction due to polarization forces decreasing as the inverse fourth power of the distance but find that, because of the contributions of the higher angular momenta, its range of applicability is markedly reduced when compared to the scattering by neutral species. The calculations of the quenching cross sections make evident the special features of ionic systems at ultralow collision energies and yield rate coefficients of the order of ${10}^{-9}{\mathrm{c}\mathrm{m}}^3{\mathrm{s}}^{-1}$, much larger than those found for the quenching of neutral molecules.