Cold and ultracold ion-neutral inelastic collisions: Spin–orbit relaxation in He+Ne+

Electronic close-coupling scattering calculations are reported on the relaxation process Ne+(2p5 2Pj1=1/2,m1)+He→Ne+(2p5 2Pj2=3/2,m2)+He using a new interaction potential derived by combining spectroscopy and ab initio theory. Collision energies range from E/kB=300 K to less than 1 μK. As E is reduced, the total inelastic cross section σj1→j2=σ1/2→3/2 passes through a resonance-dominated regime and apparently approaches the Wigner limit below 10 μK, where the cross section is found to diverge in accord with the predicted E−1/2 dependence. A complex scattering length a=−53.0−0.04145i Å is derived from the low-energy behavior. m-dependent alignment and orientation cross sections, σj1|m1|→j2|m2| and σj1m1→j2m2, undergo rapid changes near resonances, and approach limiting behavior below 10 μK as well, while their ratios become asymptotically constant below 100 μK. Analysis of the scattering amplitude in the s-wave limit shows that the latter behavior is expected in general, with the ratios depending only on the geometry of angular momentum coupling. This geometrical effect disentangles s-wave dominance from the Wigner regime, and provides a new criterion for assessing the approach to the ultracold limit. Boltzmann averaging to simulate ultracold trapping conditions shows that a remnant of the resonance region and the characteristic limiting behavior remain observable.