- Rare Gas Collisions

We have modeled the collision of lithium dimer with the rare gases Ne and Xe at collision speeds as low as 30 cm/s using classical mechanics on ab initio potentials. In these very low energy collisions, the attractive part of the potential leads to Langevin capture, with a cross section that rises with declining collision speed according to σ_Langevin ∼ v_rel^-2/3.

The total inelastic cross section also varies according to σ_inelastic ∼ v_rel^-2/3 at low collision speeds. The cross section ratio σ_inelastic /σ_Langevin is strongly modulated by the initital rotation action j_i and can vary from 0.2 to nearly 0.8. This implies that energy is not simply distributed randomly as a result of trapping behind the angular momentum barrier. We seek to explain this modulation in terms of the trapping time and other characteristics of the dynamics.

The results are interesting in themselves for the classical dynamics that they reveal. If they persist in our quantum calculations, then we can expect them to be experimentally observable, implying that the amount of energy transfer in ultracold collisions could be controlled by adjusting the initial rotational quantum number of the molecule.

We acknowledge with gratitude our use of the unpublished Li_2-Xe potential function of Kirk Peterson.