Direct Excitation in Heavy-Atom Collisions: A Propensity Rule for Charge Cloud Orientation

The Massey criterion prescribes maximum electronic excitation of atoms in heavy-particle collisions for collision velocities v , where Δε a ℏ v ≃ π. Here Δε is the energy defect and a is the effective interaction length. Experiments with planar symmetry have revealed a preferred way of rotation of the excited charge cloud in this velocity region. We demonstrate by analysis of a simple, yet realistic model why excitation favours states with a specific orientation. A general propensity rule is derived and its validity evaluated for a specific case, the Na-He system. Implications for future experiments are pointed out. In particular, the propensity role predicts very different collision behaviours of oppositely oriented atoms, as prepared, e.g. , by circular polarized laser light.