Fine-structure transitions in Li(2p)-He collisions

Cross sections for intermultiplet transitions in Li(2p 2 P)-He(1 1 S) collisions are calculated for impact energies E between 6*10 -5 eV and 6*10 -2 eV by a close-coupled wave treatment. A special study is made of the onset of spin uncoupling as E is increased. The total cross sections indicate that the spin-uncoupled limit (in which the spin of the valence electron is unaffected by the collision) is reached at 3*10 -4 eV. However partial-wave analysis indicates that there are significant departures from the limit up to 3*10 -3 eV. A correlation between peaks in the calculated cross sections and resonances in elastic scattering by the 2 Pi molecular potential is studied, and is interpreted as showing the essentially molecular nature of the collision.