Coherence study of S → D excitations: Li(2s → 3d) excitation in Li-He collisions

Collision-induced coherent S→D excitation is investigated. A polarisation analysis of the subsequent D→P transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Indeed, two distinct solutions are obtained. The electron distributions associated with these two are mirror images of one another in the plane determined by the normal to the scattering plane and the direction of maximum intensity of linearly polarised light, as measured from the direction of the normal. The Li(2s→3d) excitation induced by collisions with helium is studied both theoretically and experimentally in the keV energy range. The measurements confirm the existence of a theoretically predicted low-energy structure in the circular polarisation. The results are interpreted by generalising concepts previously developed for S→P excitation in quasi-one-electron systems. (author)