Excitation cross sections in He+-He collisions

The excitation cross sections have been measured for different energy levels of He I and He II, ion energies from 50 to 2 x 104 (lab) eV. Two types of structures have been recognised for the energy dependence of the cross sections. Profiles of the excitation functions depend systematically on angular momentum and spin multiplicity. The outgoing channels of the Σ state strongly populate the triplet levels of the He atom, and those of the Π state mainly populate the singlet levels. Rotational coupling is important for P-level excitation, as is radial coupling for D-level excitation. The characteristics of the polarisation of emitted light are consistent with assignments for the excitation processes. The two-level model calculations give excitation functions which agree qualitatively with observed ones. (author)