Excitation cross section in He+-He collisions. I. Excitation function and potential curve crossing

Abstract

The authors measured the excitation cross sections of He I 31,3S, 21P, 31,3P, 41,3P, 31,3D, 41,3D, 51,3D and He II 22P and 32P levels at ion energies from 50 to 2*104 (lab) eV. Two types of structures have been recognised on the energy dependence of the cross sections, the origins of which are ascribed to excitation from the incoming channel by radial and rotational couplings through potential degeneracies at small internuclear distances. Profiles of the excitation functions depend systematically on angular momentum and spin multiplicity; the outgoing channels of the Sigma state strongly populate the triplet levels of the He atom, following the statistical weights of the spin multiplicity and those of the Pi 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 our assignments for the excitation processes. The two-level model calculations give excitation functions which agree qualitatively with the observed ones.