Theoretical study of two-electron excitation in He2++He collisions

He 2+ +He collisions are studied at intermediate energies (0.3-30 keV) using the impact-parameter method. Two-electron excitation of quasimolecular states formed during the collision is investigated on the basis of the potential energy curves and coupling matrix elements calculated previously by the ab initio configuration interaction method. The initial channel considered is a doubly excited state (2p sigma u 2 ) 1 Sigma g + , which is correlated diabatically to the ground state of He 2+ +He in the separated-atom limit. In all, five quasimolecular states are taken into account; they are rotationally or radially coupled to each other. Total as well as differential cross sections are calculated. The travelling factor is omitted and the effect of the molecular autoionisation is neglected. Rotational coupling is confirmed to be dominant for most of the transitions. The excitation of the (2p pi u 2 ) 1 Sigma g + state is an exception and occurs mainly through the radial coupling to the initial state. The excitation cross section of the (2p pi u 2 ) 1 Delta g state, which is excited only by a two-step ( Sigma - Pi - Delta ) rotational coupling, is about 0.2 a 0 2 at a collision energy, E, of about 3 keV.