A study of double excitation in He+-He collisions

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Abstract
The authors have studied the double excitation of He in He+-He collisions at collision energies between 1.2 and 4 keV and at laboratory scattering angles between 6 degrees and 10 degrees . Under these conditions the (2p2)1D state is excited for more than 95%. The electrons emitted by the doubly excited He after the collision are measured in coincidence with the scattered ions. Both electrons and scattered ions are selected with respect to energy and angle, but the electron intensity is integrated over a certain energy range in order to detect all electrons due to decay of the (2p2)1D state, for emission by both the slow target atom or-after charge exchange-the fast projectile. From the angular distribution for a well-defined 'collision event' the sublevel population amplitudes of the 1D state are obtained. Amplitudes are presented for eight different such 'events'. This 'complete' information on the respective 'events' is analysed in terms of coupling mechanisms operating during the course of the collision. It is shown that, for all eight cases, an independent electron two-step rotational 2p sigma u=2p pi u coupling model can reproduce the main features of the observed 1D distributions. Finer details are interpreted in terms of an asymptotic Sigma - Pi coupling at large distances between the collision partners, and in terms of an additional 'inner' Sigma - Pi coupling which becomes important in the more violent events.