Production of He⁺(2p) in collisions of He with energetic H⁺, H₂⁺, H₃⁺ions and electrons

Abstract

Total cross sections for the production of He+(2p) in collisions of H+ and e- with He at energies from 75 to 800 keV and from 0.8 to 7 keV respectively were measured. The ratio of the cross sections for He+(2p) production and for single ionization is discussed to investigate the contribution of electron correlations in the two-electron transition leading to He+(2p). For the first time the impact of H2+ and H3+ ions in comparison with H+ impact has also been studied for the energy range mentioned above.

Second-order effects in (e, 2e) excitation-ionization of helium to (n= 2)

“…Measurements of excitation-ionization to He + (2p) did not therefore begin until the 1960s (Moustafa Moussa and de Heer 1967). In the intervening 30 years, and particularly since 1990, there has been a substantial accumulation of data on this process (Dashchenko et al 1975, Bloemen et al 1981, Semenyuk et al 1983, Forand et al 1985, Pedersen and Folkmann 1990, Schartner et al 1991, Fülling et al 1991, 1992, Bailey et al 1995, Hayes and Williams 1996. These data show a systematic deviation from the most sophisticated theoretical calculations presently available (Rudge 1988, Raeker et al 1994, Franz and Altick 1993 which are all based on first-order perturbation theory.…”

“…Assuming that the experiments are correct, there is obviously a clear need to proceed beyond first-order theory. The importance of higher-order terms in the perturbation series is also emphasized by comparing electron collisions with proton ‡ collisions at the same velocity (Pedersen and Folkmann 1990, Schartner et al 1991, Fülling et al 1991, 1992, Amusia et al 1993, Bruch et al 1993a, b, Bailey et al 1995 where, at sufficiently high impact energy, the two cross sections should be equal when the first-order approximation becomes valid. That the firstorder approximation is correct asymptotically, there is little doubt, but the question is 'how high is asymptopia?…”