Ionization - excitation of helium by fast protons and antiprotons

Abstract

The process of simultaneous ionization and excitation of helium by protons and antiprotons is investigated. Total and single differential cross sections for the ionization - excitation process and ionization - excitation probabilities versus the impact parameter are calculated by perturbation theory through the second order in the interaction potential. In the present calculations only the on-energy shell transitions are taken into account. The helium electron wavefunctions for the initial and final states are determined with allowance for the configuration interaction. The normalization of the helium continuous spectrum wavefunctions is examined. The different helium ionization - excitation mechanisms are analysed. The calculations predicted the exceeding of the total cross section for protons over the antiproton one for all collision energies considered. It is shown that the resonant channel coupling causes the ejected electron energy spectrum to have sharp drops at electron energies (here, n, l and L are the principal and orbital quantum numbers and the orbital angular momentum for the states of the ion lying above the one; is the energy of the state of the ion). The present cross sections are compared with those derived from other theoretical approximations.

Inner-shell excitation of lithium by fast charged projectiles

“…Experimental investigations on the double ionization and the ionization-excitation for helium [6,7] and for the hydrogen molecule [24] showed that cross sections for negative projectiles are typically higher than those for positive projectiles with equal velocity over a wide velocity range. Theoretical calculations, however, suggest that for some two-electron transitions in helium-where no experimental data are available-this should not be always the case [25,26]. It therefore seems that for inner-shell excitations of lithium also, the sign of the interference contribution to the cross section depends on the final excited state.…”

Multi-electron processes in atomic collisions – theory

“…The results of these calculations do not agree with each other, and all underestimate the experimental data [24±28]. Excepting our previous work [22] for the excitation of the 2p state, and Sidorovich [23] for the excitation of the 2s state, nobody has made an attempt to compare theoretical cross sections for same velocity positive and negative projectiles, the other calculations being only for electrons. Experimental data show the same interesting feature as for the double ionization: cross sections for electrons are obtained to be higher than for equivelocity proton projectiles in a wide velocity range.…”

“…For the ionization-excitation of the helium, calculations have been made by Rudge [19], Raeker et al [20], Franz and Altick [21], Nagy et al [22], Sidorovich [23]. The results of these calculations do not agree with each other, and all underestimate the experimental data [24±28].…”

Polarization measurements of the extreme ultraviolet (EUV) emission following excitation and ionization-excitation of helium by fast electron, proton, and molecular hydrogen (H 2 + and H 3 +) impact

“…1-2 These many-body aspects of atomic collision processes are associated with electron-electron correlations and other few-body dynamical phenomena that supercede independent-particle models. Most experiments have focused on two-electron mechanisms in helium, since this is the simplest target containing more than one electron, and it is therefore ideally suited for achieving a better theoretical understanding of the few-body problem [3][4][5][6][7][8][9][10][11] that is likely to be useful for the description of other complex systems, e.g. the dynamic behavior of atoms in molecules.…”