Two-electron transitions in high energy ion-helium collisions

Abstract

Recent calculations and experiments on two-electron transitions in helium are reviewed. For double ionization and excitation ionization there is a large difference (about a factor of two) between the cross sections produced by protons and matiprotons. This difference is due to interference between first and second Born amplitudes, and correlation is required for first Born to be nonzero. The double ionization to single ionization ratio and the proton/antiproton difference is much larger for probabilities at small impact parameter than for the cross sections. The situation is qualitatively different for double excitation, where experimental and theoretical evidence points to a very small interference term and hence little positive projectile/negative projectile difference.