Third-order pseudostate calculation of excitation–ionization by protons and antiprotons

Techniques the authors developed to include correlation in the double-excitation of atoms are applied to a second two-electron process, excitation-ionization by proton versus antiproton impact. The second-order cross section, in the closure approximation and using pseudostates for the unbound electron, reproduces only a small percentage of the experimental factor of 2 difference for positively versus negatively charged projectiles. Inclusion of a portion of the third-order term yields a significant contribution near 0.5 MeV.