Electron correlation and the cross sections for electron capture from He: The impulse approximation

Electron-capture cross sections have been evaluated for high-velocity protons impinging on a ground-state helium target. The scattering matrix element was formulated within the quantum-mechanical impulse approximation, and the resulting cross sections were examined as a function of various levels of sophistication for the description of the target atom. The helium wave functions used here were of a simple one-parameter (lambda) variational form, a Hartree--Fock (HF) equivalent function and, finally, a configuration-interaction (CI) description. The projectile energy ranged from 25 keV to 3.5 MeV and, where possible, the cross sections were compared with experiment