High ionization probabilities in 30-, 100-, and 300-keV proton-helium collisions

Absolute doubly differential electron emission yields were measured as a function of the projectile scattering angle for proton-helium collisions at incident energies of 30, 100, and 300 keV. Coincidence spectra were taken for electron emission angles of 40^∘{}, 60^∘{}, and 90^∘{} at electron energies of 10--500 eV. All investigations were performed at impact parameters small compared to the He 1s²-orbital radius. The experimental data are compared to results of different theoretical models. Large deviations are found especially between the measured values and values calculated by methods based on first-order perturbation theory. These deviations are assigned to polarization effects exceeding pure binding corrections.