Two-dimensional electron-momentum distributions for transfer ionization in fast proton-helium collisions

The momentum distribution of the electron in the reaction p + He -> H + He2+ + e is measured for projectile energies E-p = 300 and 630 keV/u at very small scattering angles of the hydrogen. We present two-dimensional distributions parallel (k(parallel to)) and perpendicular (k(perpendicular to)) to the projectile beam as well as distributions k(parallel to) for fixed k(perpendicular to). Theoretical calculations were carried out within the plane wave first Born approximation, which includes both mechanisms of the electron emission, namely, the shake-off and the sequential capture and ionization. It is shown that electron correlations in the initial ground-state wave function of the target play the most important role in the explanation of the experimentally observed enhanced backward electron emission.