Evidence for electron-electron scattering in simultaneous capture and ionization by 1 MeV protons in He

We report on angular distributions of electron emission at 600 eV and 300 eV in 1 MeV proton-He collisions, in coincidence with electron capture by the projectile. The angular distribution of electrons in the vicinity of the beam velocity (600 eV) exhibits peaks at ~ 58° and ~ 90°, while the distribution of 300 eV electrons peaks at ~ 68° and ~ 110°. The forward peaks are due to a process in which the proton interacts with each of the two target electrons leading to capture and ionization. The features of peaks at ~ 90° and ~ 110° are ascribed to a two-step process of proton-electron scattering in the first step and a localized interaction of electron-electron scattering (correlation) in a second step, in which one of the electrons is deflected into a bound state of H° while the other electron is ionized. The magnitudes of our measured absolute cross sections for the proton-electron-electron scattering processes agree within factors of two to eight with recent theoretical predictions.