We report fully differential cross section (FDCS) calculations and absolute measurements for ion-atom impact ionization. Using the COLTRIMS (cold target recoil ion momentum spectroscopy) method we have obtained absolute FDCS both in the scattering plane as well as out of the scattering plane for 100 MeV amu(- 1) C6+ ionization of helium FDCS results are presented for different projectile scattering angles and ejected electron energies. The measurements are compared with a theoretical calculation employing an asymptotically exact three body final state wavefunction that contains all active two particle subsystem interactions to infinite order in perturbation theory. For the active electron a Hartree-Fock (HF) bound state wavefunction is used for the initial state and numerical continuum state eigenfunctions of a HF potential for the ion are used for the final state In the scattering plane these theoretical results are in very good agreement with experiment for small and intermediate momentum transfer. However some significant discrepancies are found for large momentum transfer and outside the scattering plane. These discrepancies disappear upon comparison with successively less differential cross sections.
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