He Single Ionization in Collisions with ``Fixed-in-Space'' MeV H$_2^+$ Ions

Two center effects in collisions of fast ions with H$_2$ molecules have been studied intensively. We investigated in a kinematical complete experiment the ionization of He in collisions with H$_2^+$-molecular ions at 0.5 and 1.0 MeV at the Max Planck Institute for Nuclear Physics at Heidelberg. The momenta of the recoiling He ions and the electrons produced in the collisions were measured using a ``Reaction Microscope.'' The fragments of the H$^2_+$ were separated by a dipole magnet after the interaction region and detected by two position sensitive MCP detectors. From this information the orientation and the internuclear distance of the molecular ion at the instance of the collision could be determined. Pronounced structures are found both in the experimental data and theoretical calculations, indicating that the emitted He electron shows a slight preferential emission parallel to the molecular axis. According to our theoretical calculations the effects are due to two-center interference, which turned out to be strongly dependent on the type of approximated molecular wave function used in the calculations.