Ionization of He by Intermediate Energy Protons

Ionization of He by 15-100 keV H+ has been investigated using COLTRIMS. With this technique we obtained momentum distributions of emitted electrons and recoil ions in coincidence. This approach allows the distributions to be plotted for experimentally determined scattering plane and impact parameter. Dörner et al. (1) reported that, for 5-15 keV H+ on He, the electron momentum distributions lie mainly in the scattering plane and display a slightly asymmetric "pi" shaped structure when projected into this plane. For H+ energies above 200 keV this molecular structure was observed to vanish (2,3) and the distributions become very target centered. The purpose of this study was to study the evolution of the spectra in the transition region between molecular and perturbative regions. Our results show that for 40 keV H+ and higher the molecular structure seen at lower energies vanishes, with all electrons lying in the target region. No evidence was found for further oscillation of the asymmetry of the distribution around the internuclear line reported by Dörner et al.. This work is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (1) Dörner et. al., Phys. Rev. Lett. 77,4520(1996) (2) V. Mergel et. al., Phys. Rev. Lett. 74,2200(1995) (3) Abdallah et. al., 7th Workshop on Fast Ion-Atom Collisions, Debrecen(1998)