Recoil-ion momentum distributions for transfer ionization in fast proton-He collisions

We present high-luminosity experimental investigations of the transfer ionization (TI:p+He→H⁰+He²⁺+e⁻) process in collisions between fast protons and neutral helium atoms in the earlier inaccessibly high-energy range 1.4–5.8 MeV. The protons were stored in the heavy-ion storage and cooler ring CRYRING, where they intersected a narrow supersonic helium gas jet. We discuss the longitudinal recoil-ion momentum distribution, as measured by means of cold-target recoil-ion momentum spectroscopy and find that this distribution splits into two completely separated peaks at the high end of our energy range. These separate contributions are discussed in terms of the earlier proposed Thomas TI (TTI) and kinematic TI mechansims. The cross section of the TTI process is found to follow a σ∝v^−b dependence with b=10.78±0.27 in accordance with the expected v⁻¹¹ asymptotic behavior. Further, we discuss the probability for shake-off accompanying electron transfer and the relation of this TI mechanism to photodouble ionization. Finally the influence of the initial-state electron velocity distribution on the TTI process is discussed.