Double-to-Single Target Ionization Ratio for Electron Capture in Fast p -He Collisions

We have used the ion storage ring CRYRING and its internal gas-jet target and recoil-ion-momentum spectrometer to measure absolute cross sections for transfer ionization (TI: p+He→H⁰+He²⁺+e^-) in 2.5--4.5 MeV p-He collisions with separate Thomas (TTI) and kinematic (KTI) TI contributions. The probability for electron emission in kinematical capture decreases with increasing velocity and appears to approach the photoionization shakeoff value ($1.63%$) [T. {}berg, Phys. Rev. A 2, 1726 (1970)]. The velocity dependence of the TTI cross section is consistent with the theoretically predicted v^-11 scaling [J. S. Briggs and K. Taulbjerg, J. Phys. B 12, 2565 (1979)].