Excitation-ionization and double ionization of helium by Compton scattering

Multielectron transitions induced by neutron impact on helium

We explore excitation and ionization by neutron impact as a novel tool for the investigation of electron-electron correlations in helium. We present single- and double-ionization spectra calculated in accurate numerical ab initio simulations for incoming neutrons with kinetic energies of up to 150 keV. The resulting electron spectra are found to be fundamentally different from photoionization or charged particle impact due to the intrinsic many-body character of the interaction. In particular, doubly excited resonances that are strongly suppressed in electron or photon impact become prominent. The ratio of double to single ionization is found to differ significantly from those of photon and charged-particle impact.

Double ionization of lithiumlike ions by Compton scattering of high-energy photons

The double ionization of lithiumlike ions by Compton scattering of photons is investigated in the asymptotic high-energy region. To leading order of the nonrelativistic perturbation theory, the total cross section for double Compton effect is calculated, taking into account the channels of simultaneous and sequential emission of two electrons. Relationships between the cross sections for double ionization of He- and Li-like ions with the same nuclear charge Z are established. This can open wide perspectives for experimental investigations of ionization processes involving low-lying excited states. The universal scaling is found for the ratio of double-to-single ionization in the lithium isoelectronic sequence.

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. Aberg, 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)]].