Fully Differential Study of Non-PCI Higher-Order Contributions to Ionization of Helium by Proton Impact

We have measured fully momentum analyzed projectiles and recoil-ions in coincidence for 75 keV p + He collisions. We extracted fully differential cross sections (FDCS) for electrons ejected into the scattering plane as a function of the emission angle q_e. The electron energy E_e and the momentum transfer q were fixed. Earlier we reported studies of FDCS for electron energies corresponding to electron speeds near the projectile speed (velocity matching). In this regime, higher-order contributions are dominated by the post-collision interaction (PCI). There, the projectile interacts with the active electron twice. In the first interaction, enough energy is transferred to the electron to lift it to the continuum. In the second interaction, the projectile and the ejected electron focus each other towards the initial beam axis. We found two signatures of PCI: a) the binary peak was shifted in the forward direction relative to q and b) a pronounced peak structure in the initial projectile beam direction (forward peak) was observed.

The motivation for this project was to study higher-order effects other than PCI, those involving an interaction between the projectile and the target nucleus (PT interaction). We suppressed PCI by selecting electron energies of 1 and 75.4 eV, which are far from the velocity-matching.

At 1 eV we found that one of the PCI signatures, the forward peak, was absent in the FDCS, but at 75.4 eV it was strongly reduced compared to the velocity-matching regime, but a significant residue remained. The forward shift of the binary peak at 1 eV was as prominent as in the velocity-matching regime and even more pronounced than at 75.4 eV. However, earlier we pointed out that not every forward shift is indicative of PCI, but rather it can be caused by the PT interaction.



The data reveal two separate regimes, one at very small E_e and one in the velocity-matching regime and at large E_e. We interpret these regions as being caused by PCI (velocity-matching) and higher-order effects involving the PT interaction (small E_e), respectively. We thus conclude that at 1 eV the data are dominated by non-PCI higher-order contributions and by PCI at 75.4 eV.