Three-Dimensional Fully Differential Single Ionization Cross Sections for 75 keV p + He collisions

Single ionization of a neutral atom by a charged particle impact is a most suitable process to study the fundamentally important few-body problem. The most detailed information can be obtained from fully differential cross section (FDCS). FDCS for single ionization can be obtained by measuring the momenta of any two of three particles involved in the interaction. In our experiment we measured the complete momentum vector of the projectile and the ionized target. The electron momentum is then readily determined by momentum conservation. The ionized target momentum was obtained using cold-target recoil-ion momentum spectroscopy (COLTRIMS) while the projectile momentum was obtained using ion energy-loss spectroscopy. Using this experimental technique, we studied single ionization of He by proton impact at 75 keV. Such collision system corresponds to a perturbation of 0.6, defined as Q_p/v_p (where Q_p and v_p are the projectile charge and velocity, respectively) and small relative speed between the projectile and the ionized electron. At these kinematic conditions, the post-collision interaction (PCI) is particularly important. The experimental results are compared to various theoretical models including a CDW calculation with the n-n interaction included. Good agreement in shape but poor agreement in magnitude is found.