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We have measured fully differential single ionization cross sections for 75 keV p He collisions. At this relatively small projectile velocity, signatures of the projectile -residual-target-ion interaction, which are not observable for fast projectiles and for electron impact, are revealed rather sensitively. In fact, this interaction appears to be more important than the postcollision interaction, which so far was assumed to be the most important factor in higher-order effects for slow ion impact. These features are not well reproduced by our three-distorted-wave calculations.Ionization processes in atomic collisions have been studied extensively for several decades. The extraordinary relevance of this type of research to the yet unsolved and fundamentally important few-body problem has been emphasized frequently (e.g., [1,2]). Because the Schrödinger equation is not analytically solvable for more than two mutually interacting particles, detailed experimental studies are essential to guide theoretical modeling efforts. Investigations of atomic few-body systems are particularly important because the underlying fundamental interaction (electromagnetic) is basically understood for two mutually interacting particles. As a result, comparisons of experimental data with calculations are essentially testing the treatment of the few-body aspects in theory. Therefore, such studies can also help to advance our understanding of basic concepts in other areas of physics. For example, our understanding of the underlying fundamental interactions in nuclear systems (strong and weak) is still rather incomplete. Once successful techniques for the description of few-body aspects in atomic systems are developed and tested by experiment, they can be applied to nuclear scattering theory. Comparisons with experimental data can then be used to extract more detailed information about the nuclear potential.In the case of ionization of atoms by electron impact, fully differential cross sections (FDCS), which offer the most sensitive tests of theoretical calculations, have been measured routinely since the pioneering work of Ehrhardt et al. [3,4]. For very energetic electron impact, data were routinely very well reproduced even by the relatively simple first Born approximation (FBA) [5,6]. For many years, it was therefore generally held that single ionization is basically understood, at least for small perturbations (projectile charge to velocity ratio) where the assumptions of first-order perturbation theory were expected to be valid.For ion impact the measurement of FDCS for single ionization is much more challenging because it is very difficult to measure the scattered projectile momentum directly. The first experimental FDCS were reported only about a few years ago [2,7,8], where the momenta of the ejected electron and the recoil ion were measured and the scattered projectile momentum was deduced from the conservation laws. Similar to electron impact, the data at small for electrons ejected into the scattering plane (defined by the initi...

We have studied fully differential cross sections (FDCS) for single ionization in 75 keV p + He collisions by measuring the recoil-ion momentum and the projectile momentum in coincidence. The measured FDCS are completely dominated by the binary peak while the recoil peak is absent. This is a clear signature of the post-collision interaction between the outgoing projectile and the ejected electron. Furthermore, our continuum distorted wave calculation demonstrates the importance of the projectile-target nucleus interaction. While the shape of the experimental FDCS is well reproduced by this calculation, there are significant discrepancies in magnitude.

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