Ejected-Electron-Energy and Angular Dependence of Fully Differential Ionization Cross Sections in Medium-Velocity Proton Collisions with He and H₂

We have measured fully momentum-analyzed recoiling target ions and scattered projectiles, produced in ionization of He and H ₂ by 75 keV proton impact, in coincidence. The momentum of the ejected electrons was deduced from momentum conservation. From the data we extracted fully differential ionization cross sections as a function of the polar electron emission angle (for fixed electron energies) and as a function of the electron energy (for fixed electron emission angles). Comparison between experiment and various distorted wave calculations confirms that under kinematic conditions where the post-collision interaction plays an important role, the few-body dynamics underlying the ionization process are still poorly understood.