Fully Differential Measurements on Single Ionization of He by 75 keV Proton Impact

Fully momentum analyzed scattered projectiles and recoil ions in 75 keV p + He collisions were measured in coincidence. The momentum of the ionized electrons was deduced from momentum conservation. We obtained fully differential three-dimensional angular distributions of electrons with an energy of 5.5 eV. Through the use of position sensitive detectors, data were recorded for a broad range of scattering angles simultaneously. According to the first Born approximation, peak structures in the direction of the momentum transfer q (difference between initial and final projectile momentum) and --q were expected (binary and recoil peak). Instead, we observe a peak which is shifted backwards at small scattering angles and forward at large scattering angles relative to q and no significant recoil peak. These observations as well as unexpected features outside the scattering plane are a manifestation of a surprisingly important role of the projectile -- target nucleus interaction.