Recoil-Ion Momentum Distribution in Positron-Impact Ionization Collisions

We employ a Classical Trajectory Monte Carlo simulation and a Continuum Distorted Wave - Eikonal Initial State model to study the distribution of the recoil ion momentum (RIM) in the single ionization of H2 molecules by positron impact. We observe that this distribution reaches a finite value at the kinematical threshold, an effect that can be related to the familiar ``electron capture the continuum'' peak of electron momentum spectroscopy. We also analyze a second threshold of the distribution which--up to our knowledge--has never been reported in the literature. We propose that this new effect represents a fingerprint of a strong orientation of low-energy electrons into the direction of motion of the electron-positron centre-of-mass.