Threshold effects in the ionization of atoms by positron impact

We employ different theoretical models, both classical and quantum-mechanical, to explore the recoil-ion momentum distribution in positron–atom collisions. We pay special attention to the vicinity of the kinematical threshold between ionization and positronium formation. We demonstrate that it is intertwined by dynamical constraints to the formation of highly excited and low-lying continuum electron–positron states. Finally we discuss how the study of recoil-ion momentum distribution, which is characteristic of a reaction microscopy technique, might represent an alternative approach to the standard spectroscopy of electrons and positrons.