Ionization of H2 by fast proton impact studied with COLTRIMS

Full text: Basically, two pathways contribute to single ionization of H2 by charged particle impact: First, in what we call pure ionization, an H2+ ion remains after the removal of one electron during the collision. Second, with a small probability of a few percent of all ionization events [1], the molecule dissociates into an H+ and an H atom (dissociative ionization). Until now experimental studies focussed either on electron emission characteristics or on the measurement of the nuclear fragments. We present results for single ionization of H2 in collisions with 6 MeV protons obtained with a combined recoil-ion electron momentum spectrometer (reaction-microscope). The emitted electron as well as the recoil ion (H2+ or H+) have been recorded in coincidence and their momentum vectors have been analyzed. Thus, we have recorded electron energy spectra for both reaction pathways. The electron energy distributions, resolved down to 0.1 eV, have been compared with predictions of a CDW-EIS calculation [2] (continuum-distorted-wave eikonal-initial-state) which takes into account interference effects caused by the electron emission from the two centers of the molecule [3]. Surprisingly, significant differences are observed though the kinetic energy released in the dissociation amounts to only a few tens of meV. Within the same frame, FDCSs have been obtained for pure ionization, which, to our knowledge, have not been accessible so far for ion impact on H2. Sizeable contributions from purely molecule-specific electron emission channels have been identified. The observed differences are due to the autoionization of singly or doubly excited states of H2, effects that explicitly involve the coupling between the electronic and the nuclear motion of the molecule, even beyond the Born-Oppenheimer approximation [4]