Breakup ofH2in Singly Ionizing Collisions with Fast Protons: Channel-Selective Low-Energy Electron Spectra

Dissociative as well as nondissociative single ionization of ${\mathrm{H}}_2$ by 6 MeV proton impact has been studied in a kinematically complete experiment by measuring the momentum vectors of the electron and the ${\mathrm{H}}^{+}$ fragment or the $\mathrm{H}_2{}^{+}$ target ion, respectively. For the two ionization pathways, the electron spectra reveal the role of autoionization of the doubly and singly excited states of ${\mathrm{H}}_2$. The latter explicitly involve the coupling between the electronic and the nuclear motion of the molecule. This is a clear manifestation of a breakdown of the Born-Oppenheimer approximation.