Breakup of ${\mathrm{H}}_2$ in 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.

Figure 1

Schematic potential curves for ${\mathrm{H}}_2$ and $\mathrm{H}_2{}^{+}$ illustrating the different single ionization pathways (for detailed potential curves see 24, 25).

Figure 2

(a) Electron energy distributions for single ionization of ${\mathrm{H}}_2$ by 6 MeV proton impact. Triangles: experimental results for pure ionization; circles: experimental results for dissociative ionization (for ${\mathrm{H}}^{+}$ energies smaller than 40 meV); solid lines: CDW-EIS results. (b) Ratios of experimental to theoretical cross sections.

Figure 3

FDCS for electrons emitted into the scattering plane for pure ionization of ${\mathrm{H}}_2$ by 6 MeV proton impact. Upper row: The electron energy $E_e=2.6\mathrm{e}\mathrm{V}$ is fixed and the values of the momentum transfer are $q=0.5$, 0.8 a.u.. Lower row: $E_e=0.2\mathrm{e}\mathrm{V}$, same values of $q$. Solid lines: CDW-EIS results. The cross sections are given in ${10}^{-18}{\mathrm{c}\mathrm{m}}^2/\mathrm{a}.\mathrm{u}{.}^2$.