Photodouble ionization dynamics for fixed-in-space H2

The Coulomb explosion of the hydrogen molecule, after absorption of a 76 eV photon, has been studied by momentum imaging the two electrons and the two protons. Absolute fully differential cross sections of high statistical quality are obtained. A subset of the overall data, namely, equal electron-energy sharing, is used to investigate the effects of molecular orientation on the photoelectron angular distribution. Departures from the first-order heliumlike model are evident in detection geometries where electronelectron correlation is ‘‘frozen.’’ During the last decade experimental techniques have been steadily improving, enabling studies of the correlated electron pair dynamics produced in photodouble ionization (PDI). PDI of the simplest two-electron molecule, H2 ,i s significantly more complex than the well-studied case of helium and introduces new physical effects. First, there is no unique double ionization threshold; the threshold energy depends on internuclear separation as the upper repulsive potential curve is purely Coulombic. Second, the ground state electronic configuration is inevitably more complex as a result of the two-center nuclear potential. Third, PDI in H2 is followed by a so-called ‘‘Coulomb explosion’’ as the two protons rapidly separate in opposite directions. Since the photo-fragmentation process is rapid compared to molecular rotation, the relative momentum of the two escaping protons defines the molecular alignment at the instant of double ionization. Energy- and angleresolved detection of all four particles —with a welldefined light polarization state —completely defines the PDI dynamics and allows one to study fully differential cross sections (FDCS) within the molecular frame using such ‘‘fixed-in-space’’ molecules. These measurements provide the most stringent tests for theory and the greatest possible physical insight into this prototypical 4-body process.