SINGLE PHOTON DOUBLE IONIZATION OF THE OXYGEN MOLECULE

Double ionization of the oxygen molecule by 40.8 eV photons has been studied by the photoelectron-photoelectron coincidence technique, photoelectron spectroscopy and mass spectroscopy. The coincidence spectra show that at 40.8 eV double photoionization is predominantly indirect via intermediate singly charged states, and involves an atomic autoionization step at large internuclear distance. Structure corresponding to atomic autoionization is apparent in the 40.8 eV photoelectron spectrum, requiring a reassignment of signals previously assigned to states of the molecular ion. The atomic autoionization step allows the O2(2+) dication to be formed well outside the Franck-Condon zone for a direct transition from the neutral geometry, and explains an apparent discrepancy between earlier photoionization experiments and the results of Auger spectroscopy and theoretical investigations.