VARIABLE PHOTON ENERGY PHOTOELECTRON-SPECTROSCOPY OF OSO4 AND PSEUDOPOTENTIAL CALCULATIONS OF THE VALENCE IONIZATION ENERGIES OF OSO4 AND RUO4

Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetraoxide in the ionization energy range 12-18 eV. The photon energies used ranged between 24 and 115 eV. The ionization cross sections of the 2t2, 1e, and 2a1 orbitals show evidence of substantial metal character. By use of an ab initio many-body Green's function formalism that takes into account the effect of electron correlation and relaxation, ionization energies of OsO4 and RuO4 have been calculated. Considerations of the spectral features and the predictions of the calculation lead to an assignment of ion state ordering of 2T1 < 2T2 < 2A1 < 2E < 2T2. The presence of a substantial p-d resonance feature in the cross section of the upper 2T2 and 2E ion states shows that the 2t2 and le orbitals have significant Os 5d character. The lower 2T2 ion state (arising from ionization from the 3t2 orbital) undergoes a spin-orbit splitting of 0.4 eV resulting from an Os 6p contribution to the 3t2 molecular orbital.