Core-valence double ionization of SF6 involving S2p, F1s and S1s inner shells
Core-valence double ionization electron spectra near the S2p, F1s and S1s edges of SF6 are presented, analyzed and compared with conventional valence photoelectron spectra and quantum chemical calculations. The core-valence spectra are energetically stretched out, revealing salient structures between 15 and 40 eV that are sufficiently separated for molecular orbital analysis. The spectra offer new insights into the electronic structure, showing that the core hole substantially rearranges the molecular orbital order. This effect can be traced to orbital localization and nodal structure, as well as to valence-to-core penetration. The singlet and triplet splittings of the dicationic states progressively decrease in all three core-valence spectra towards deeper core levels, with larger splittings for S2p than S1s, reflecting greater valence-to-core penetration and exchange interaction for S2p. By large, the MO interpretation holds in frozen or self-consistent-field representations, except for the inner parts of the F1s spectra, which require analysis in terms of MO breakdown effects. An intensity model for CV spectra is derived using an independent-channel approach, where primary core ionization is treated by dipolar coupling to a continuum and the valence electron is promoted to a second continuum through shake-off. Full spin coupling and spin selectivity between the discrete doubly ionized states and the two continuum electrons are maintained. The primary molecular photoelectron intensity is expressed as a weighted sum of atomic subshell cross sections using a one-center atomic orbital projection of the relevant molecular orbital. The calculated spectra agree well with experiment and allow detailed assignment. In particular, symmetry breaking at the F1s core-ionization site is identified and discussed.