THE SIGMA-ASTERISK ABSORPTION PEAK AT THE OXYGEN 1S EDGE OF O-2 - EXCHANGE SPLITTING, ULTRAFAST DISSOCIATION, AND ATOMIC-LIKE AUGER-SPECTRA

The x-ray absorption spectrum of solid O-2 at the O 1s absorption edge is analyzed, using its polarization dependence and the Anger de-excitation spectra. Rydberg states are quenched in the solid, and the exchange splitting of the Is ionization threshold (1.1 eV) can be observed. Below the ionization threshold, core electrons can be excited into unoccupied antibonding pi* and o* orbitals. We conclude from the relative intensities and from the polarization dependence, that the exchange splitting of the o* final-state is small (<1 eV). This is confirmed by LDA calculations of core-excited states. The calculated dr potential surfaces are repulsive, which explains the large width of this absorption peak. Because of ultrafast dissociation (similar to 5 fs), core hole decay is likely to occur at large interatomic distances. Anger spectra at the o* absorption of O-2 do indeed show peaks that would be expected for free core-excited oxygen atoms.