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-excit ation spectra. Rydberg states are quenched in the solid, and the excha nge splitting of the Is ionization threshold (1.1 eV) can be observed. Below the ionization threshold, core electrons can be excited into un occupied antibonding pi and o* orbitals. We conclude from the relativ e 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 absorpt ion peak. Because of ultrafast dissociation (similar to 5 fs), core ho le decay is likely to occur at large interatomic distances. Anger spec tra at the o absorption of O-2 do indeed show peaks that would be exp ected for free core-excited oxygen atoms.