HIGH-RESOLUTION AND SYMMETRY-RESOLVED OXYGEN K-EDGE SPECTRA OF O2

High-resolution ion-yield and electron-yield spectra and "symmetry-resolved" ion-yield spectra have been measured at the O K-edge of O2. The 1s-pi* ((3)PI) and 1s-sigma* ((3)SIGMA-) bound-state resonances are clearly resolved, where the sigma* resonance is split into two features. Many Rydberg transitions converging to the (4)SIGMA- and (2)SIGMA- ionization thresholds are found on the 1s-sigma* resonance, but above the (4)SIGMA- threshold the Rydberg series converging to the (2)SIGMA- threshold show only a broad band. They are interpreted with the help of ab initio SCF-CI and frozen-core calculations for the core-excited states with four open shells. The Rydberg series are assigned to the 1s-3s-sigma, 1s-3p-pi, and 1s-np-sigma (n = 3, 4, ...) transitions. The splitting of the sigma* resonance arises from the exchange interaction; however, in contrast with the Rydberg states, the core-to-valence (1s-sigma*) excited state related to the (2)SIGMA- spin coupling is lower in energy than the 1s-sigma* state related to the (4)SIGMA- one.