Prominent role of multielectron processes in K-shell double and triple photodetachment of oxygen anions.

The photon-ion merged-beam technique was used at a synchrotron light source for measuring the absolute cross sections of the double and triple photodetachment of O- ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13 000, the position, strength, and width of the below-threshold 1s2s22p6 S2 resonance as well as the positions of the 1s2s22p5 P3 and 1s2s22p5 P1 thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multiconfiguration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shakeup (and shakedown) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed. © 2016 American Physical Society.