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 ${\mathrm{O}}^{-}$ 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 $1s2s^{2}2p^6 {}^{2}S$ resonance as well as the positions of the $1s2s^{2}2p^5 {}^{3}P$ and $1s2s^{2}2p^5 {}^{1}P$ 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.

• Received 15 August 2016

DOI:https://doi.org/10.1103/PhysRevA.94.041401