Multiple photodetachment of oxygen anions via K -shell excitation and ionization: Direct double-detachment processes and subsequent deexcitation cascades

Experimental cross sections for $m$-fold photodetachment ($m=2--5$) of oxygen anions via $K$-shell excitation and ionization were measured in the photon-energy range of 525--1500 eV using the photon-ion merged-beams technique at a synchrotron light source. The measured cross sections exhibit clear signatures of direct double detachment, including double $K$-hole creation. The shapes of the double-detachment cross sections as a function of photon energy are in accord with Pattard's [J. Phys. B 35, L207 (2002)] empirical scaling law. We have also followed the complex de-excitation cascades that evolve subsequently to the initial double-detachment events by systematic large-scale cascade calculations. The resulting theoretical product charge-state distributions are in good agreement with the experimental findings.