Multiply Excited States in the Two- and Three-Electron Oxygen Ion

High-resolution measurements of x-ray decay from excited states in highly stripped oxygen ions have yielded many transitions not attributable to the Lyman series in hydrogenic oxygen or to the analogous heliumlike oxygen series. These additional transitions are explained by considering the "screening" effects associated with adding a $2s$, $2p$, or $3p$ "spectator" electron to configurations governing the single-electron decays in excited heliumlike and lithiumlike oxygen. By comparing Hartree-Fock calculations for term energies, many of the observed transitions can be assigned to the doubly excited configurations $2pnp$ and $1smsnp$ in the heliumlike and lithiumlike sequencies, respectively.