Parts-per-million-accurate determination of the K α photoionization resonance of Be-like oxygen with resolution of its O 16 – O 18 isotopic shift

We determine with high accuracy the energy of the inner-shell transition 1 s 2 2 s 2 S 0 1 → 1 s 2 s 2 2 p 3 / 2 P 1 1 O K α 4 + 16 at 554.372(3) eV ( λ = 22.36480 ( 12 ) Å ) as well as its small shift of 2.2 ± 1.3 meV ( Δ λ = 0.089 ( 52 ) m Å ) for the O 18 isotope. This transition blends with a K α line of O 5 + used in astrophysical diagnostics, potentially affecting its reliability. In contrast to our experimental uncertainty of ± 3 meV, advanced electronic structure predictions for this four-electron system, including quantum electrodynamic (QED) corrections on the order of 100 meV, still scatter by more than ± 250 meV. Ions generated and stored in an electron beam ion trap were excited at the ELETTRA synchrotron facility with monochromatic soft x rays, with photon energies corrected by an additional spectrometer. Upon resonant excitation of O 4 + and their subsequent autoionization, we separate the photoions of each isotope by a time-of-flight measurement. This way, we resolve soft x-ray isotopic shifts of a few meV, obtain very accurate data on essential astrophysical ions, and test calculations down to the level of QED contributions.