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

We determine with high accuracy the energy of the inner-shell transition 1s(2)2s(2 1)S(0) -> 1s2s(2)2p(3/2) P-1(1) O-16(K⍺)4+ at 554.372(3) eV (lambda= 22.36480(12) Å) as well as its small shift of 2.2 +/- 1.3 meV (Delta lambda= 0.089(52) mÅ) for the O-18 isotope. This transition blends with a K⍺ line of O5+ 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 O4+ 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.