K -shell photoionization of O 4+ and O 5+ ions: experiment and theory

We report that absolute cross-sections for the K-shell photoionization of Be-like (O4+) and Li-like (O5+) atomic oxygen ions were measured for the first time (in their respective K-shell regions) by employing the ion–photon merged-beam technique at the SOLEIL synchrotron-radiation facility in Saint-Aubin, France. High-resolution spectroscopy with E/ΔE ≈ 3200 (≈170 meV, full width at half-maximum) was achieved with photon energy from 550 to 670 eV. Rich resonance structure observed in the experimental spectra is analysed using the R-matrix with pseudo-states (RMPS) method. Results are also compared with the screening constant by unit nuclear charge (SCUNC) calculations. We characterize and identify the strong 1s → 2p resonances for both ions and the weaker 1s → np resonances (n ≥ 3) observed in the K-shell spectra of O4+.