Abstract
Absolute cross-sections for the K-shell photoionization of Be-like (O-4 (+)) and Li-like (O-5 (+)) 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/Delta E approximate to 3200 (approximate to 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 +.