Absolute photoionization cross section measurements of O II ions from 29.7 to 46.2 eV

Absolute photoionization cross sections have been measured for a mixture of ground-state and metastable O II (O⁺) ions at photon energies ranging from 29.9 to 46.0 eV (414.7 to 269.5 Å). All measurements were performed by merging an O⁺ beam with synchrotron radiation from an undulator beam line at the Advanced Light Source (ALS). At a resolution of 17 meV, more than 70 spectral features have been resolved, most of them identified and characterized. These measurements are compared with two independent R-matrix calculations and the data in TOPbase. All three calculations agree within 25% on the direct photoionization cross section, and with the absolute measurements within 40%. Some differences are noted in the predicted resonance positions among the three close-coupling R-matrix calculations, the TOPbase data being the least accurate. The estimated total experimental uncertainty varies from 15% to 20%. Such measurements benchmark theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.