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.
Aguilar, A., Covington, A. M., Hinojosa, G., Phaneuf, R. A., Álvarez, I., Cisneros, C., Bozek, J. D., Dominguez, I., Sant'Anna, M. M., Schlachter, A. S., Nahar, S. N., & McLaughlin, B. M. (2003). Absolute photoionization cross section measurements of O II ions from 29.7 to 46.2 eV. Astrophysical Journal, Supplement Series, 467-477. https://doi.org/10.1086/368077