| The first measurements of photoionization of Fe3+, Fe 5+, and Fe7+ ions were made using the ion-photon beam endstation on beamline 10.0.1 at the Advanced Light Source, where ion beams from an ECR source were merged with monochromatic beams of synchrotron radiation. The photoion-yield spectra contain numerous resonances due to photoexcitation of autoionizing states, including extremely broad features in Fe3+ and Fe5+ that are thought to result from fast super-Coster-Kronig transitions following 3p-3d excitations. Absolute photoionization cross-section measurements were also performed for these ions using beams with unknown fractions of ground and metastable states. A Rydberg series from the 2G1 metastable state in Fe3+ was identified and assigned. The absolute cross-section measurements were compared with different theoretical calculations. Large differences were found between the absolute cross-section measurements in this study and the TOPbase atomic database widely used in astrophysics. A new R-matrix calculation shows better correspondence to the Fe3+ experimental data, but substantial differences remain. The presence of unknown fractions of primary ions in metastable and ground states significantly complicates the analysis of the data and comparison with theory. Hartree-Fock calculations of autoionizing states of Fe7+ were performed, but found to be inadequate to accurately predict the energies of the resonances observed in photoionization. |
