Photoionization measurements for the iron isonuclear sequence Fe3+, Fe5+, and Fe7+

Cross sections for single photoionization of Fe{sup 3+}, Fe{sup 5+}, and Fe{sup 7+} ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to 3p-nd transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe{sup 3+} and Fe{sup 5+} that are believed to result from unresolved fast super-Coster-Kronig transitions following 3p-3d excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to 3p-nd transitions from the {sup 2}G{sub 1} metastable state in Fe{sup 3+} was identified. The data are compared with recently published measurements on Fe{sup 3+} and Fe{sup 5+} using a similar technique at lower spectral resolution.