Photoionization of Xe4+ to Xe7+ ions was studied by combining an electron cyclotron resonance ion source with synchrotron radiation. Multiconfiguration Dirac-Fock calculations were performed to interpret the data. Many autoionization lines were measured and identified, resulting from excitation of a 4d electron into nf and np orbitals followed by Auger decay of the excited states. Continuum photoionization is negligible for the higher members of the isonuclear series.

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Photoionization of multiply charged ions of the Ba isonuclear sequence up to Ba6+ has been studied in a beam-beam experiment. A very strong increase in the resonance structures was observed when moving from Ba2+ to Ba6+. Absolute values of the photoionization cross sections were measured for Ba2+ and Ba3+ ions. The interpretation of the results is provided using theoretical multiconfiguration Dirac-Fock and relativistic random phase approximation calculations, showing that the collapse of the nf orbitals occurs for Ba4+.

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