Photoionization cross sections for ions of the cerium isonuclear sequence

Photoionization cross sections for Ceq+ (1≤q≤9) ions were measured in the 105-180 eV energy range of the 4d inner-shell giant resonance by merging a mass-to-charge-ratio-selected ion beam with a beam of monochromatized synchrotron radiation. The Cowan atomic structure code was used as an aid to interpret the experimental data. Four Rydberg series for 4d→nf (n-4) and 4d→np (n-6) autoionizing excitations were assigned using the quantum-defect theory in the Ce3+ photoionization cross section. The experimental data show the collapse of the nf wave functions (n-4) with increasing ionization stage as outer-shell electrons are stripped from the parent ion. The nf orbital collapse occurs partially for Ce2+ and Ce3+ ions and completely for Ce4+, where these wave functions penetrate the core region of the ion. A strong contribution to the total oscillator strength was observed in multiple photoionization channels for Ce+, Ce2+, and Ce3+, whereas most of the 4d excitations of the higher charge states decay by ejection of one electron.