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.