Absolute Measurements and Theoretical Calculations of Photoionization Cross Sections along the Isonuclear Sequence of Multiply Charged Barium Ions

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⁺.