Absolute Single Photoionization Cross-Section Measurements of Rb³⁺

Absolute single photoionization cross-section measurements of Rb³⁺ ions were performed using the photo-ion, merged-beams technique with synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory. Measurements of Rb³⁺ were made in the photon energy range 46.52 eV to 62.51 eV with a nominal photon energy resolution of 30 meV. These measurements span the ³P₂ ground state ionization threshold as well as the ³P₁, ³P₀, ¹D₂, and ¹S₀ metastable state ionization thresholds. We identified multiple Rydberg resonance series in the auto-ionization resonance structure of Rb³⁺ and used these identifications to verify the reported ionization potential of Rb³⁺ and accurately determine the excited-state energy levels of low-lying metastable states of both Rb³⁺ and Rb⁴⁺. The ionization potential of Br⁺ and the metastable-state energy levels were found to agree with the tabulated energy levels reported by NIST to within the energy uncertainties of the NIST values and the present measurements. The measurements are also compared to Breit-Pauli R-matrix calculations with positive agreement between theory and experiment.