Absolute single photoionization cross-section measurements of Se ³⁺ and Se ⁵⁺

Absolute single photoionization cross-section measurements of Se ³⁺ and Se ⁵⁺ ions were performed using monochromatized synchrotron radiation and the photo-ion merged-beams technique. Se ^{3 +} measurements were made at a photon energy resolution of 20 ± 3 meV from 41.90 to 54.56 eV, spanning the ²P _1/2 ground state ionization threshold. Numerous autoionizing resonances arising from both the ²P _1/2 ground state and ²P _3/2 metastable state are identified using quantum defect analysis. For Se ^{5 +}, measurements were made at a photon energy resolution of 60 ± 10 meV from 81 to 101 eV, spanning the ²S _1/2 ground state ionization threshold, and at a photon energy resolution of 28 ± 3 meV from 100.175 to 106.675 eV in a region rich with autoionizing resonances. The direct ionization threshold of the Se ^{5 + 2}S _1/2 ground state is experimentally determined to be 81.780 ± 0.010 eV.