Photoionization of selenium ions for the determination of elemental abundances in astrophysical nebulae

Absolute single photoionization cross section measurements are presented for Se+, Se2+, Se3+ and Se 5+. These measurements were performed at undulator beamline 10.0.1 of the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory using the merged-beams technique. All ions except Se5+ were measured from the energy region of the low-lying metastable state thresholds to at least 10 eV above the ionization potential. Theoretical calculations for Se5+ indicated strong resonances above 100 eV would dominate the photoionization spectrum, therefore this region was explored for this ion. Rydberg series of resonances are identified using quantum defect theory and the experimental results are compared to theoretical photoionization cross section calculations using fully relativistic Dirac Atomic R-matrix Code (DARC). These results are also used to improve the accuracy of the National Institute of Standards and Technology tabulated database of atomic structure.The results for each ion are additionally used to examine specific aspects of the measurement and analysis processes. Results for Se+ are used to determine the approximate fraction of higher-order radiation in the photon beam at low energies. Results for Se2+ are used to analyze the relative precision of the online version of the Cowan Hartree-Fock atomic structure code available from Los Alamos National Laboratory. Results for Se3+ are used to analyze the non-linearity in the photon energy response of the monochromator of Beamline 10 at the Advanced Light Source. Results for Se5+ are used to examine the effects of impurities in the ion source discharge and the process of selecting an appropriate photon energy resolution.