Photoionization of singly charged sulfur ions in the extreme-ultraviolet region: Absolute continuum cross section and resonance structures

Abstract

The absolute single- and double-photoionization cross sections of singly charged sulfur ions have been measured from 20 to 200 eV by merging a 2 keV beam of ${\mathrm{S}}^{+}$ ions with a synchrotron-radiation beam from an undulator. The continuum cross section for single photoionization is strongly influenced by a broad Fano profile near the ${\mathrm{S}}^{+}{\mathrm{}(}^{4}S)$ ionization threshold at 23.34 eV and a Cooper minimum located near 35 eV. Structures resulting from the excitation of $3s$ or $2p$ electrons, either from the ${\mathrm{S}}^{+}\mathrm{}{(}^{4}S)$ ground-state ion or from the ${\mathrm{S}}^{+}{(}^{2}D)$ metastable ion, are resolved, identified, and characterized. Significant deviations exist between the experimental data and the photoionization cross sections calculated as part of the Opacity Project.