Near L-edge Single and Multiple Photoionization of Singly Charged Iron Ions

Abstract

Absolute cross-sections for m -fold photoionization ( m = 1 , … , 6 ) of Fe + by a single photon were measured employing the photon–ion merged-beams setup PIPE at the PETRA III synchrotron light source, operated by DESY in Hamburg, Germany. Photon energies were in the range 680–920 eV, which covers the photoionization resonances associated with 2 p and 2 s excitation to higher atomic shells as well as the thresholds for 2 p and 2 s ionization. The corresponding resonance positions were measured with an uncertainty of ±0.2 eV. The cross-section for Fe + photoabsorption is derived as the sum of the individually measured cross-sections for m -fold ionization. Calculations of the Fe + absorption cross-sections were carried out using two different theoretical approaches, Hartree–Fock including relativistic extensions and fully relativistic multiconfiguration Dirac–Fock. Apart from overall energy shifts of up to about 3 eV, the theoretical cross-sections are in good agreement with each other and with the experimental results. In addition, the complex de-excitation cascades after the creation of inner-shell holes in the Fe + ion were tracked on the atomic fine-structure level. The corresponding theoretical results for the product charge-state distributions are in much better agreement with the experimental data than previously published configuration-average results. The present experimental and theoretical results are valuable for opacity calculations and are expected to pave the way to a more accurate determination of the iron abundance in the interstellar medium.