  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 2p and 2s excitation to higher atomic shells as well
as the thresholds for 2p and 2s 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 have been carried out using two different theoretical approaches,
Hartree-Fock including relativistic extensions and fully relativistic
Multi-Configuration 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 deexcitation cascades
after the creation of inner-shell holes in the Fe+ ion have been 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.
