Autoionization resonances in the photoabsorption spectra of [Fe.sup.n+] iron ions

The photoabsorption cross sections of a neutral iron atom, as well as positive [Fe.sup.+] and [Fe.sup.2+] ions, are calculated in the relativistic random-phase approximation with exchange in the energy range 20-160 eV. The wavefunctions of the ground and excited states are calculated in the single -configuration Hartree-Fock-Dirac approximation. The resultant photoabsorption spectra are compared with experimental data and with the results of calculations based on the nonrelativistic spin-polarized version of the random-phase approximation with exchange. Series of autoionization resonance peaks, as well as giant autoionization resonance lines corresponding to discrete transitions 3p [right arrow] 3d, are clearly observed in the photoabsorption cross sections. The conformity of the positions of calculated peaks of giant autoionization resonances with experimental data is substantially improved by taking into account additionally the correlation electron-electron interaction based on the model of the dynamic polarization potential.