Photoionization cross sections and oscillator strengths for Fe

Abstract

Ab initio calculations are carried out for the photoionization cross sections and oscillator strengths for Fe III in the close-coupling approximation employing the R-matrix method. A 49-state eigenfunction expansion for Fe IV, with states dominated by the ground configuration 3${\mathit{d}}^{5}$ and the excited configurations 3d${\mathrm{}}^{4}$4s and 3d${\mathrm{}}^{4}$4p, is employed to ensure an extensive treatment of autoionizing resonances and other electron correlation effects related to channel couplings that considerably affect the effective cross sections. Of particular interest are the wide resonances due to photoexcitation within the core states that enhance the background cross sections by orders of magnitude at high energies. Coupled wave functions are obtained for 805 bound states of Fe III below the first ionization threshold; all of the 199 LS terms observed experimentally have been identified. Photoionization cross sections, with detailed autoionizing resonances, are obtained for the computed bound states as a function of photoelectron energy. The present results also include the oscillator strengths for 11 979 transitions between the bound states of Fe III that lie below the ionization threshold. In addition, the partial photoionization cross sections for 239 bound states, with the residual ion in the ground state, are also obtained. These cross sections are computed for applications to the determination of electron-ion recombination rate coefficients and population of levels in collisional-radiative models that do not assume local thermodynamic equilibrium. The present radiative data are compared with the observed energy values, lifetimes, and the photoionization cross sections calculated by others. \textcopyright{} 1996 The American Physical Society.