On the Structure of the Iron K Edge

It is shown that the commonly held view of sharp Fe K edges must be modified if the decay pathways of the resonances converging to the K thresholds are adequately taken into account. These resonances display damped Lorentzian profiles of nearly constant widths along the spectroscopic series that are smeared to impose continuity across the threshold. By modeling the effects of K damping on opacities, it is found that the broadening of the K edge grows with the ionization level of the plasma and that the appearance at high ionization of a localized absorption feature around 7.2 keV is identified as the Kβ unresolved transition array.