Double ionization yields from the photoionization of Fe II and Fe I

Photoionization cross sections for ground state Fe II and Fe I (3p 6 3d 6 4s x , x = 1 and 2) up to photon energies corresponding to ejection of an inner-shell 3p electron are calculated using the R -matrix method, in a first attempt at estimating the double ionization yield. Although single ionization at low energies is dominated by the 3d electron ionization channel with an inner-shell edge at the 3p ejection threshold (~60 eV): γ + 3p 6 3d 6 4s x →e - + 3p 6 3d 5 4s x , 3p 5 3d 6 4s x double ionization involves many channels associated with the two-electron correlation interaction. In simpler atoms this can be modelled using pseudo-state bases, but these would be computationally prohibitive for Fe ions, so we investigate instead final (physical) state expansions of the form 3p 6 3d 5 4s x , 3p 6 3d 6 4s x -1 , 3p 6 3d 5 4s x -1 nl , 3p 6 3d 4 4s x nl , 3p 5 3d 6 4s x with nl = 4p, 4d, 5s; many of these ` nl ' states and all of the 3p 5 3d 6 4s x states lie in the continuum, and the final-state resolved cross section summed over these states is interpreted as the double ionization yield. We give some tests of convergence with respect to the continuum state expansion, by including other states and pseudo-states. The double ionization cross section is found to be 10-20% of the total, generally larger for Fe I than for Fe II due to excitation of a 3d4s electron pair in addition to 3d3d, and dominated above 4 Rydberg by 3p hole resonances due to 3p→3d, 4s photoexcitation of the core.