Photoionization of Fe$^{7+}$ in the $3p-3d$ resonance energy region*

Photoionization of Fe$^{7+}$ in the energy range of $3s$ and $3p$ inner-shell excitations were studied using photoion spectroscopy with monochromatized synchrotron radiation. The resonance structure in the range 150 -- 180~eV is attributed to [$3s^23p^63d, 3s^23p^64s$] -- [$3s^23p^54s5s$, $3s^23p^53d5s, 3s^23p^53d6s$, $3s^23p^53d4d, 3s^23p^53d5d$, $3s3p^63d4p$] transitions. Relativistic many-body perturbation theory was used to evaluate multipole (M1 and E2) matrix elements to obtain lifetimes of the $3s^23p^63d^3D_{5/2}$ and $3s^23p^64s\ ^2S_{1/2}$ metastable levels. These calculations started from an argonlike closed-shell Dirac-Fock potential. Matrix elements were calculated using both relativistic many-body perturbation theory, complete through second and third orders, and the relativistic all-order method restricted to single and double excitations. To reproduce resonance structure in the photoionization cross section, a large-scale COWAN calculation including about 30 configurations was used.