Dielectronic recombination of highly ionized iron

Dielectronic recombination of the iron ions ${\mathrm{Fe}}^{15+}$, ${\mathrm{Fe}}^{23+}$, and ${\mathrm{Fe}}^{25+}$ has been studied in the isolated-resonance, distorted-wave approximation. The cross-section calculations include the dielec- tronic transitions associated with the 3s→3l and 3s→4l excitations in ${\mathrm{Fe}}^{15+}$, the 2s→2p and 2s→3l excitations in ${\mathrm{Fe}}^{23+}$, and the 1s→2l excitations in ${\mathrm{Fe}}^{25+}$. The effects of external electric fields have been included by employing intermediate-coupled, field-mixed eigenvectors for the doubly excited Rydberg states, determined by diagonalizing a Hamiltonian matrix which includes the internal electrostatic and spin-orbit terms, as well as the Stark matrix elements. The field effects are found to be quite large in ${\mathrm{Fe}}^{15+}$, relatively small in ${\mathrm{Fe}}^{23+}$, and negligible in ${\mathrm{Fe}}^{25+}$. The calculations indicate that there are large resonances near threshold in ${\mathrm{Fe}}^{23+}$ that are unaffected by external fields and may be measurable in new experiments currently being designed. In addition, the contributions of radiative recombination and the possible interference between radiative and dielectronic recombination in low-lying resonances are considered. Even though the radiative recombination cross sections may be appreciable near threshold in ${\mathrm{Fe}}^{15+}$ and ${\mathrm{Fe}}^{23+}$, the interference between these processes appears to be completely negligible.