Electron-ion recombination of Fe IV

Ab initio calculations are presented for total and partial recombination cross sections and rate coefficients for e+Fe V-->Fe IV, employing a unified treatment that incorporates both the radiative and dielectronic recombinations (RR and DR) in a self-consistent and accurate manner. The theoretical treatment is based on the close-coupling approximation using the R-matrix method. Recombination calculations for heavy atoms such as Fe IV, with 3d open-shell ground configuration, require an extensive eigenfunction expansion for the target ion, as well as a carefully optimized basis set of electron-ion bound configurations that represent short range correlation effects. The large-scale calculations are preceded by electron scattering and photoionization calculations using a 31-term eigenfunction expansion dominated by the configurations 3d⁴, 3d³4s, and 3d³4p of Fe V. Photorecombination cross sections and DR collision strengths are thereby obtained, including individual photorecombination cross sections for a large number of bound states of Fe IV that couple to the ground state 3d^{4 5}D of the target ion Fe V-all possible bound states up to n=10 (740 LS terms of Fe IV). The cross sections include autoionizing resonances, also up to the n=10 complex, accounting for the unified (RR + DR) contribution into the n<=10 (low-n) bound states of Fe IV. Recombination into the high-n states, 10<=n<=∞, is obtained through the DR collision strengths for the corresponding series of resonances in the electron-ion continua. The convergence of the close-coupling expansion, with respect to the target ion states and the (electron plus ion) correlation functions is discussed with reference to other highly complex atomic systems. Maxwellian average at a range of temperatures yields the total rate coefficient, as well as partial contributions directly to state-specific recombination rate coefficients. The new close-coupling rates differ considerably from those heretofore obtained from simpler approximations. We expect the present data to be of importance in the modeling of astrophysical and laboratory plasmas where iron is often a prominent constituent.