Unified treatment of electron-ion recombination in the close-coupling approximation

A unified treatment for electron-ion recombination has been developed and applied to detailed calcu lations for cross sections and total recombination rate coefficients for a number of atoms and ions. The ab initio calculations are carried out in the close-coupling approximation employing the R-matrix method for the bound and the continuum states of the electron-ion system. All final states of the recom bined system are taken into account and the present method subsumes both the radiative and the dielect ronic recombination (RR and DR) processes over all energy and temperature ranges of practical impor tance. Recombined states of the electron-ion system are divided into two groups: (a) n ~ no and (b) no < n ~ 00. Photoionization cross sections are calculated for all bound states of group (a), typically a few hundred bound states, referred to as low-n states, including a detailed energy resolution of the several infinite series of autoionizing resonances converging onto the various excited states of the core ion included in the close-coupling expansion. High-n states of group (b) are treated primarily through the theory of DR developed by Bell and Seaton (J. Phys. B 18, 1589 (1985)). DR collision strengths with detailed resonance structures are presented. The detailed and the resonance-averaged DR collision strengths show characteristic peaks at the target thresholds of the core ion corresponding to dipole allowed transitions. Individual bound states of the e +ion system with dominant contribution to low energy recombination are described. The present results demonstrate the importance of (i) recombina tions to excited states (particularly the metastable states), and (ii) low-energy autoionizing resonances, both of which result in large contributions to effective electron-ion recombination. The individual con tributions of the excited bound states of the e + ion system are calculated and their relative contribution to the total are discussed. The general pattern of the recombination rate, as a function of electron tem perature, is studied along an isoelectronic sequence. It is found that while the low-energy (temperature) recombination increases with ion charge z, the relative high-energy (temperature) contribution to the to tal decreases; i.e., viewed as independent processes, the RR part increases while the DR decreases with z. Total recombination rate coefficients for several atoms and ions (C II, S II, C II, N II, 0 III, F IV, N e v, and Si IX) are obtained over the entire temperature range of possible interest in applications. Comparisons are made with earlier works on RR and DR.