Intermediate-coupling effects in the dielectronic recombination of oxygen ions

We have calculated {Δ}n=0 dielectronic recombination (DR) cross sections and rate coefficients for Oq+, q=1--5, in configuration-mixing LS-coupling and intermediate-coupling approximations, using the program autostructure, as well as in a partitioned configuration-average (PCA) approximation. The intermediate-coupling cross sections (rate coefficients) are about 45% (25%) greater than the LS-coupling results in the case of O⁺ and O⁵⁺, due to the effect of core fine-structure interactions, while the increase is no more than 10% (5%) for O²⁺, O³⁺, and O⁴⁺. There is good agreement between the zero-field PCA results and the intermediate-coupling results when DR takes place through a single core term, as in O⁺, O⁴⁺, and O⁵⁺, but the PCA approximation gives poorer results when DR proceeds through more than one core term, as in O²⁺ and O³⁺. The maximum-field-enhanced PCA results are about a factor of 3 greater than the zero-field results. In the case of the rate coefficients the general formula of Burgess was found to overestimate the zero-field results by a factor of between 1.3 and 1.8 due to the neglect of autoionization into excited states or the averaging over incident angular momentum.