Interacting resonance effects along an isoelectronic sequence: Electron-impact excitation of Mg-like ions

We have carried out a series of isolated-resonance and R-matrix close-coupling calculations for the electron-impact excitation of the 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$S→3s3p ${}_{}{}^3{}_{}{}^{}$P transition in the Mg-like ions ${\mathrm{Ar}}^{6+}$, ${\mathrm{Ti}}^{10+}$, ${\mathrm{Fe}}^{14+}$, and ${\mathrm{Se}}^{22+}$. We obtain good agreement between the results of the two theoretical methods for three-state calculations, i.e., for the description of just a single Rydberg series of resonances. However, when two or more Rydberg series are present we observe large differences, due to interacting resonance effects, over a wide range of energies for all of the ions examined. For the case of ${\mathrm{Fe}}^{14+}$, we have continued the perturbative studies of interacting resonances of Griffin et al. [Phys. Rev. Lett. 72, 3491 (1994)] and also find the predominant effect to be due to direct configuration interaction between the resonant states rather than being due to their coupling through a common continuum. Our inclusion of these interacting resonance effects removes the major discrepancies between the perturbative and nonperturbative results for ${\mathrm{Fe}}^{14+}$ reported previously [Badnell et al., Phys. Rev. A 48, R2519 (1993)]. This is of importance for the study of indirect processes which are strongly radiation damped, for which the perturbative approach is widely used and the standard formulation of the close-coupling approximation is not applicable.