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 3s2 1S --> 3s3p 3P transition in the Mg-like ions Ar6+, Ti10+, Fe14+ , and S e22+. We obtain good agreement between the results of the two theoreti cal 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 inter acting resonance effects, over a wide range of energies for all of the ions examined. For the case of Fe14+, We have continued the perturbat ive studies of interacting resonances of Griffin et al. [Phys. Rev. Le tt. 72, 3491 (1994)] and also find the predominant effect to be due to direct configuration interaction between the resonant states rather t han being due to their coupling through a common continuum. Our inclus ion of these interacting resonance effects removes the major discrepan cies between the perturbative and nonperturbative results for Fe14+ re ported previously [Badnell et al., Phys. Rev. A 48, R2519 (1993)]. Thi s is of importance for the study of indirect processes which are stron gly radiation damped, for which the perturbative approach is widely us ed and the standard formulation of the close-coupling approximation is not applicable.