Breakdown of the isolated-resonance approximation for the electron-impact excitation of positive ions

We have carried out a series of calculations, the results of which demonstrate the breakdown of the isolated-resonance approximation for the electron-impact excitation of positive ions. The specific results presented here, namely for the 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$S→3s3p ${}_{}{}^3{}_{}{}^{}$P transition in ${\mathrm{Fe}}^{14+}$, are not unique. All of the Mg-like ions that we have investigated so far exhibit the same breakdown. There is no reason to believe either that the Mg-like sequence is unique. This has important implications for the description of resonant processes that are strongly radiation damped, for which the isolated-resonance approximation is widely used and the standard formulation of the close-coupling approximation is not applicable.