Comprehensive theoretical and experimental analysis of Coster-Kronig electron spectra from 64-MeV ${\mathrm{S}}^{12+}$ ions excited through He gas and C-foil targets

In Coster-Kronig electrons from the autoionizng Rydberg states of 64 MeV ${\mathrm{S}}^{12+}$ ions excited through He gas and very thin C-foil targets, several peaks have been observed in the electron energy range up to 2.5 eV. To obtain a better understanding on low-energy electron production mechanisms in such collisions, we synthesize the expected electron spectrum by assuming that the spectral (energy) distribution of each line can be expressed with a Gaussian profile with proper resolution and compare that with the observed spectrum. Two theoretical methods, namely, the perturbation theory of the Z-expansion (MZ code) and multiconfiguration Hartree-Fock method (Cowan code) are used to calculate Auger electron energies and rates. It is found that the $1s2p15l–1s2s{}^{1}S,\hspace{0.5em}1s2p13l–1s2s{}^{3}S,\hspace{0.5em}{1s}^{2}2p9l–{1s}^{2}2s,\hspace{0.5em}{1s}^{2}2s2p9l–{1s}^2{2s}^2,\hspace{0.5em}{1s}^2{2p}^{2}7l–{1s}^{2}2s2p{}^{3}P,$ and ${1s}^2{2p}^{2}9l–{1s}^{2}2s2p{}^{1}P$ decays give a significant contribution in the electron spectrum near threshold in the range of 0–2.5 eV. Synthetic spectra have been found to reproduce the observed spectra reasonably well if the ion charge distributions are taken into account properly. Based upon the synthetic electron spectra from particular core configurations best fitted to the observed spectra, we have inferred the ion charge “inside” foils which can be compared with the well-established equilibrium charge “after” foil.