Electron-correlation effects on the 3 C to 3 D line-intensity ratio in the Ne-like ions Ar 8 + to Kr 26 +

We present a theoretical and experimental investigation of the $3d\\ensuremath{\\rightarrow}2p$ resonance to the intercombination line ratio in low- to mid-$Z$ neonlike ions of astrophysical interest, i.e., of the $2{p}_{1/2}2{p}_{3/2}^{4}3{d}_{3/2}{}^{1}{P}_{1}^{o}\\ensuremath{\\rightarrow}2{p}^{6}{\\phantom{\\rule{0.16em}{0ex}}}^{1}{S}_{0}$ and $2{p}_{1/2}^{2}2{p}_{3/2}^{3}3{d}_{5/2}\\phantom{\\rule{0.16em}{0ex}}{}^{3}{D}_{1}^{o}\\ensuremath{\\rightarrow}2{p}^{6}{\\phantom{\\rule{0.16em}{0ex}}}^{1}{S}_{0}$ transitions commonly labeled $3C$ and $3D$, respectively. In particular, we have employed the configuration-interaction method with three different numbers of basis states and the many-body perturbation theory method to calculate oscillator strengths and energies for neonlike ions from $Z=18$ to 36. Combining our calculations with a systematic study of previous works in the literature, we show that these methods can predict accurate and converged energies for these transitions. We also find convergence for the oscillator strengths, but the ratio of oscillator strengths, which can be compared to experimental values of the relative intensity ratios of these lines, appears to converge to values higher than measured. We speculate that this is due to the role of electron-electron correlations. While the amount of electron correlations associated with the intercombination line $3D$ appears to be well described, it seems that the contributions from highly excited states are not sufficiently accounted for in the case of the resonance line $3C$. In order to augment the body of available experimental data for neonlike ions, we present a measurement of the $3C$ and $3D$ lines in neonlike ${\\mathrm{Ar}}^{8+}$. We report a wavelength of $41.480\\ifmmode\\pm\\else\\textpm\\fi{}0.001$ \\AA{} for line $3C$ and $42.005\\ifmmode\\pm\\else\\textpm\\fi{}0.001$ \\AA{} for line $3D$. The intensity ratio of the two lines was determined to be $I(3C)/I(3D)=11.32\\ifmmode\\pm\\else\\textpm\\fi{}1.40$.