Abstract
We present a theoretical and experimental investigation of the 3d -> 2p resonance to the intercombination line ratio in low-to mid-Z neonlike ions of astrophysical interest, i.e., of the 2p(1/2)2p(3/2)(4)3d(3/2)(1)P(1)(0) -> 2p(6) S-1(0) and 2p(1/2)(2)2p(3/2)(3)3d(5/2)(3)D(1)(0) -> 2p(6) S-1(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 Ar8+. We report a wavelength of 41.480 +/- 0.001 angstrom for line 3C and 42.005 +/- 0.001 angstrom for line 3D. The intensity ratio of the two lines was determined to be I(3C)/I(3D) = 11.32 +/- 1.40.