Electron-correlation effects on the3Cto3Dline-intensity ratio in the Ne-like ionsAr8+

Abstract

We present a theoretical and experimental investigation of the 3d → 2p resonance to intercombination line ratio in low-to mid-Z neonlike ions of astrophysical interest, i.e., of the 2p 1/2 2p 4 3/2 3d 3/2 1 P o 1 → 2p 6 1 S 0 and 2p 2 1/2 2p 3 3/2 3d 5/2 3 D o 1 → 2p 6 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 Z = 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 Ar 8+. We report a wavelength of 41.480 ± 0.001Å for line 3C and 42.005 ± 0.001Å for line 3D. The intensity ratio of the two lines was determined to be I(3C)/I(3D) = 11.32 ± 1.40.

X-ray fluorescence spectrum of highly charged Fe ions driven by strong free-electron-laser fields

“…However, small differences in the results obtained from distinct atomic many-body calculations were reported. Our CI-DFS method predicts a value of 3.56, and other recent theoretical predictions are in the range 3.43 -3.66 [23,[47][48][49][50][51][52]. We plot, for Gaussian pulses in Fig.…”

Resonance-to-intercombination-line ratios of neonlike ions in the relativistic regime

“…3 as a function of Z together with the values from the previous measurement with Z ≥ 28. Here, we compare the measured values with two theoretical calculations by Santana et al -one based on a CI model that considers 816 states up to triply excited n = 3 levels and the other based on the fully relativistic second order MBPT method [9]. In addition, the figure includes the theoretical values from the fully relativistic distorted-wave model of Zhang and Sampson [33] and the values from a configuration-interaction calculation by Hibbert et al [34].…”

“…Therefore, discrepancies between experiment and theory, if caused by uncalculated correlation effects, should be largest for Ar 8+ and diminish for Kr 26+ . Indeed, this is seen in a recent study presented by Santana et al [9] in which they employed configurationinteraction (CI) calculations with a limited set of configurations. When using second-order many-body perturbation theory (MBPT), which accounts for a very large fraction of the correlation effects, the discrepancy between theory and experiment disappeared as expected for the lowest-Z neonlike ions.…”

“…The observed ratio of the two lines does not readily agree with theoretical predictions. Over the past twenty years, it has been shown that this disagreement extends over a wide range of neonlike ions between Ar 8+ and Kr 26+ [3][4][5][6][7][8][9]. In the case of Fe 16+ , it was shown that both the absolute and relative electron-impact excitation cross sections [10] as well as the relative oscillator strengths [11] differ from theory.…”

Guest Editor’s Notes on the “Atoms” Special Issue on “Perspectives of Atomic Physics with Trapped Highly Charged Ions”

“…A photo-excitation experiment using the FLASH-EBIT at the LCLS X-ray light source moreover corroborated the Livermore EBIT finding (in agreement with astrophysical observations and tokamak work) that there is significant disagreement between the present-day theoretical predictions of the 3C/3D line ratio in Fe XVII and experiment under even the cleanest experimental conditions realized so far [41]. By the way, Fe XVII may be the best/most researched member of the isoelectronic sequence in this case, because of its astrophysical implications, but theory (atomic structure computation) is still struggling with the 3C/3D line ratio for the whole sequence [42].…”