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The exponentially correlated Hylleraas–configuration interaction method (E-Hy-CI) is a generalization of the Hylleraas–configuration interaction method (Hy-CI) in which the single rij of an Hy-CI wave function is generalized to a form of the generic type rijνije−ωijrij. This work continues the exploration, begun in the first two papers in this series (on the helium atom and on ground and excited S states of Li II), of whether wave functions containing both linear and exponential rij factors converge more rapidly than either one alone. In the present study, we examined not only 1s2 1S states but 1s2p 1P states for the He I, Li II, Be III, C V and O VII members of the He isoelectronic sequence as well. All 1P energies except He I are better than previous results. The wave functions obtained were used to calculate oscillator strengths, including upper and lower bounds, for the He-sequence lowest (resonance) 1S→1P transition. Interpolation techniques were used to make a graphical study of the oscillator strength behavior along the isoelectronic sequence. Comparisons were made with previous experimental and theoretical results. The results of this study are oscillator strengths for the 1s2 1S→ 1s2p1P He isoelectronic sequence with rigorous non-relativistic quantum mechanical upper and lower bounds of (0.001–0.003)% and probable precision ≤ 0.0000003, and were obtained by extending the previously developed E-Hy-CI formalism to include the calculation of transition moments (oscillator strengths).

The excitation energies of the Be-like ions (4 ≤ Z ≤ 74) doubly excited state 1 s 2 2 p 2 and doubly core-excited state 2 s 2 p 3 are calculated using the multi-configuration Dirac–Hartree–Fock method. The contributions of relativistic effects, Breit interaction, and QED effects to the excitation energies are analyzed and discussed in detail. A reasonable electron correlation model has been constructed using the active set method. The electron correlation and the finite size of the atomic nucleus correction have also been appropriately considered. The calculated results agree with the available theoretical and experimental data. It is found that the contribution of relativistic effects, Breit interaction, and QED effects to the excitation energy becomes significant with increasing Z . The contribution of the relativistic effects reaches about 8.5% at Z =74. The 3 P 0 level of the doubly excited state 1 s 2 2 p 2 is sensitive to the Breit interaction and the QED effects, but these two contributions cancel each other out. For the doubly core-excited state 2 s 2 p 3 , the Breit interaction and the QED effects are almost equally important, and their maximum contribution is about 0.5%. These results are expected to help future experimental and theoretical work. Graphical abstract

PyNeb is a Python package widely used to model emission lines in gaseous nebulae. We take advantage of its object-oriented architecture, class methods, and historical atomic database to structure a practical environment for atomic data assessment. Our aim is to reduce the uncertainties in the parameter space (line ratio diagnostics, electron density and temperature, and ionic abundances) arising from the underlying atomic data by critically selecting the PyNeb default datasets. We evaluate the questioned radiative-rate accuracy of the collisionally excited forbidden lines of the N- and P-like ions (O ii, Ne iv, S ii, Cl iii, and Ar iv), which are used as density diagnostics. With the aid of observed line ratios in the dense NGC 7027 planetary nebula and careful data analysis, we arrive at emissivity ratio uncertainties from the radiative rates within 10%, a considerable improvement over a previously predicted 50%. We also examine the accuracy of an extensive dataset of electron-impact effective collision strengths for the carbon isoelectronic sequence recently published. By estimating the impact of the new data on the pivotal [N ii] and [O iii] temperature diagnostics and by benchmarking the collision strength with a measured resonance position, we question their usefulness in nebular modeling. We confirm that the effective-collision-strength scatter of selected datasets for these two ions does not lead to uncertainties in the temperature diagnostics larger than 10%.

Highly charged ions (HCIs) are less sensitive to external perturbations and are therefore attractive for the development of ultrastable clocks. However, only a few HCI candidates are known to provide optical clock transitions. In this work, we discover a large family of HCI clocks, with more than 100 suitable optical clock transitions hidden in the fine-structure terms of open-shell ions over 70 elements. Their projected instabilities and accuracies are σ τ ~ 1 0 − 17 / τ and δ ν / ν  < 10 −20 , respectively, surpassing state-of-the-art optical clocks by several orders of magnitude. This indicates that having a high-performance optical clock transition is not a property of particular elements, but a virtue host by most elements from the periodic table. Furthermore, at given configurations, the clock transitions in heavy ions scale up to the XUV and soft-x-ray region, and thus enable the development of ultrastable clocks based on shorter wavelengths. The existence of multiple clock transitions in different charge states of a single element, as well as in a whole isoelectronic sequence would significantly enrich the search for new physics and the test of nuclear theories via high-precision spectroscopy. Highly charged ions are attractive for the development of next-generation atomic clocks. In this work, the authors identify more than 100 highly charged ion clock candidates whose performance could surpass state-of-the-art optical clocks by several orders of magnitude.

The Shannon entropy in an LS-coupled configuration space has been calculated through a transformation from that in a jj-coupled configuration space for a Ni-like isoelectronic sequence. The sudden change of Shannon entropy, information exchange, eigenlevel anticrossing, and strong configuration interaction have been presented for adjacent levels. It is shown that eigenlevel anticrossing is a sufficient and necessary condition for the sudden change of Shannon entropy, and both of them are a sufficient condition for information exchange, which is the same as the case of the jj-coupled configuration space. It is found that the structure of sudden change from jj-coupled into LS-coupled configuration spaces through the LS-jj transformation is invariant for Shannon entropy along the isoelectronic sequence. What is more, in an LS-coupled configuration space, there are a large number of information exchanges between energy levels whether with or without strong configuration interaction, and most of the ground and single excited states of Ni-like ions are more suitable to be described by a jj-coupled or other configuration basis set instead of an LS-coupled configuration basis set according to the configuration mixing coefficients and their Shannon entropy. In this sense, Shannon entropy can also be used to measure the applicability of a configuration basis set or the purity of atomic state functions in different coupling schemes.

Synopsis Emission and absorption features from C-like ions are served as temperature and density diagnostics of astrophysical plasmas. R-matrix electron-impact excitation data for C-like ions in literature cover merely a handful of ions. Here we present a systematic intermediate coupling frame transformation R-matrix calculations for C-like ions from N II to Kr XXXI. The effective collision strengths cover a wide range of temperature. Our results are compared with archival databases and literature results.

Simulated (or measured) photoabsorption spectra often provide the first indication of how matter interacts with light when irradiated by some radiation source. In addition to the direct, often slowly varying photoabsorption cross-section as a function of the incident photon frequency, such spectra typically exhibit numerous resonances and edges arising from the interaction of the radiation field with the subvalence or even inner-shell electrons. Broadly speaking, these resonances reflect photoexcitation, with its subsequent fluorescence, or the autoionization of bound electrons. Here, a (relativistic) cascade model is developed for estimating the photoabsorption of (many) atoms and multiply charged ions with a complex shell structure across the periodic table. This model helps distinguish between level- and shell-resolved, as well as total photoabsorption, cross-sections, starting from admixtures of selected initial-level populations. Examples are shown for the photoabsorption of C+ ions near the 1s − 2p excitation threshold and for Xe2+ ions in the photon energy range from 10 to 200 eV. While the accuracy and resolution of the predicted photoabsortion spectra remain limited due to the additive treatment of resonances and because of missing electronic correlations in the representation of the levels involved, the present implementation is suitable for ions with quite different open-shell structures and may support smart surveys of resonances along different isoelectronic sequences.

Synopsis The K-shell dielectronic (DR), trielectronic (TR), and quadruelectronic (QR) recombination have been systematically studied for highly charged B-like ions with Z=14-50 by using the FAC code. Strong configuration mixing are found that significant influence the Auger rate, and TR strengths, and leads to irregular changes of TR/DR strength ratio along B-like sequence.

This study reports on the absolute photoionization cross sections for the magnesium-like Cl5+ ion over the 190–370 eV photon energy range, corresponding to the L-shell (2s and 2p subshells) excitation regime. The experiments were performed using the Multi-Analysis Ion Apparatus (MAIA) on the PLéIADES beamline at the SOLEIL synchrotron radiation storage ring facility. Single and double ionization ion yields, produced by photoionization of the 2p subshell of the Cl5+ ion from the 2p63s2 1S0 ground state and the 2p63s3p 3P0,1,2 metastable levels, were observed, as well as 2s excitations. Theoretical calculations of the photoionization cross sections using the Multi-Configuration Dirac-Fock and R-matrix approaches were carried out, and the results were compared with the experimental data. The Cl5+ results were examined within the overall evolution of L-shell excitation for the early members of the Mg-like isoelectronic sequence (Mg, Al+, Si2+, S4+, Cl5+). Characteristic photon energies for P3+ were estimated by interpolation.

Synopsis A detailed investigation of Si-like dielectronic recombination is performed for the entire Si-like isoelectronic sequence using a perturbative multi-configurational Breit-Pauli method.