Configuration Interaction Effects in Unresolved 5p^{6}5d^{N+1}−5p^{5}5d^{N+2}+5p^{6}5d^{N}5f^{1} Transition Arrays in Ions Z = 79–92*Atoms* **2017**, *5*(2), 20; doi:10.3390/atoms5020020 - 21 May 2017**Abstract **

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Configuration interaction (CI) effects can greatly influence the way in which extreme ultraviolet (EUV) and soft X-ray (SXR) spectra of heavier ions are dominated by emission from unresolved transition arrays (UTAs), the most intense of which originate from *Δn = 0, 4p*^{6}

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Configuration interaction (CI) effects can greatly influence the way in which extreme ultraviolet (EUV) and soft X-ray (SXR) spectra of heavier ions are dominated by emission from unresolved transition arrays (UTAs), the most intense of which originate from *Δn = 0, 4p*^{6}4d^{N+1}−*4p*^{5}4d^{N+2}+4p^{6}4d^{N}4f^{1} transitions. Changing the principle quantum number *n*, from 4 to 5, changes the origin of the UTA from *Δn = 0, 4p*^{6}4d^{N+1}−*4p*^{5}4d^{N+2}+4p^{6}4d^{N}4f^{1} to *Δn = 0, 5p*^{6}5d^{N+1}−*5p*^{5}5d^{N+2}+5p^{6}5d^{N}5f^{1} transitions. This causes unexpected and significant changes in the impact of configuration interaction from that observed in the heavily studied *n = 4 – n = 4* arrays. In this study, the properties of *n = 5* – *n = 5* arrays have been investigated theoretically with the aid of Hartree-Fock with configuration interaction (HFCI) calculations. In addition to predicting the wavelengths and spectral details of the anticipated features, the calculations show that the effects of configuration interaction are quite different for the two different families of *Δn = 0* transitions, a conclusion which is reinforced by comparison with experimental results.
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Inter-Series Interactions on the Atomic Photoionization Spectra Studied by the Phase-Shifted Multichannel-Quantum Defect Theory*Atoms* **2017**, *5*(2), 21; doi:10.3390/atoms5020021 - 20 May 2017**Abstract **

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Development in mathematical formulations of parameterizing the resonance structures using the phase-shifted multichannel quantum defect theory (MQDT) and their use in analyzing the effect of inter-series interactions on the autoionizing Rydberg spectra is reviewed. Reformulation of the short-range scattering matrix into the form

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Development in mathematical formulations of parameterizing the resonance structures using the phase-shifted multichannel quantum defect theory (MQDT) and their use in analyzing the effect of inter-series interactions on the autoionizing Rydberg spectra is reviewed. Reformulation of the short-range scattering matrix into the form analogous to **S** = **S**_{B}S_{R} in scattering theory are the crucial step in this development. Formulation adopts different directions and goals depending on whether autoionizing series converge to the same limit (degenerate) or to different limits (nondegenerate) because of the different nature of the perturbation. For the nondegenerate case, finding the simplest form of profile index functions of the autoionizing spectra with the minimal number of parameters is the main goal and some results are reviewed. For the degenerate case where perturbation acts uniformly throughout the entire series, isolation of the overlapped autoionizing series into the unperturbed autoionizing series is the key objective in research and some results in that direction are reviewed.
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Radiative Rates and Electron Impact Excitation Rates for Transitions in He II*Atoms* **2017**, *5*(2), 19; doi:10.3390/atoms5020019 - 2 May 2017**Abstract **

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We report calculations of energy levels, radiative rates, collision strengths and effective collision strengths for transitions among the lowest 25 levels of the $n\le $ 5 configurations of He II. The general-purpose relativistic atomic structure package (grasp) and Dirac atomic R-matrix

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We report calculations of energy levels, radiative rates, collision strengths and effective collision strengths for transitions among the lowest 25 levels of the $n\le $ 5 configurations of He II. The general-purpose relativistic atomic structure package (grasp) and Dirac atomic R-matrix code (darc) are adopted for the calculations. Radiative rates, oscillator strengths and line strengths are reported for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2) and magnetic quadrupole (M2) transitions among the 25 levels. Furthermore, collision strengths and effective collision strengths are listed for all 300 transitions among the above 25 levels over a wide energy (temperature) range up to 9 Ryd (10${}^{5.4}$ K). Comparisons are made with earlier available results, and the accuracy of the data is assessed.
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Elementary Processes and Kinetic Modeling for Hydrogen and Helium Plasmas*Atoms* **2017**, *5*(2), 18; doi:10.3390/atoms5020018 - 2 May 2017**Abstract **

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We report cross-sections and rate coefficients for excited states colliding with electrons, heavy particles and walls useful for the description of H${}_{2}$ /He plasma kinetics under different conditions. In particular, the role of the rotational states in resonant vibrational excitations of the

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We report cross-sections and rate coefficients for excited states colliding with electrons, heavy particles and walls useful for the description of H${}_{2}$ /He plasma kinetics under different conditions. In particular, the role of the rotational states in resonant vibrational excitations of the H${}_{2}$ molecule by electron impact and the calculation of the related cross-sections are illustrated. The theoretical determination of the cross-section for the rovibrational energy exchange and dissociation of H${}_{2}$ molecule, induced by He atom impact, by using the quasi-classical trajectory method is discussed. Recombination probabilities of H atoms on tungsten and graphite, relevant for the determination of the nascent vibrational distribution, are also presented. An example of a state-to-state plasma kinetic model for the description of shock waves operating in H${}_{2}$ and He-H${}_{2}$ mixtures is presented, emphasizing also the role of electronically-excited states in affecting the electron energy distribution function of free electrons. Finally, the thermodynamic properties and the electrical conductivity of non-ideal, high-density hydrogen plasma are finally discussed, in particular focusing on the pressure ionization phenomenon in high-pressure high-temperature plasmas.
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In a previous paper (Bhatia A.K. 2016), a hybrid theory for the scattering of positrons from hydrogen atoms was applied to calculate *S*-wave phase shifts, annihilation, and positronium formation cross sections. This approach is now being applied to calculate *P*-wave positron-hydrogen

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In a previous paper (Bhatia A.K. 2016), a hybrid theory for the scattering of positrons from hydrogen atoms was applied to calculate *S*-wave phase shifts, annihilation, and positronium formation cross sections. This approach is now being applied to calculate *P*-wave positron-hydrogen scattering. The present results, obtained using short-range correlation functions along with long-range correlations in the Schrödinger equation at the same time, agree very well with the results obtained in an earlier calculation by Bhatia et al. (1974), using the Feshbach projection operator formalism. In these earlier calculations, the correction due to the long-range correlations was applied to the variational results. In spite of the fact that this ad hoc correction destroyed the variational bound, the final results have been considered accurate. Annihilation cross-sections, positronium formation cross-sections, calculated in the distorted-wave approximation, are also presented.
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Multiconfiguration Dirac-Hartree-Fock Calculations with Spectroscopic Accuracy: Applications to Astrophysics*Atoms* **2017**, *5*(2), 16; doi:10.3390/atoms5020016 - 14 April 2017**Abstract **

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Atomic data, such as wavelengths, spectroscopic labels, broadening parameters and transition rates, are necessary for many applications, especially in plasma diagnostics, and for interpreting the spectra of distant astrophysical objects. The experiment with its limited resources is unlikely to ever be able to

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Atomic data, such as wavelengths, spectroscopic labels, broadening parameters and transition rates, are necessary for many applications, especially in plasma diagnostics, and for interpreting the spectra of distant astrophysical objects. The experiment with its limited resources is unlikely to ever be able to provide a complete dataset on any atomic system. Instead, the bulk of the data must be calculated. Based on fundamental principles and well-justified approximations, theoretical atomic physics derives and implements algorithms and computational procedures that yield the desired data. We review progress and recent developments in fully-relativistic multiconfiguration Dirac–Hartree–Fock methods and show how large-scale calculations can give transition energies of spectroscopic accuracy, i.e., with an accuracy comparable to the one obtained from observations, as well as transition rates with estimated uncertainties of a few percent for a broad range of ions. Finally, we discuss further developments and challenges.
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Shannon Information Entropy in Position Space for the Ground and Singly Excited States of Helium with Finite Confinements*Atoms* **2017**, *5*(2), 15; doi:10.3390/atoms5020015 - 24 March 2017**Abstract **

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We provide benchmark values for Shannon information entropies in position space for the ground state and ls2s ^{1}S^{e} excited state of helium confined with finite confinement potentials by employing the highly correlated Hylleraas-type wave functions. For the excited state, a “tilt”

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We provide benchmark values for Shannon information entropies in position space for the ground state and ls2s ^{1}S^{e} excited state of helium confined with finite confinement potentials by employing the highly correlated Hylleraas-type wave functions. For the excited state, a “tilt” (small oscillation) on the curve of Shannon entropy as a function of width size for the confinement potential is observed. Justified by the behavior of the electron density, the localization or delocalization of the helium wave functions confined with repulsive and attractive finite oscillator (FO) potentials are examined.
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Electroweak Decay Studies of Highly Charged Radioactive Ions with TITAN at TRIUMF

*Atoms* **2017**, *5*(1), 14; doi:10.3390/atoms5010014 - 21 March 2017**Abstract **

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Several modes of electroweak radioactive decay require an interaction between the nucleus and bound electrons within the constituent atom. Thus, the probabilities of the respective decays are not only influenced by the structure of the initial and final states in the nucleus, but

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Several modes of electroweak radioactive decay require an interaction between the nucleus and bound electrons within the constituent atom. Thus, the probabilities of the respective decays are not only influenced by the structure of the initial and final states in the nucleus, but can also depend strongly on the atomic charge. Conditions suitable for the partial or complete ionization of these rare isotopes occur naturally in hot, dense astrophysical environments, but can also be artificially generated in the laboratory to selectively block certain radioactive decay modes. Direct experimental studies on such scenarios are extremely difficult due to the laboratory conditions required to generate and store radioactive ions at high charge states. A new electron-beam ion trap (EBIT) decay setup with the TITAN experiment at TRIUMF has successfully demonstrated such techniques for performing spectroscopy on the radioactive decay of highly charged ions.
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Direct Observation of the M1 Transition between the Ground Term Fine Structure Levels of W VIII*Atoms* **2017**, *5*(1), 13; doi:10.3390/atoms5010013 - 8 March 2017**Abstract **

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We present a direct observation of the M1 transition between the fine structure splitting in the $4{f}^{13}5{s}^{2}5{p}^{6}$ ${}^{2}F$ ground term of W VIII. The spectroscopic data of few-times ionized tungsten ions are important for

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We present a direct observation of the M1 transition between the fine structure splitting in the $4{f}^{13}5{s}^{2}5{p}^{6}$ ${}^{2}F$ ground term of W VIII. The spectroscopic data of few-times ionized tungsten ions are important for the future ITER diagnostics, but there is a serious lack of data. The present study is part of an ongoing effort to solve this problem. Emission from the tungsten ions produced and trapped in a compact electron beam ion trap is observed with a Czerny–Turner visible spectrometer. Spectra in the EUV range are also observed at the same time to help identify the previously-unreported visible lines. The observed wavelength $574.47\pm 0.03$ nm (air), which corresponds to the fine structure splitting of 17,402.5 ± 0.9 cm${}^{-1}$ , shows reasonable agreement with the previously reported value 17,410 ± 5 cm${}^{-1}$ obtained indirectly through the analysis of EUV spectra [Ryabtsev et al., Atoms **3** (2015) 273].
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Spectral Analysis of Moderately Charged Rare-Gas Atoms*Atoms* **2017**, *5*(1), 12; doi:10.3390/atoms5010012 - 7 March 2017**Abstract **

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This article presents a review concerning the spectral analysis of several ions of neon, argon, krypton and xenon, with impact on laser studies and astrophysics that were mainly carried out in our collaborative groups between Argentina and Brazil during many years. The spectra

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This article presents a review concerning the spectral analysis of several ions of neon, argon, krypton and xenon, with impact on laser studies and astrophysics that were mainly carried out in our collaborative groups between Argentina and Brazil during many years. The spectra were recorded from the vacuum ultraviolet to infrared regions using pulsed discharges. Semi-empirical approaches with relativistic Hartree–Fock and Dirac-Fock calculations were also included in these investigations. The spectral analysis produced new classified lines and energy levels. Lifetimes and oscillator strengths were also calculated.
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Resonance Transitions in the Spectra of the Ag^{6+}–Ag^{8+} Ions*Atoms* **2017**, *5*(1), 11; doi:10.3390/atoms5010011 - 4 March 2017**Abstract **

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The spectrum of silver, excited in a vacuum spark, was recorded in the region 150–350 Å on a 3-m grazing incidence spectrograph. The resonance 4d^{k}–(4d^{k−1}5p + 4d^{k−1}4f + 4p^{5}4d^{k+1}) was studied in the

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The spectrum of silver, excited in a vacuum spark, was recorded in the region 150–350 Å on a 3-m grazing incidence spectrograph. The resonance 4d^{k}–(4d^{k−1}5p + 4d^{k−1}4f + 4p^{5}4d^{k+1}) was studied in the Ag^{6+}–Ag^{8+} spectra (Ag VII–Ag IX) with k = 5–3, respectively. Several hundred lines were identified with the aid of the Cowan code and orthogonal operator technique calculations. The energy levels were found and the transition probabilities were calculated.
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The Role of the Hyperfine Structure for the Determination of Improved Level Energies of Ta II, Pr II and La II*Atoms* **2017**, *5*(1), 10; doi:10.3390/atoms5010010 - 28 February 2017**Abstract **

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For the determination of improved energy levels of ionic spectra of elements with large values of nuclear magnetic dipole moment (and eventually large values of nuclear quadrupole moments), it is necessary to determine the center of gravity of spectral lines from resolved hyperfine

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For the determination of improved energy levels of ionic spectra of elements with large values of nuclear magnetic dipole moment (and eventually large values of nuclear quadrupole moments), it is necessary to determine the center of gravity of spectral lines from resolved hyperfine structure patterns appearing in highly resolved spectra. This is demonstrated on spectral lines of Ta II, Pr II and La II. Blend situations (different transitions with accidentally nearly the same wave number difference between the combining levels) must also be considered.
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The Cu II Spectrum*Atoms* **2017**, *5*(1), 9; doi:10.3390/atoms5010009 - 24 February 2017**Abstract **

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New wavelength measurements in the vacuum ultraviolet (VUV), ultraviolet and visible spectral regions have been combined with available literature data to refine and extend the description of the spectrum of singly ionized copper (Cu II). In the VUV region, we measured 401 lines

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New wavelength measurements in the vacuum ultraviolet (VUV), ultraviolet and visible spectral regions have been combined with available literature data to refine and extend the description of the spectrum of singly ionized copper (Cu II). In the VUV region, we measured 401 lines using a concave grating spectrograph and photographic plates. In the UV and visible regions, we measured 276 lines using a Fourier-transform spectrometer. These new measurements were combined with previously unpublished data from the thesis of Ross, with accurate VUV grating measurements of Kaufman and Ward, and with less accurate older measurements of Shenstone to construct a comprehensive list of ≈2440 observed lines, from which we derived a revised set of 379 optimized energy levels, complemented with 89 additional levels obtained using series formulas. Among the 379 experimental levels, 29 are new. Intensities of all lines observed in different experiments have been reduced to the same uniform scale by using newly calculated transition probabilities (*A*-values). We combined our calculations with published measured and calculated *A*-values to provide a set of 555 critically evaluated transition probabilities with estimated uncertainties, 162 of which are less than 20%.
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Calculation of Rates of 4p–4d Transitions in Ar II*Atoms* **2017**, *5*(1), 8; doi:10.3390/atoms5010008 - 21 February 2017**Abstract **

Recent experimental work by Belmonte et al. (2014) has given rates for some 4p–4d transitions that are significantly at variance with the previous experimental work of Rudko and Tang (1967) recommended in the NIST tabulations. To date, there are no theoretical rates with

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Recent experimental work by Belmonte et al. (2014) has given rates for some 4p–4d transitions that are significantly at variance with the previous experimental work of Rudko and Tang (1967) recommended in the NIST tabulations. To date, there are no theoretical rates with which to compare. In this work, we provide such theoretical data. We have undertaken a substantial and systematic configuration interaction calculation, with an extrapolation process applied to ab initio mixing coefficients, which gives energy differences in agreement with experiment. The length and velocity forms give values that are within 10%–15% of each other. Our results are in sufficiently close agreement with those of Belmonte et al. that we can confidently recommend that their results are much more accurate than the early results of Rudko and Tang, and should be adopted in place of the latter.
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Core Effects on Transition Energies for 3d^{k} Conﬁgurations in Tungsten Ions*Atoms* **2017**, *5*(1), 7; doi:10.3390/atoms5010007 - 8 February 2017**Abstract **

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Allenergylevelsofthe3dk,k=1,2,...,8,9,conﬁgurationsfortungstenions,computedusing the GRASP2K fully relativistic code based on the variational multiconﬁguration Dirac–Hartree–Fock method, are reported. Included in the calculations are valence correlation where all 3s,3p,3d orbitals are considered to be valence orbitals, as well as core–valence and core–core effects from the 2s,2p subshells.

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Allenergylevelsofthe3dk,k=1,2,...,8,9,conﬁgurationsfortungstenions,computedusing the GRASP2K fully relativistic code based on the variational multiconﬁguration Dirac–Hartree–Fock method, are reported. Included in the calculations are valence correlation where all 3s,3p,3d orbitals are considered to be valence orbitals, as well as core–valence and core–core effects from the 2s,2p subshells. Results are compared with other recent theory and with levels obtained from the wavelengths of lines observed in the experimental spectra. It is shown that the core correlation effects considerably reduce the disagreement with levels linked directly to observed wavelengths, but may differ signiﬁcantly from the NIST levels, where an unknown shift of the levels could not be determined from experimental wavelengths. For low values of k, levels were in good agreement with relativistic many-body perturbation levels, but for 2 < k < 8, the present results were in better agreement with observation.
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JJ2LSJ Transformation and Unique Labeling for Energy Levels*Atoms* **2017**, *5*(1), 6; doi:10.3390/atoms5010006 - 27 January 2017**Abstract **

The JJ2LSJ program, which is important not only for the GRASP2K package but for the atom theory in general, is presented. The program performs the transformation of atomic state functions(ASFs) from a jj-coupled CSF basis into an LSJ-coupled CSF basis. In addition, the

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The JJ2LSJ program, which is important not only for the GRASP2K package but for the atom theory in general, is presented. The program performs the transformation of atomic state functions(ASFs) from a jj-coupled CSF basis into an LSJ-coupled CSF basis. In addition, the program implements a procedure that assigns a unique label to all energy levels. Examples of how to use the JJ2LSJ program are given. Several cases are presented where there is a unique labeling problem.
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High-Precision Measurements of the Bound Electron’s Magnetic Moment*Atoms* **2017**, *5*(1), 4; doi:10.3390/atoms5010004 - 21 January 2017**Abstract **

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Highly charged ions represent environments that allow to study precisely one or more bound electrons subjected to unsurpassed electromagnetic fields. Under such conditions, the magnetic moment (*g*-factor) of a bound electron changes significantly, to a large extent due to contributions from

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Highly charged ions represent environments that allow to study precisely one or more bound electrons subjected to unsurpassed electromagnetic fields. Under such conditions, the magnetic moment (*g*-factor) of a bound electron changes significantly, to a large extent due to contributions from quantum electrodynamics. We present three Penning-trap experiments, which allow to measure magnetic moments with ppb precision and better, serving as stringent tests of corresponding calculations, and also yielding access to fundamental quantities like the fine structure constant *α* and the atomic mass of the electron. Additionally, the bound electrons can be used as sensitive probes for properties of the ionic nuclei. We summarize the measurements performed so far, discuss their significance, and give a detailed account of the experimental setups, procedures and the foreseen measurements.
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Hyperfine Structure and Isotope Shifts in Dy II*Atoms* **2017**, *5*(1), 5; doi:10.3390/atoms5010005 - 20 January 2017**Abstract **

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Using fast-ion-beam laser-fluorescence spectroscopy (FIBLAS), we have measured the hyperfine structure (hfs) of 14 levels and an additional four transitions in Dy II and the isotope shifts (IS) of 12 transitions in the wavelength range of 422–460 nm. These are the first precision

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Using fast-ion-beam laser-fluorescence spectroscopy (FIBLAS), we have measured the hyperfine structure (hfs) of 14 levels and an additional four transitions in Dy II and the isotope shifts (IS) of 12 transitions in the wavelength range of 422–460 nm. These are the first precision measurements of this kind in Dy II. Along with hfs and IS, new undocumented transitions were discovered within 3 GHz of the targeted transitions. These atomic data are essential for astrophysical studies of chemical abundances, allowing correction for saturation and the effects of blended lines. Lanthanide abundances are important in diffusion modeling of stellar interiors, and in the mechanisms and history of nucleosynthesis in the universe. Hfs and IS also play an important role in the classification of energy levels, and provide a benchmark for theoretical atomic structure calculations.
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Combining Multiconfiguration and Perturbation Methods: Perturbative Estimates of Core–Core Electron Correlation Contributions to Excitation Energies in Mg-Like Iron*Atoms* **2017**, *5*(1), 3; doi:10.3390/atoms5010003 - 12 January 2017**Abstract **

Large configuration interaction (CI) calculations can be performed if part of the interaction is treated perturbatively. To evaluate the combined CI and perturbative method, we compute excitation energies for the $3l3{l}^{\prime}$ , $3l4{l}^{\prime}$ and $3$

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Large configuration interaction (CI) calculations can be performed if part of the interaction is treated perturbatively. To evaluate the combined CI and perturbative method, we compute excitation energies for the $3l3{l}^{\prime}$ , $3l4{l}^{\prime}$ and $3s5l$ states in Mg-like iron. Starting from a CI calculation including valence and core–valence correlation effects, it is found that the perturbative inclusion of core–core electron correlation halves the mean relative differences between calculated and observed excitation energies. The effect of the core–core electron correlation is largest for the more excited states. The final relative differences between calculated and observed excitation energies is 0.023%, which is small enough for the calculated energies to be of direct use in line identifications in astrophysical and laboratory spectra.
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An Investigation on the *He*^{−}(1*s*2*s*^{2}^{2}*S*) Resonance in Debye Plasmas*Atoms* **2017**, *5*(1), 2; doi:10.3390/atoms5010002 - 11 January 2017**Abstract **

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The effect of Debye plasma on the $1s2{s}^{2}\phantom{\rule{3.33333pt}{0ex}}{}^{2}S$ resonance states in the scattering of electron from helium atom has been investigated within the framework of the stabilization method. The interactions among the charged particles in Debye plasma

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The effect of Debye plasma on the $1s2{s}^{2}\phantom{\rule{3.33333pt}{0ex}}{}^{2}S$ resonance states in the scattering of electron from helium atom has been investigated within the framework of the stabilization method. The interactions among the charged particles in Debye plasma have been modelled by Debye–Huckel potential. The $1s2s$ excited state of the helium atom has been treated as consisting of a $H{e}^{+}$ ionic core plus an electron moving around. The interaction between the core and the electron has then been modelled by a model potential. It has been found that the background plasma environment significantly affects the resonance states. To the best of our knowledge, such an investigation of $1s2{s}^{2}\phantom{\rule{3.33333pt}{0ex}}{}^{2}S$ resonance states of the electron–helium system embedded in Debye plasma environment is the first reported in the literature.
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