## Extended Calculations with Spectroscopic Accuracy: Energy Levels and Transition Properties for the Fluorine-like Isoelectronic Sequence with Z = 24-30

Si, R.; Li, S.; Guo, X. L.; Chen, Z. B.; Brage, T.; Jönsson, P.; Wang, K.; Yan, J.; Chen, C. Y.; Zou, Y. M.

We have performed extensive multiconfiguration Dirac-Hartree-Fock calculations and second-order many-body perturbation calculations for F-like ions with Z = 24-30. Energy levels and transition rates for electric dipole (E1), electric-quadrupole (E2), electric-octupole (E3), magnetic dipole (M1), and magnetic-quadrupole (M2) transitions, as well as radiative lifetimes, are provided for the lowest 200 levels belonging to the 1{s}22{s}22{p}5, 1{s}22s2{p}6, 1{s}22{s}22{p}43l, 1{s}22s2{p}53l, 1{s}22{p}63l, and 1{s}22{s}22{p}44l configurations of each ion. The results from the two sets of calculations are in excellent agreement. Extensive comparisons are also made with other theoretical results and observed data from the CHIANTI and NIST databases. The present energies and wavelengths are believed to be accurate enough to aid line identifications involving the n = 3 and n = 4 configurations, for which observations are largely missing. The calculated wavelengths and transition data will be useful in the modeling and diagnostics of astrophysical and fusion plasmas.

## MCDHF and RCI calculations of energy levels, lifetimes and transition rates for 3l3l’, 3l4l’, and 3s5l states in Ca IX – As XXII and Kr XXV

Gustafsson, S.; Jönsson, P.; Froese Fischer, C.; Grant, I. P.

Multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations and relativistic configuration interaction (RCI) calculations were performed for states of the 3l3l’, 3l4l’ and 3s5l configurations in the Mg-like ions Ca IX – As XXII and Kr XXV. Valence and core-valence electron correlation effects are accounted for through large configuration state function expansions. Calculated excitation energies are in very good agreement with observations for the lowest levels. For higher lying levels observations are often missing and present energies aid line identification in spectra. Lifetimes and transition data are given for all ions. There is an excellent agreement for both lifetimes and transition data with recent multiconfiguration Hartree-Fock Breit Pauli calculations.

## Improved Cr II log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937

Lawler, J. E.; Sneden, C.; Nave, G.; Den Hartog, E. A.; Emrahoğlu, N.; Cowan, J. J.

New emission branching fraction (BF) measurements for 183 lines of the second spectrum of chromium (Cr ii) and new radiative lifetime measurements from laser-induced fluorescence for 8 levels of Cr+ are reported. The goals of this study are to improve transition probability measurements in Cr ii and reconcile solar and stellar Cr abundance values based on Cr i and Cr ii lines. Eighteen spectra from three Fourier Transform Spectrometers supplemented with ultraviolet spectra from a high-resolution echelle spectrometer are used in the BF measurements. Radiative lifetimes from this study and earlier publications are used to convert the BFs into absolute transition probabilities. These new laboratory data are applied to determine the Cr abundance log ε in the Sun and metal-poor star HD 84937. The mean result in the Sun is < {log}\varepsilon ({Cr} {{II}})> = 5.624 ± 0.009 compared to < {log}\varepsilon ({Cr} {{I}})> = 5.644 ± 0.006 on a scale with the hydrogen abundance log ε(H) = 12 and with the uncertainty representing only line-to-line scatter. A Saha (ionization balance) test on the photosphere of HD 84937 is also performed, yielding < {log}\varepsilon ({Cr} {{II}})> = 3.417 ± 0.006 and <log ε(Cr i, lower level excitation potential E. P. > 0 eV)> = 3.374 ± 0.011 for this dwarf star. We find a correlation of Cr with the iron-peak element Ti, suggesting an associated nucleosynthetic production. Four iron-peak elements (Cr along with Ti, V, and Sc) appear to have a similar (or correlated) production history—other iron-peak elements appear not to be associated with Cr.

## Experimental and theoretical oscillator strengths of Mg I for accurate abundance analysis

Pehlivan Rhodin, A.; Hartman, H.; Nilsson, H.; Jonsson, P.

Context. With the aid of stellar abundance analysis, it is possible to study the galactic formation and evolution. Magnesium is an important element to trace the alpha-element evolution in our Galaxy. For chemical abundance analysis, such as magnesium abundance, accurate and complete atomic data are essential. Inaccurate atomic data lead to uncertain abundances and prevent discrimination between different evolution models. Aims. We study the spectrum of neutral magnesium from laboratory measurements and theoretical calculations. Our aim is to improve the oscillator strengths ( f -values) of Mg I lines and to create a complete set of accurate atomic data, particularly for the near-IR region. Methods. We derived oscillator strengths by combining the experimental branching fractions with radiative lifetimes reported in the literature and computed in this work. A hollow cathode discharge lamp was used to produce free atoms in the plasma and a Fourier transform spectrometer recorded the intensity-calibrated high-resolution spectra. In addition, we performed theoretical calculations using the multiconfiguration Hartree-Fock program ATSP2K. Results. This project provides a set of experimental and theoretical oscillator strengths. We derived 34 experimental oscillator strengths. Except from the Mg I optical triplet lines (3p 3^P^o_0,1,2 – 4s 3^S_1), these oscillator strengths are measured for the first time. The theoretical oscillator strengths are in very good agreement with the experimental data and complement the missing transitions of the experimental data up to n = 7 from even and odd parity terms. We present an evaluated set of oscillator strengths, gf, with uncertainties as small as 5%. The new values of the Mg I optical triplet line (3p 3^P^o_0,1,2 – 4s 3^S_1) oscillator strength values are ~0.08 dex larger than the previous measurements.

Yu, Yanmei; Derevianko, Andrei

We tabulate spontaneous emission rates for all possible 811 electric-dipole-allowed transitions between the 75 lowest-energy states of Ca I. These involve the $4sns$ ($n=4-8$), $4snp$ ($n=4-7$), $4snd$ ($n=3-6$), $4snf$ ($n=4-6$), $4p^2$, and $3d4p$ electronic configurations. We compile the transition rates by carrying out ab initio relativistic calculations using the combined method of configuration interaction and many-body perturbation theory. The results are compared to the available literature values. The tabulated rates can be useful in various applications, such as optimizing laser cooling in magneto-optical traps, estimating various systematic effects in optical clocks and evaluating static or dynamic polarizabilities and long-range atom-atom interaction coefficients and related atomic properties.

## Hyperfine structure and isotope shifts in Mo II

S.D. Rosner, R.A. Holt

We have applied fast-ion-beam laser-fluorescence spectroscopy to measure the hyperfine structures of seven even-parity and six odd-parity levels in Mo II and the isotope shifts of nine transitions in the wavelength range 424.5–443.4 nm. These are the first measurements of these parameters and almost the first such measurements in Mo II. These atomic data are essential for astrophysical studies of chemical abundances, allowing correction for saturation and the effects of blended lines. They also provide important constraints on stellar diffusion modeling, particularly in chemically peculiar (CP) stars, and serve as valuable input to semi-empirical theoretical models of hyperfine structure and isotope shifts.

Article -> http://www.nrcresearchpress.com/doi/10.1139/cjp-2016-0192#.WCwazeErI7w

## The Opacity Project: computational methods

James Colgan

Los Alamos National Laboratory, Los Alamos, NM 87545, USA

This viewpoint relates to an article by K A Berrington et al (1987 J. Phys. B: At. Mol. Opt. Phys. 20 6379–97) and was published as part of a series of viewpoints celebrating 50 of the most influential papers published in the Journal of Physics series, which is celebrating its 50th anniversary. ==> Complete viewpoint