[en] A recent measurement of the lifetime of the excited 3p⁵ ${}^2{P}_{1/2}$ state in the S⁻ negative ion, which is dominated by a forbidden magnetic dipole transition to the ${}^2{P}_{3/2}$ ground state, reveals a discrepancy with earlier theoretical predictions. To investigate this we have performed systematic and large-scale multiconfiguration Dirac–Hartree–Fock calculations for this system. After including a careful treatment of correlation and relativistic effects, we predict a well-converged value for this lifetime, with an uncertainty considerably less than 1%, thereby removing the apparent conflict between theory and experiment. We also show that this result corresponds to the non-relativistic limit in the LS coupling approximation for the magnetic dipole transition within this ² P term. In addition we demonstrate the usefulness of the latter approach for ² P transitions in O⁻, Se⁻ and Te⁻, as well as for analogous M1 transitions within ² D terms in Ni⁻ and Pt⁻ ions. (paper)

[en] We present a theoretical investigation of the hyperfine structure of the 5s5d ³D₂-5s4f ³F⁰_2,3 transitions in In II. Earlier work has failed in determining hyperfine constants for the upper levels of these transitions. We show that this is due to strong off-diagonal hyperfine interaction, which not only changes the position of the individual hyperfine lines but also introduces large intensity redistributions among the different hyperfine levels. We present hyperfine dependent gf-values and show that off-diagonal hyperfine interaction reduces some of the gf-values by two orders of magnitude, while others are increased by up to more than a factor of 6. We also discuss the influence on the hyperfine structure of an accurate representation of the level-splitting of the 5s4f configuration. We show that the hyperfine interaction in ³F⁰₃ and ¹F⁰₃ is very hard to determine accurately even in a large-scale calculation, and we derive a semi-empirical method for adjusting our results using an experimentally known, diagonal hyperfine constant for 5s4f ¹F⁰₃. The resulting theoretical synthetic spectra reproduce the experimental values to high accuracy and facilitate the identification of all observed lines.

[en] Advances in merged-beams instruments have allowed experimental studies of the mutual neutralization (MN) processes in collisions of both Li⁺ and Na⁺ ions with D⁻ at energies below 1 eV. These experimental results place constraints on theoretical predictions of MN processes of Li⁺ and Na⁺ with H⁻, important for non-LTE modeling of Li and Na spectra in late-type stars. We compare experimental results with calculations for methods typically used to calculate MN processes, namely the full quantum (FQ) approach, and asymptotic model approaches based on the linear combination of atomic orbitals (LCAO) and semiempirical (SE) methods for deriving couplings. It is found that FQ calculations compare best overall with the experiments, followed by the LCAO, and the SE approaches. The experimental results together with the theoretical calculations, allow us to investigate the effects on modeled spectra and derived abundances and their uncertainties arising from uncertainties in the MN rates. Numerical experiments in a large grid of 1D model atmospheres, and a smaller set of 3D models, indicate that neglect of MN can lead to abundance errors of up to 0.1 dex (26%) for Li at low metallicity, and 0.2 dex (58%) for Na at high metallicity, while the uncertainties in the relevant MN rates as constrained by experiments correspond to uncertainties in abundances of much less than 0.01 dex (2%). This agreement for simple atoms gives confidence in the FQ, LCAO, and SE model approaches to be able to predict MN with the accuracy required for non-LTE modeling in stellar atmospheres.

[en] Unexpected transitions are induced by weaker interactions not included in the gross structure model of the ion under investigation. We discuss different examples of such decay channels, starting with relativistic spin-induced transitions. These represented an important field of study a few decades ago, and we illustrate how some challenging cases can be treated very accurately with today's computational techniques, while close degeneracy sometimes still prevents ab initio methods from obtaining accurate results. For hyperfine induced transitions we review some recent results and discuss remaining challenges for experiment and theory. Finally, we discuss the newly opened field of accurate calculations for transitions induced by an external magnetic field and point to some examples of where these are accessible for experimental tests. (paper)

[en] We present a measured value for the degree of pseudo-degeneracy between two fine-structure levels in Fe⁹⁺ from line intensity ratios involving a transition induced by an external magnetic field. The extracted fine-structure energy difference between the $3p^{4}3d{}^4{D}_{5/2}$ and ${}^4{D}_{7/2}$ levels, where the latter is the upper state for the magnetic-field induced line, is needed in our recently proposed method to measure magnetic-field strengths in the solar corona. The intensity of the $3p^{4}3d{}^4{D}_{7/2}\to <!--\; ???\; -->3p^5{}^2{P}_{3/2}$ line at 257.262 Å is sensitive to the magnetic field external to the ion. This sensitivity is in turn strongly dependent on the energy separation in the pseudo-degeneracy through the mixing induced by the external magnetic field. Our measurement, which uses an Electron Beam Ion Trap with a known magnetic-field strength, indicates that this energy difference is 3.5 cm⁻¹. The high abundance of Fe⁹⁺ and the sensitivity of the line’s transition probability to field strengths below 0.1 T opens up the possibility of diagnosing coronal magnetic fields. We propose a new method to measure the magnetic field in the solar corona, from similar intensity ratios in Fe⁹⁺. In addition, the proposed method to use the line ratio of the blended line $3p^{4}3d{}^4{D}_{7/2,5/2}\to <!--\; ???\; -->3p^5{}^2{P}_{3/2}$ with another line from Fe x as the density diagnostic should evaluate the effect of the magnetic-field-induced transition line.

[en] We report on theoretical results of magnetic field induced transitions (MITs) in Ne- and Be-like ions without nuclear spin for two applications. Firstly, MITs are promising candidates in the determination of magnetic fields in plasmas. In our work on Ne-like ions we present accurate theoretical MIT rates for 2p^{6 1}S_o – 2p⁵3s ³P_o,2 [1]. Furthermore, for Be-like ions, it has been proposed to extract the rate of the E1M1 two-photon transition 2s²¹S₀ – 2s2p ³P₀ by measuring the lifetime of the ³P₀ state using a storage ring, which involves an external magnetic field. The MIT rates are carefully evaluated and shown to be of the same order as the E1M1 rates [2].

[en] For all involved in astronomy, the importance of monitoring and determining astrophysical magnetic-field strengths is clear. It is also a well-known fact that the corona magnetic fields play an important part in the origin of solar flares and the variations of space weather. However, after many years of solar corona studies, there is still no direct and continuous way to measure and monitor the solar magnetic-field strength. We present here a scheme that allows such a measurement, based on a careful study of an exotic class of atomic transitions, known as magnetic induced transitions, in Fe⁹⁺. In this contribution we present a first application of this methodology and determine a value of the coronal field strength using the spectroscopic data from Hinode.

[en] We propose a new method to determine magnetic fields, by using the magnetic-field-induced electric dipole transition $3{\mathrm{p}}^{4}3\mathrm{d}{}^4{D}_{7/2}$ $\to <!--\; ???\; -->3{\mathrm{p}}^5{}^2{P}_{3/2}$ in ${\mathrm{F}\mathrm{e}}^{9+}$ ions. This ion has a high abundance in astrophysical plasma and is therefore well suited for direct measurements of even rather weak fields in, e.g., solar flares. This transition is induced by an external magnetic field and its rate is proportional to the square of the magnetic field strength. We present theoretical values for what we will label the reduced rate and propose that the critical energy difference between the upper level in this transition and the close-to-degenerate $3{\mathrm{p}}^{4}3\mathrm{d}{}^4{D}_{5/2}$ should be measured experimentally since it is required to determine the relative intensity of this magnetic line for different magnetic fields.

[en] Energies, transition rates, line strengths and lifetimes have been computed for all levels of the 4p ⁶ and 4p ⁵4d configurations of W³⁸⁺ by using the multi-configuration Dirac–Hartree–Fock (MCDHF) method as well as relativistic many-body perturbation theory. We investigate systematically correlation, relativistic and quantum electro-dynamical (QED) effects of different properties, including excitation energies and transition rates. We demonstrate that it is important to include the core-valence correlation of rather deep subshells (including 3d and 3p) to reach close to spectroscopic accuracy for the transition energies. We also show that high-multipole transitions (E3, M2) are important for the lifetime of some metastable levels of 4p ⁵4d (${}^3{F}_3,{}^1{D}_2,{}^3{D}_2$). The present results are in good agreement with experiments and of considerably higher accuracy than those achieved in previous theoretical works. (paper)