[en] We report on large-scale ab initio calculations for intercombination lines in Si I-like ions. Two measurable quantities, the lifetime of the 3s3p³⁵S₂ level and the branching ratio of the two lines are discussed, to infer the importance of different features of our calculations. The effects of core polarization, two-body spin-dependent operators and Dirac contra Breit-Pauli approaches are discussed. Earlier calculations are reviewed and evaluated. The calculated lifetimes are in good agreement with experiments, while a discrepancy persists for branching ratios

[en] We present calculations of autoionization widths using a fully numerical multiconfigurational Hartree-Fock method (MCHF). Energy positions and autoionization rates are given for several doubly-excited states lying below the n = 2 threshold in He I and H^-. Particular attention is given to orthogonality considerations in the computation of the autoionization widths. The method is extended to the 1s2p²²D, 1s2s(³S)3d ²D and 1s2s(³S)4d ²D states of Li I. To the author's knowledge, our MCHF value, 12.5 meV, is the first result obtained for the autoionization width of the 1s2s(³S)4d ²D state. Comparison is made with other theoretical results and with experimental values and we point out several inconsistencies between the two sets of results. (author)

[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] Theoretical investigations of the hyperfine quenching of the two metastable levels 1s²2s²2p⁶3s3p ³P₀ and ³P₂ have been performed along the magnesium-like iso-electronic sequence ranging from Z = 12 to Z = 31. It is shown that the lifetime of the latter level is sensitive to hyperfine quenching in the beginning of the sequence, but that this sensitivity decreases going to higher Z. It is also shown that the branching fractions of the 3s²¹S₀-3s3p ³P₂ and 3s3p ³P₁-3s3p ³P₂ transitions are very sensitive to hyperfine quenching, which could lead to some dramatic effects in the spectrum. Our predicted transition rates of the hyperfine induced 3s²¹S₀-3s3p ³P₀ transition are compared to earlier results and some disagreements are found and explained. The present predicted lifetime of 3s3p ³P₀ in ²⁷Al⁺ is slightly longer than the recent measurement, but in better agreement than earlier predictions.

[en] Full text: In this contribution we will discuss a single spectral line in W₂₇₊ - an M1 line between the two ground state fine structure levels 4d₁₀4f ₂F_7/2 and ₂F_5/2. This is the only transition within the ground configuration of this ion and therefore a challenge to identify spectroscopically. We have studied this line both experimentally using an EBIT and theoretically using the GRASP2K code. These investigations were done independently and the results only compared when both methods had reached a conclusive wavelength, from careful and systematic considerations. One of the motivation for this work was the fact that earlier predictions for this transition showed a spread of over 400 Å for the wavelength. Our experiments were performed using the Shanghai permanent magnet EBIT and an Andor 303 Shamrock spectrometer equipped with an Andor Newton CCD camera. Tungsten was injected into the EBIT using the vaporous compound W(CO)₆. Spectra were recorded at several electron beam energies to isolate the W₂₇₊ line. The spectra were wavelength calibrated using a number of calibration lamps and the final experimental wavelength was determined to be 3377.43 ± 0.26 Å. To confirm the line was really from W₂₇₊ we determined the lifetime of the upper level of the ₂F term and compared our value with one from a calculation. As the line is from an M1 transition the lifetime of the upper level it is trivial to calculate the rate and thereby the lifetime. Our theoretical studies are systematic in two senses – first we investigate the contributions to the wavelength from different core subshells. These studies implies it is important to include core valence correlation even between the valence 4f and deep subshells – as a matter of fact, the correlation contribution from 3d is more pronounced than from 4d. The second dimension of the systematic approach is an increase of the active set of orbitals, and thereby the space of configuration state functions (CSFs) to show convergence of the final results. Finally, after these independent investigation we are able to do single-line spectroscopy to identify this transition, since the agreement between our experimental and theoretical wavelength is within the order 0.1%, since the experimental vacuum wavelength is 3378.43 Å whereas the calculated wavelength is 3374.73 Å. This is probably the best agreement for such a transition in the visible region for a highly charged high Z ion. We will also discuss cases in which this agreement could be even better, allowing for rigorous tests of minute effects – in the realm of Breit and QED interactions. (author)

[en] This paper includes a discussion and classification of what is generally referred to as 'Forbidden lines' in atomic physics. Historical and important examples are reviewed, in light of their challenge to computations and remaining disagreements between experiment and theory. We also review some recent results on what could be labeled the third generation of unexpected transitions, induced by hyperfine quenching.

[en] Negative ions homologous to N⁻ are interesting systems, since there are several bound terms within the ground configuration. The forbidden transitions between them, often dominated by the magnetic dipole contributions, are affected by term mixing—a relativistic effect. When increasing the nuclear charge in the homologous sequence of negative ions, this effect increases. In this paper we use systematic multiconfiguration Dirac–Hartree–Fock calculations to study these effects and predict affinities, level splittings and transition rates between bound states of N⁻, P⁻, As⁻, Sb⁻ and Bi⁻. By monitoring the line strengths we are able to predict the deviation from the non-relativistic LS-coupled values for transitions between levels of the lowest ³P term. For Sb⁻ the ¹D term is also bound, and the prediction for the rates of its transition to the ³P-levels is a challenge for theory. For Bi⁻ less is known experimentally and we analyze its structure and the relativistic contributions to it. (paper)

[en] Based on the multi-configuration Dirac–Hartree–Fock method and using the GRASPVU package, a theoretical investigation was performed to study the lifetimes of hyperfine levels of the first excited level 3d⁹4s ³D₃ in Ni-like ions (Z = 72–79) for all stable isotopes with nuclear spin. Comparisons between hyperfine-induced electric quadrupole transition rates and the pure magnetic octupole transition rates show that the extra electric quadrupole transition channel caused by the nuclear magnetic dipole and electric quadrupole hyperfine interaction is important for most hyperfine levels in each individual ion. Lifetimes of most hyperfine levels are sensitive to this extra decay channel. Extreme cases are found in ¹⁸¹Ta, ¹⁸⁵Re and ¹⁸⁷Re, where lifetimes of some hyperfine levels are shortened by more than an order of magnitude. (paper)

[en] In this paper we give a short introduction to the use of atomic spectroscopy in plasma diagnostics. Both older works and exciting new branches of atomic physics, which have relevance to diagnostics, are discussed. In particular we focus on forbidden lines in Be-like ions, lines sensitive to magnetic fields and levels which have a lifetime dependence on the nuclear spin of the ion, i.e. f-dependent lifetimes. Finally we mention a few examples of where tokamaks, instead of needing atomic data, actually provide new data and lead to developments in atomic structure studies.