A possibility for a determination of the fine structure constant in experiments on the bound-electron g-factor is examined. It is found that studying a specific difference of the g-factors of B- and H-like ions of the same spinless isotope in the Pb region to the currently accessible experimental accuracy of 7 x 10(-10) would lead to a determination of the fine structure constant to an accuracy which is better than that of the currently accepted value. Further improvements of the experimental and theoretical accuracy could provide a value of the fine structure constant which is several times more precise than the currently accepted one.

The hyperfine splitting of the ground state of H-, Li-, and B-like ions is investigated in details within the range of nuclear numbers Z = 7 − 28. The rigorous QED approach together with the large-scale configuration-interaction Dirac-Fock-Sturm method are employed for the evaluation of the interelectronicinteraction contributions of first and higher orders in 1/Z. The screened QED corrections are evaluated to all orders in αZ utilizing an effective potential approach. The influence of nuclear magnetization distribution is taken into account within the single-particle nuclear model. The specific differences between the hyperfinestructure level shifts of H-and Li-like ions, where the uncertainties associated with the nuclear structure corrections are significantly reduced, are also calculated.

We have measured the ground-state g-factor of boronlike argon 40 Ar 13+ with a fractional uncertainty of 1.4 × 10 −9 with a single ion in the newly developed Alphatrap double Penning-trap setup. The here obtained value of g = 0.663 648 455 32(93) is in agreement with our theoretical prediction of 0.663 648 12(58). The latter is obtained accounting for quantum electrodynamics, electron correlation, and nuclear effects within the state-of-the-art theoretical methods. Our experimental result distinguishes between existing predictions that are in disagreement, and lays the foundations for an independent determination of the fine-structure constant.

The fully relativistic theory of the g factor of hydrogenlike ions with nonzero nuclear spin is considered. The hyperfine-interaction correction to the atomic g factor is calculated for both point and extended chargedistribution models for nuclei. Both the magnetic dipole and the electric quadrupole interactions are taken into account. This correction is combined with corrections resulting from QED, nuclear recoil, and nuclear size, to obtain theoretical high-precision values for the g factor of hydrogenlike ions with nonzero nuclear spin. The results can be used for a precise determination of nuclear magnetic moments from g factor experiments.

Tin is the chemical element with the largest number of stable isotopes. Its complete proton shell, comparable with the closed electron shells in the chemically inert noble gases, is not a mere precursor to extended stability; since the protons carry the nuclear charge, their spatial arrangement also drives the nuclear electromagnetism. We report high-precision measurements of the electromagnetic moments and isomeric differences in charge radii between the lowest 1/2+, 3/2+, and 11/2− states in 117–131Sn, obtained by collinear laser spectroscopy. Supported by state-of-the-art atomic-structure calculations, the data accurately show a considerable attenuation of the quadrupole moments in the closed-shell tin isotopes relative to those of cadmium, with two protons less. Linear and quadratic mass-dependent trends are observed. While microscopic density functional theory explains the global behaviour of the measured quantities, interpretation of the local patterns demands higher-fidelity modelling.

We measure optical spectra of Nd-like W, Re, Os, Ir, and Pt ions of particular interest for studies of a possibly varying fine-structure constant. Exploiting characteristic energy scalings we identify the strongest lines, confirm the predicted 5s-4f level crossing, and benchmark advanced calculations. We infer two possible values for optical M2=E3 and E1 transitions in Ir 17þ that have the highest predicted sensitivity to a variation of the fine-structure constant among stable atomic systems. Furthermore, we determine the energies of proposed frequency standards in Hf 12þ and W 14þ . DOI: 10.1103/PhysRevLett.114.150801 PACS numbers: 06.20.Jr, 31.15.am, 31.15.bw, 32.30.Jc Highly charged ions (HCIs) are currently in the focus of theoretical studies analyzing their applications to frequency metrology and tests of a variation of the fine-structure constant α [1][2][3][4][5][6][7][8][9][10][11][12]. In most of the proposed HCIs with atomic number Z ¼ 55-98 and in charge states from 7 to 35, the complex electronic structures are experimentally unknown, and accurate calculations are extremely difficult. In view of novel techniques for sympathetically cooling HCIs in Paul traps [13][14][15] aiming at quantum logic spectroscopy on highly forbidden transitions [16], such data are urgently required.Observations from quasar absorption spectra have suggested a spatial variation of the value of the fine-structure constant α over cosmological dimensions [17], characterized by a dipolar distribution with a value of 10 −6 GLyr −1 . Laboratory experiments [18][19][20][21] have not yet reached the accuracy needed to test this dipolar pattern, which translates to a temporal variation of 10 −19 yr −1 [22] due to the motion of the Earth. Future optical clocks based on HCIs [3] could improve such tests. Interconfiguration transitions in HCIs have a high sensitivity to a variation of α due to large relativistic contributions to their binding energies. Advantageously, they have a strongly suppressed sensitivity to external perturbations [12]. However, interconfiguration transitions quickly shift from the optical laser range into the extreme ultraviolet or x-ray region with increasing charge state [23]. Nonetheless, at level crossings with two or more nearly degenerate electronic configurations, forbidden, and thus narrow, optical transitions with an enhanced sensitivity [1,2] arise. In particular, the Nd-like system Ir 17þ offers narrow lines between three electronic configurations 4f 14 , 4f 13 5s 1 , and 4f 12 5s 2 with the highest ever predicted sensitivity in a stable atomic system [2]. However, calculations for this system are exceptionally difficult, and the predicted energies for intraconfiguration M1 transitions ideally suited for clock applications [6] can exhibit errors on the 10% level, as can be seen below. For interconfiguration transitions, uncertainties at the eV level are expected. Thus, optical line identification becomes extremely difficult.In this Letter, we demonstrate a method to reliably identify transitions in...

Line intensities and oscillator strengths for the controversial 3C and 3D astrophysically relevant lines in neonlike Fe 16+ ions are calculated. We show that, for strong x-ray sources, the modeling of the spectral lines by a peak with an area proportional to the oscillator strength is not sufficient and non-linear dynamical effects have to be taken into account. Furthermore, a large-scale configurationinteraction calculation of oscillator strengths is performed with the inclusion of higher-order electroncorrelation effects. The dynamical effects give a possible resolution of discrepancies of theory and experiment found by recent measurements, which motivates the use of light-matter interaction models also valid for strong light fields in the analysis and interpretation of astrophysical and laboratory spectra.PACS numbers: 31.15.am,32.30.Rj,42.50.Ct Astrophysical spectra recorded by space observatories provide the only means to determine the element composition, temperature, density, and velocity of distant celestial objects such as stars, x-ray binaries, black hole accretion discs, or active galactic nuclei [1][2][3][4][5][6][7][8][9]. Such x-ray (or optical) spectra are often composed of a series of peaks associated with a range of elements, ionic charge states, and transitions. Therefore, a large amount of reliable atomic data is needed to disentangle the physical properties of the emitting objects. Such data-transition energies and probabilities, oscillator strengths, collisional and recombination cross sections, etc.-may be obtained from laboratory astrophysics experiments (see e.g. [3, 10-16]) or, more economically, from theoretical calculations (see e.g. [17][18][19][20]).The x-ray emission lines of highly charged Fe ions are among the brightest in astrophysical spectra. Within the last decade, several observations were performed with the space laboratories Chandra and XMM-Newton (see e.g. [5,6,21,22]). The line-strength ratio of two 2p → 3d lines in Fe 16+ , customarily denoted as 3C [2p 6 (J = 0) → (2p 5 ) 1/2 3d 3/2 (J = 1), transition energy of 826 eV] and 3D [2p 6 (J = 0) → (2p 5 ) 3/2 3d 5/2 (J = 1), at 812 eV], was observed, but the results disagreed with theoretical predictions [17][18][19][20]. Initially this disagreement was considered to originate from the co-existence of different charge states of Fe, and later, after laboratory measurements, as an effect of electron-impact excitation of the ion. Furthermore, since several theoretical calculations of transition probabilities in highly charged ions agreed well with the experiments (see, e.g. [23][24][25]), there was no reason to assume that essential contributions had not been included in the predictions. The question was out of focus until the first laser spectroscopic experiment in the x-ray regime [3], enabled by the advent of x-ray freeelectron-laser (XFEL) facilities [26]. This experiment at the Linac Coherent Light Source (LCLS, Ref.[27]) gave hints for an incorrect atomic structure theory: a disagreement between all state-of-the-art ...

For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p − 3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios fð3CÞ=fð3DÞ disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of fð3CÞ=fð3DÞ ¼ 3.09ð8Þð6Þ supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.