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Kromer K, Lyu C, Door M, Filianin P, Harman Z, Herkenhoff J, Indelicato P, Keitel CH, Lange D, Novikov YN, Schweiger C, Eliseev S, Blaum K. Observation of a Low-Lying Metastable Electronic State in Highly Charged Lead by Penning-Trap Mass Spectrometry. PHYSICAL REVIEW LETTERS 2023; 131:223002. [PMID: 38101362 DOI: 10.1103/physrevlett.131.223002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/21/2023] [Revised: 10/09/2023] [Accepted: 10/17/2023] [Indexed: 12/17/2023]
Abstract
Highly charged ions (HCIs) offer many opportunities for next-generation clock research due to the vast landscape of available electronic transitions in different charge states. The development of extreme ultraviolet frequency combs has enabled the search for clock transitions based on shorter wavelengths in HCIs. However, without initial knowledge of the energy of the clock states, these narrow transitions are difficult to be probed by lasers. In this Letter, we provide experimental observation and theoretical calculation of a long-lived electronic state in Nb-like Pb^{41+} that could be used as a clock state. With the mass spectrometer PENTATRAP, the excitation energy of this metastable state is directly determined as a mass difference at an energy of 31.2(8) eV, corresponding to one of the most precise relative mass determinations to date with a fractional uncertainty of 4×10^{-12}. This experimental result agrees within 1σ with two partially different ab initio multiconfiguration Dirac-Hartree-Fock calculations of 31.68(13) eV and 31.76(35) eV, respectively. With a calculated lifetime of 26.5(5.3) days, the transition from this metastable state to the ground state bears a quality factor of 1.1×10^{23} and allows for the construction of a HCI clock with a fractional frequency instability of <10^{-19}/sqrt[τ].
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Affiliation(s)
- Kathrin Kromer
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Chunhai Lyu
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Menno Door
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Pavel Filianin
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Zoltán Harman
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Jost Herkenhoff
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Paul Indelicato
- Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-PSL Research University, Collège de France, Paris, France
| | | | - Daniel Lange
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Yuri N Novikov
- Department of Physics, St Petersburg State University, St. Petersburg 198504, Russia
- NRC "Kurchatov Institute"-Petersburg Nuclear Physics Institute, Gatchina 188300, Russia
| | | | - Sergey Eliseev
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
| | - Klaus Blaum
- Max-Planck-Institut für Kernphysik, 69117 Heidelberg, Germany
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2
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King SA, Spieß LJ, Micke P, Wilzewski A, Leopold T, Benkler E, Lange R, Huntemann N, Surzhykov A, Yerokhin VA, Crespo López-Urrutia JR, Schmidt PO. An optical atomic clock based on a highly charged ion. Nature 2022; 611:43-47. [DOI: 10.1038/s41586-022-05245-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2022] [Accepted: 08/17/2022] [Indexed: 11/06/2022]
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3
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Stark J, Warnecke C, Bogen S, Chen S, Dijck EA, Kühn S, Rosner MK, Graf A, Nauta J, Oelmann JH, Schmöger L, Schwarz M, Liebert D, Spieß LJ, King SA, Leopold T, Micke P, Schmidt PO, Pfeifer T, Crespo López-Urrutia JR. An ultralow-noise superconducting radio-frequency ion trap for frequency metrology with highly charged ions. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2021; 92:083203. [PMID: 34470420 DOI: 10.1063/5.0046569] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2021] [Accepted: 07/13/2021] [Indexed: 06/13/2023]
Abstract
We present a novel ultrastable superconducting radio-frequency (RF) ion trap realized as a combination of an RF cavity and a linear Paul trap. Its RF quadrupole mode at 34.52 MHz reaches a quality factor of Q ≈ 2.3 × 105 at a temperature of 4.1 K and is used to radially confine ions in an ultralow-noise pseudopotential. This concept is expected to strongly suppress motional heating rates and related frequency shifts that limit the ultimate accuracy achieved in advanced ion traps for frequency metrology. Running with its low-vibration cryogenic cooling system, electron-beam ion trap, and deceleration beamline supplying highly charged ions (HCIs), the superconducting trap offers ideal conditions for optical frequency metrology with ionic species. We report its proof-of-principle operation as a quadrupole-mass filter with HCIs and trapping of Doppler-cooled 9Be+ Coulomb crystals.
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Affiliation(s)
- J Stark
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - C Warnecke
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Bogen
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Chen
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - E A Dijck
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Kühn
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - M K Rosner
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - A Graf
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - J Nauta
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - J-H Oelmann
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - L Schmöger
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - M Schwarz
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - D Liebert
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - L J Spieß
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S A King
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - T Leopold
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - P Micke
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - P O Schmidt
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - T Pfeifer
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
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4
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Beloy K, Dzuba VA, Brewer SM. Quadruply Ionized Barium as a Candidate for a High-Accuracy Optical Clock. PHYSICAL REVIEW LETTERS 2020; 125:173002. [PMID: 33156679 DOI: 10.1103/physrevlett.125.173002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/04/2020] [Accepted: 09/24/2020] [Indexed: 06/11/2023]
Abstract
We identify Ba^{4+} (Te-like) as a promising candidate for a high-accuracy optical clock. The lowest-lying electronic states are part of a ^{3}P_{J} fine structure manifold with anomalous energy ordering, being nonmonotonic in J. We propose a clock based on the 338.8 THz electric quadrupole transition between the ground (^{3}P_{2}) and first-excited (^{3}P_{0}) electronic states. We perform relativistic many-body calculations to determine relevant properties of this ion. The lifetime of the excited clock state is found to be several seconds, accommodating low statistical uncertainty with a single ion for practical averaging times. The differential static scalar polarizability is found to be small and negative, providing suppressed sensitivity to blackbody radiation while simultaneously allowing cancellation of Stark and excess micromotion shifts. With the exception of Hg^{+} and Yb^{+}, sensitivity to variation of the fine structure constant is greater than other optical clocks thus far demonstrated.
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Affiliation(s)
- K Beloy
- National Institute of Standards and Technology, Boulder, Colorado 80305, USA
| | - V A Dzuba
- School of Physics, University of New South Wales, Sydney 2052, Australia
| | - S M Brewer
- Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA
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5
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Liang S, Lu Q, Wang X, Yang Y, Yao K, Shen Y, Wei B, Xiao J, Chen S, Zhou P, Sun W, Zhang Y, Huang Y, Guan H, Tong X, Li C, Zou Y, Shi T, Gao K. A low-energy compact Shanghai-Wuhan electron beam ion trap for extraction of highly charged ions. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2019; 90:093301. [PMID: 31575235 DOI: 10.1063/1.5112154] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/03/2019] [Accepted: 08/13/2019] [Indexed: 06/10/2023]
Abstract
A low-energy, compact, and superconducting electron beam ion trap (the Shanghai-Wuhan EBIT or SW-EBIT) for extraction of highly charged ions is presented. The magnetic field in the central drift tube of the SW-EBIT is approximately 0.21 T produced by a pair of high-temperature superconducting coils. The electron-beam energy of the SW-EBIT is in the range of 30-4000 eV, and the maximum electron-beam current is up to 9 mA. Acting as a source of highly charged ions, the ion-beam optics for extraction is integrated, including an ion extractor and an einzel lens. A Wien filter is then used to measure the charge-state distribution of the extracted ions. In this work, the tungsten ions below the charge state of 15 have been produced, extracted, and analyzed. The charge-state distributions and spectra in the range of 530-580 nm of tungsten ions have been measured simultaneously with the electron-beam energy of 279 eV and 300 eV, which preliminarily indicates that the 549.9 nm line comes from W14+.
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Affiliation(s)
- Shiyong Liang
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Qifeng Lu
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Xincheng Wang
- School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
| | - Yang Yang
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Ke Yao
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Yang Shen
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Baoren Wei
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Jun Xiao
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Shaolong Chen
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Pengpeng Zhou
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Wei Sun
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Yonghui Zhang
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Yao Huang
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Hua Guan
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Xin Tong
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Chengbin Li
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Yaming Zou
- Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China
| | - Tingyun Shi
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
| | - Kelin Gao
- State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
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6
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Micke P, Kühn S, Buchauer L, Harries JR, Bücking TM, Blaum K, Cieluch A, Egl A, Hollain D, Kraemer S, Pfeifer T, Schmidt PO, Schüssler RX, Schweiger C, Stöhlker T, Sturm S, Wolf RN, Bernitt S, Crespo López-Urrutia JR. The Heidelberg compact electron beam ion traps. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2018; 89:063109. [PMID: 29960545 DOI: 10.1063/1.5026961] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Electron beam ion traps (EBITs) are ideal tools for both production and study of highly charged ions (HCIs). In order to reduce their construction, maintenance, and operation costs, we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA, and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar16+ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e.g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe24+, achieving an electron-energy resolving power of E/ΔE > 1500 at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating the resonant photoexcitation of highly charged oxygen.
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Affiliation(s)
- P Micke
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Kühn
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - L Buchauer
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - J R Harries
- National Institutes for Quantum and Radiological Science and Technology, SPring-8, Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
| | - T M Bücking
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - K Blaum
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - A Cieluch
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - A Egl
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - D Hollain
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Kraemer
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - T Pfeifer
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - P O Schmidt
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - R X Schüssler
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Ch Schweiger
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - T Stöhlker
- GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany
| | - S Sturm
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - R N Wolf
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - S Bernitt
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
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Schmöger L, Schwarz M, Baumann TM, Versolato OO, Piest B, Pfeifer T, Ullrich J, Schmidt PO, López-Urrutia JRC. Deceleration, precooling, and multi-pass stopping of highly charged ions in Be⁺ Coulomb crystals. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2015; 86:103111. [PMID: 26520944 DOI: 10.1063/1.4934245] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Preparing highly charged ions (HCIs) in a cold and strongly localized state is of particular interest for frequency metrology and tests of possible spatial and temporal variations of the fine structure constant. Our versatile preparation technique is based on the generic modular combination of a pulsed ion source with a cryogenic linear Paul trap. Both instruments are connected by a compact beamline with deceleration and precooling properties. We present its design and commissioning experiments regarding these two functionalities. A pulsed buncher tube allows for the deceleration and longitudinal phase-space compression of the ion pulses. External injection of slow HCIs, specifically Ar(13+), into the linear Paul trap and their subsequent retrapping in the absence of sympathetic cooling is demonstrated. The latter proved to be a necessary prerequisite for the multi-pass stopping of HCIs in continuously laser-cooled Be(+) Coulomb crystals.
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Affiliation(s)
- L Schmöger
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - M Schwarz
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - T M Baumann
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - O O Versolato
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - B Piest
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - T Pfeifer
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - J Ullrich
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - P O Schmidt
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
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Windberger A, Crespo López-Urrutia JR, Bekker H, Oreshkina NS, Berengut JC, Bock V, Borschevsky A, Dzuba VA, Eliav E, Harman Z, Kaldor U, Kaul S, Safronova UI, Flambaum VV, Keitel CH, Schmidt PO, Ullrich J, Versolato OO. Identification of the predicted 5s-4f level crossing optical lines with applications to metrology and searches for the variation of fundamental constants. PHYSICAL REVIEW LETTERS 2015; 114:150801. [PMID: 25933300 DOI: 10.1103/physrevlett.114.150801] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2015] [Indexed: 06/04/2023]
Abstract
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^{17+} 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^{12+} and W^{14+}.
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Affiliation(s)
- A Windberger
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | | | - H Bekker
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - N S Oreshkina
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - J C Berengut
- School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia
| | - V Bock
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - A Borschevsky
- Centre for Theoretical Chemistry and Physics, The New Zealand Institute for Advanced Study, Massey University Auckland, Private Bag 102904, 0745 Auckland, New Zealand
| | - V A Dzuba
- School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia
| | - E Eliav
- School of Chemistry, Tel Aviv University, 69978 Tel Aviv, Israel
| | - Z Harman
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - U Kaldor
- School of Chemistry, Tel Aviv University, 69978 Tel Aviv, Israel
| | - S Kaul
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - U I Safronova
- Physics Department, University of Nevada, Reno, Nevada 89557, USA
| | - V V Flambaum
- School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia
| | - C H Keitel
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - P O Schmidt
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
- Institut für Quantenoptik, Leibniz Universität Hannover, Welfengarten 1, 30167 Hannover, Germany
| | - J Ullrich
- Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
| | - O O Versolato
- Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
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