Isotope Shifts of Energy Levels in Helium-Like Highly Charged Ions
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TROSCOPY AND PHYSICS OF ATOMS AND MOLECULES
Isotope Shifts of Energy Levels in Helium-Like Highly Charged Ions N. A. Zubovaa, *, M. Yu. Kaygorodova, Yu. S. Kozheduba, A. V. Malysheva, R. V. Popova, I. M. Savelyeva, I. I. Tupitsyna, and V. M. Shabaeva a St.
Petersburg State University, St. Petersburg, 199034 Russia *e-mail: [email protected]
Received March 18, 2020; revised March 18, 2020; accepted March 25, 2020
Abstract—Isotope shifts of n = 1 and n = 2 levels and corresponding transitions in helium-like highly charged ions are calculated with high precision. The total value of the isotope shift is determined mainly by the sum of the field and mass shifts. The field shift is calculated by using the Dirac–Fock–Sturm method taking into account interaction of configurations. Quantum-electrodynamic corrections to this contribution are taken into account approximately by using the corresponding one-electron expressions. The mass shift is calculated both within the framework of the Breit approximation and by taking into account quantum-electrodynamic contributions that become substantial for heavy ions. In the case of thorium and uranium ions, corrections due to nuclear polarization and deformation are taken into account additionally. Keywords: isotopic shifts, relativistic theory of atoms, charge radii of nuclei DOI: 10.1134/S0030400X20080408
1. INTRODUCTION The first precision measurements of isotope shifts in highly charged ions were carried out in [1–3]. Higher precision was achieved in the experiments on measuring isotope shift in a boron-like argon ion [4] and a lithium-like neodymium ion [5]. The achieved precision allowed testing relativistic calculations of mass shift in highly charged ions for the first time and came close to the value of the quantum-electrodynamic (QED) contribution to the nuclear recoil effect by order of magnitude. Investigation of the latter contribution in heavy ions is of special interest, because its calculation requires using quantum electrodynamics in the strong-coupling region beyond the approximation in which nucleus is considered to be a source of classical field (the Furry picture) rather than a moving particle. In addition, experimental and theoretical values of the isotope shift in lithium-like neodymium were used in [5] to determine the difference of charge radii of the corresponding isotopes. It is expected that the experimental precision of measuring isotope shifts in heavy ions up to uranium ( Z = 92 ) can be improved by approximately an order of magnitude in facilities of the FAIR megaproject which is currently under construction [6, 7]. These facts are certainly indicate of the importance of the corresponding theoretical calculations. We carried out precision calculations of the isotope shifts of the energy levels of lithium-, boron-, and beryllium-like highly charged ions previously [8–10]. The present work aims at extending these calculations
to the case of helium-like highly charged ions. As of today, the most accurate calculations of the n = 1 and n = 2 energy level
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