A scheme to infer the temporal coherence of EUV frequency combs generated from intracavity highorder harmonic generation is put forward. The excitation dynamics of highly charged Mg-like ions, which interact with EUV pulse trains featuring different carrier-envelope-phase fluctuations, are simulated. While demonstrating the microscopic origin of the macroscopic equivalence between excitations induced by pulse trains and continuous-wave lasers, we show that the coherence time of the pulse train can be determined from the spectrum of the excitations. The scheme will provide a verification of the comb temporal coherence at timescales several orders of magnitude longer than current state of the art, and at the same time will enable high-precision spectroscopy of EUV transitions with a relative accuracy up to δω=ω ∼ 10 −17 .

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A scheme is put forward to generate fully coherent x-ray lasers based on population inversion in highly charged ions, created by fast inner-shell photoionization using broadband x-ray free-electronlaser (XfeL) pulses in a laser-produced plasma. numerical simulations based on the Maxwell-Bloch theory show that one can obtain high-intensity, femtosecond x-ray pulses of relative bandwidths Δω/ω = 10 −5-10 −7 , by orders of magnitude narrower than in x-ray free-electron-laser pulses for discrete wavelengths down to the sub-ångström regime. Such x-ray lasers can be applicable in the study of x-ray quantum optics and metrology, investigating nonlinear interactions between x-rays and matter, or in high-precision spectroscopy studies in laboratory astrophysics.

The absolute atomic mass of $$^{208}$$ 208 Pb has been determined with a fractional uncertainty of $$7\times 10^{-11}$$ 7 × 10 - 11 by measuring the cyclotron-frequency ratio R of $$^{208}$$ 208 Pb$$^{41+}$$ 41 + to $$^{132}$$ 132 Xe$$^{26+}$$ 26 + with the high-precision Penning-trap mass spectrometer Pentatrap and computing the binding energies $$E_{\text {Pb}}$$ E Pb and $$E_{\text {Xe}}$$ E Xe of the missing 41 and 26 atomic electrons, respectively, with the ab initio fully relativistic multi-configuration Dirac–Hartree–Fock (MCDHF) method. R has been measured with a relative precision of $$9\times 10^{-12}$$ 9 × 10 - 12 . $$E_{\text {Pb}}$$ E Pb and $$E_{\text {Xe}}$$ E Xe have been computed with an uncertainty of 9.1 eV and 2.1 eV, respectively, yielding $$207.976\,650\,571(14)$$ 207.976 650 571 ( 14 )  u ($$\text {u}=9.314\,941\,024\,2(28)\times 10^{8}$$ u = 9.314 941 024 2 ( 28 ) × 10 8  eV/c$$^2$$ 2 ) for the $$^{208}$$ 208 Pb neutral atomic mass. This result agrees within $$1.2\sigma $$ 1.2 σ with that from the Atomic-Mass Evaluation (AME) 2020, while improving the precision by almost two orders of magnitude. The new mass value directly improves the mass precision of 14 nuclides in the region of Z = 81–84 and is the most precise mass value with $$A>200$$ A > 200 . Thus, the measurement establishes a new region of reference mass values which can be used e.g. for precision mass determination of transuranium nuclides, including the superheavies.

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