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Enhanced One-Color-Two-Photon Resonant Ionization in Highly Charged Ions by Fine-Structure Effects

Moto Togawa1,2,*, Chunhai Lyu2,†, Chintan Shah3,2,4, Marc Botz2,5, Joschka Goes2, Jonas Danisch2, Marleen Maxton2, Kai Köbnick2, Filipe Grilo6 et al.

Pedro Amaro6, Katharina Kubicek1,7, Mohammed Sekkal1,7, Awad Mohamed8, Rebecca Boll1, Alberto De Fanis1, Simon Dold1, Tommaso Mazza1, Jacobo Montano1, Nils Rennhack1, Björn Senfftleben1, Sergey Usenko1, Zoltan Harman2, Christoph H. Keitel2, Maurice Leutenegger3, Michael Meyer1, Thomas Pfeifer2, José R. Crespo López-Urrutia2, and Thomas M. Baumann1,‡

  • *Contact author: moto.togawa@xfel.eu
  • †Contact author: chunhai.lyu@mpi-hd.mpg.de
  • ‡Contact author: thomas.baumann@xfel.eu
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Phys. Rev. Lett. 135, 223003 – Published 25 November, 2025

DOI: https://doi.org/10.1103/mhks-ktxw

Abstract

Ultraintense pulses from x-ray free-electron lasers can drive, within femtoseconds, multiple processes in the inner shells of atoms and molecules in all phases of matter. The ensuing complex ionization pathways of outer-shell electrons from the neutral to the final highly charged states make a comparison with theory enormously difficult. We resolve these pathways by preparing highly charged ions in an electron beam ion trap before exposing them to the pulsed radiation. This reveals how relativistic fine-structure effects shift electronic energies, largely compensate the core-screening potential, and enable the consecutive, resonant absorption of two quasi-monochromatic x-ray photons that would generally be unfeasible. This doubly resonant channel enhances the efficiency of two-photon ionization by more than two orders of magnitude, dominating in this regime the nonlinear interaction of light and matter with possible application for future precision x-ray metrology.

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Outline

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Phys. Rev. Lett. 135, 223003– Published 25 November, 2025
  • Received 22 May 2025
  • Revised 27 October 2025
  • Accepted 5 November 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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