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Spatiotemporal ionization dynamics in nonthermal copper plasma pumped by femtosecond relativistic laser pulse

Yuichi Inubushi1,2,*, Yuya Kubota2,†, Yasuhiko Sentoku3, Toshinori Yabuuchi1,2, Kohei Miyanishi2, Keiichi Sueda2, Ichiro Inoue2, and Makina Yabashi1,2

  • *Contact author: inubushi@spring8.or.jp
  • †Present address: Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan.
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Phys. Rev. Research 8, 043005 – Published 5 October, 2026

DOI: https://doi.org/10.1103/sgyf-lrw1

Abstract

Understanding ultrafast ionization dynamics in high-density plasmas is essential for applications such as particle acceleration, short-pulse x-ray generation, and laser fusion. However, resolving ionization states inside optically opaque plasmas with femtosecond resolution remains challenging. Using femtosecond-resolved x-ray absorption spectroscopy, we directly measured inner-shell vacancies, overall charge states, and the nonthermal electron population responsible for ionization in solid-density copper plasma. In copper plasma generated by a 40-fs relativistic laser pulse, ionization proceeds within 0.5 ps. M- and outer-shell ionization persist for over 10 ps, whereas L-shell vacancies decay within 4 ps. The population of electrons capable of L-shell ionization decreases in picoseconds, in agreement with particle-in-cell simulations incorporating atomic processes. Our spectroscopic approach provides direct experimental insights into ultrafast ionization dynamics in high-density plasmas, supporting improvements in the modeling of laser-driven plasmas.

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Phys. Rev. Research 8, 043005– Published 5 October, 2026
  • Received 22 January 2026
  • Accepted 5 September 2026
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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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