Structural Dynamics (Sep 2023)

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

  • Robin Y. Engel,
  • Oliver Alexander,
  • Kaan Atak,
  • Uwe Bovensiepen,
  • Jens Buck,
  • Robert Carley,
  • Michele Cascella,
  • Valentin Chardonnet,
  • Gheorghe Sorin Chiuzbaian,
  • Christian David,
  • Florian Döring,
  • Andrea Eschenlohr,
  • Natalia Gerasimova,
  • Frank de Groot,
  • Loïc Le Guyader,
  • Oliver S. Humphries,
  • Manuel Izquierdo,
  • Emmanuelle Jal,
  • Adam Kubec,
  • Tim Laarmann,
  • Charles-Henri Lambert,
  • Jan Lüning,
  • Jonathan P. Marangos,
  • Laurent Mercadier,
  • Giuseppe Mercurio,
  • Piter S. Miedema,
  • Katharina Ollefs,
  • Bastian Pfau,
  • Benedikt Rösner,
  • Kai Rossnagel,
  • Nico Rothenbach,
  • Andreas Scherz,
  • Justine Schlappa,
  • Markus Scholz,
  • Jan O. Schunck,
  • Kiana Setoodehnia,
  • Christian Stamm,
  • Simone Techert,
  • Sam M. Vinko,
  • Heiko Wende,
  • Alexander A. Yaroslavtsev,
  • Zhong Yin,
  • Martin Beye

DOI
https://doi.org/10.1063/4.0000206
Journal volume & issue
Vol. 10, no. 5
pp. 054501 – 054501-21

Abstract

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Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L 3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale.