Electron Population Dynamics in Resonant Non-Linear X-ray Absorption in Nickel at a Free-Electron Laser

Abstract

Free-electron lasers provide bright, ultrashort, and monochromatic X-ray pulses, enabling novel spectroscopic mea-surements not only with femtosecond temporal resolution: The high fluence of their X-ray pulses can also easily enterthe regime of non-linear X-ray-matter interaction. Entering this regime necessitates a rigorous analysis and reliableprediction of the relevant non-linear processes for future experiment designs. Here we show non-linear changes in theL3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but pre-dictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations withinthe pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more thanthree orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical pro-cesses driving the non-linear changes. Our findings provide important insights for the design and evaluation of futurehigh-fluence free-electron laser experiments, and contribute to the understanding of non-linear electron dynamics inX-ray absorption processes in solids at the femtosecond timescale