  Free-electron lasers (FEL) in the extreme ultraviolet (XUV) and X-ray regime
opened up the possibility for experiments at high power densities, in
particular allowing for fluence-dependent absorption and scattering experiments
to reveal non-linear light-matter interactions at ever shorter wavelengths.
Findings of such non-linear effects in the XUV and X-ray regime are met with
tremendous interest, but prove difficult to understand and model due to the
inherent shot-to-shot fluctuations in photon intensity and the often
structured, non-Gaussian spatial intensity profile of a focused FEL beam.
Presently, the focused beam spot is characterized and optimized separately from
the actual experiment. Here, we present the first simultaneous measurement of
diffraction signals from solid samples in tandem with the corresponding
single-shot spatial fluence distribution on the actual sample. This new in-situ
characterization scheme enables fast and direct monitoring and thus control of
the sample illumination which ultimately is necessary for a quantitative
understanding of non-linear light-matter interaction in X-ray and XUV FEL
experiments.
