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Researchers from the Department of Low and Medium Energy Physics of the "Jožef Stefan" Institute (Dr. Špela Krušič and Prof. Dr. Matjaž Žitnik) took part in the measurements and data analysis of the experiment, which was carried out by an international group of experts at the SCS beamline of the free electron laser facility EuXFEL in Hamburg. They measured the absorption of short pulses (15 fs) of X-ray light with a wavelength of > 1.3 nm when passing through a 100 nm thick copper foil in the vicinity of the L3 edge. The results show an interesting dependence on the intensity of incident light, which is reported in an article just published in the journal [[ https://www.nature.com/articles/s41567-024-02587-w | Nature Physics ]]. Up to 5 TW/cm², the absorption spectrum was the same as already known from previous measurements with weak light, and at higher intensities up to 200 TW/cm², a strong pre-peak appeared due to reversibly saturated absorption into an empty 3d orbital of copper. Above this threshold light intensity, the clear structures around the edge of L3 began to disappear, until above 100 PW/cm² they completely disappeared and practically nothing in the spectrum indicated the position of the L3 edge.