Fig. 1: Experimental results in Mg.
a Simplified sketch of the experimental setup (see Methods section). The single-shot spectra of the pulses are collected with two spectrometers (depicted as a couple of gratings) upstream (Iu(Eph)) and downstream (Id(Eph)) of a Mg sample foil. The NLEs are obtained comparing the two FEL spectra, demonstrating a spectral blue-shift above the L2,3 absorption edge. On the contrary, spectral red-shift and broadening can be observed below edge. The averages of the differences ΔI = 〈Id − Iu〉 is reported for a FEL photon energy (~42.1 eV) below the Mg L2,3-edge (b) and for two photon energies (~51.5 and ~56.2 eV) above it (c, d). The red and blue curves correspond to ΔI at the highest and lowest fluence. Averaged FEL spectra 〈Iu(Eph)〉, scaled to improve readability, are shown by solid black lines. The blue-shift in c,d can also be appreciated comparing 〈Iu(Eph)〉 and 〈Id(Eph)〉 at the largest fluence (dashed black lines).
