Abstract
Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L{}_{3} edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.
Highlights
Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies
With the advent of X-ray free electron lasers (XFELs) it is possible to produce high fluence X-ray pulses of tens of femtoseconds duration that transform a solid sample into exotic states of matter with dramatically changed electronic structure and response to X-rays[1,2,3,4,5,6,7]
The photon energy of the spectra recorded with the XFEL were calibrated to align with
Summary
Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. The two most basic processes consist of two or more photons working together to couple the absorption and decay processes within the core hole lifetime, referred to as stimulated resonant scattering[13,14,15,16], or the modification of a given atomic absorption event by the perturbation caused by the other electronic excitations within the pulse duration. In both cases, the absorption spectrum, recorded as a statistical average over many pulses, will be changed in shape or intensity
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