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| Book/Dissertation / PhD Thesis | PUBDB-2023-01185 |
; ;
2023
Verlag Deutsches Elektronen-Synchrotron DESY
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2023-01185
Report No.: DESY-THESIS-2023-004
Abstract: The key feature of X-ray Free Electron Lasers (XFELs) is their capability to generate ultrashort and at least partially coherent X-ray pulses with extreme intensity. This capability holds the promise to revolutionize X-ray physics in a way similar to how lasers have revolutionized optics, as the non-linear and coherent interactions known from theoptical regime combined with the properties of X-ray radiation could enable techniques with unprecedented analyzing power. This thesis summarizes several contributions tothe development from linear to non-linear X-ray spectroscopies at XFELs. To begin with, I address the technical challenge of normalizing the spectral intensity fluctuationsof XFEL-radiation by presenting several versions of the split-beam normalization scheme. Versions suitable for both monochromatic and broadband measurements, either in transmission through liquids or metal films or in reflection from bulk-supported samples are demonstrated and their capabilities and performance are compared. Moving to non-parametric high-fluence studies, we present a non-linear absorption study at the nickel L3-edge using a monochromatic split-beam normalization scheme. We interpret the fluence-dependent spectral changes by characterizing the evolution of the electronic system during interaction with the X-ray pulse using a rate model that quantifiesthe photon absorption and electronic scattering processes. Further, we show a similar non-linear absorption experiment that utilizes a broadband split-beam normalization scheme. While we observe a comparable evolution ofthe electronic system, the broadband incident radiation leads to a strong contribution of stimulated inelastic scattering that is up to six orders of magnitude stronger than the spontaneous contribution that is exploited in conventional Resonant Inelastic X-rayScattering (RIXS). Finally, we demonstrate sum and difference frequency generation between core-resonant XFEL-photons with two infrared photons for the first time. The observed photon-energy dependence of the third-order non-linear susceptibility suggests an enhancement through coupling between the 1s2p and 1s2s excited states, thus demonstratinga key capability of wave-mixing spectroscopy methods. In summary, the presented work contributes to the development of non-linear X-rayspectroscopy on various fronts, but further developments will be needed to bring X-ray wave-mixing techniques into their preconceived position to deliver unprecedented insights into molecular and solid-state dynamics.
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