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@PHDTHESIS{Engel:580279,
      author       = {Engel, Robin},
      othercontributors = {Beye, Martin and Huse, Nils},
      title        = {{F}rom {L}inear to {N}on-{L}inear {X}-ray {S}pectroscopy on
                      {M}aterials using {SASE}-{FEL}s},
      school       = {Universität Hamburg},
      type         = {Dissertation},
      address      = {Hamburg},
      publisher    = {Verlag Deutsches Elektronen-Synchrotron DESY},
      reportid     = {PUBDB-2023-01185, DESY-THESIS-2023-004},
      series       = {DESY-THESIS},
      pages        = {193},
      year         = {2023},
      note         = {Dissertation, Universität Hamburg, 2023},
      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.},
      cin          = {FS-FLASH},
      cid          = {I:(DE-H253)FS-FLASH-20140814},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / 6G2 - FLASH (DESY) (POF4-6G2) / PHGS,
                      VH-GS-500 - PIER Helmholtz Graduate School
                      $(2015_IFV-VH-GS-500)$},
      pid          = {G:(DE-HGF)POF4-632 / G:(DE-HGF)POF4-6G2 /
                      $G:(DE-HGF)2015_IFV-VH-GS-500$},
      experiment   = {EXP:(DE-H253)F-PG2-20150101 /
                      EXP:(DE-H253)XFEL-SCS-20150101 /
                      EXP:(DE-H253)FLASH(machine)-20150101},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.3204/PUBDB-2023-01185},
      url          = {https://bib-pubdb1.desy.de/record/580279},
}