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@PHDTHESIS{Shakya:646135,
      author       = {Shakya, Yashoj},
      othercontributors = {Santra, Robin and Inhester, Ludger},
      title        = {{U}ltrafast {Q}uantum-{C}lassical {D}ynamics:
                      {A}pplications in {X}-ray {S}pectroscopy and {M}ethod
                      {D}evelopment},
      school       = {University of Hamburg},
      type         = {Dissertation},
      reportid     = {PUBDB-2026-00735},
      pages        = {114},
      year         = {2022},
      note         = {Dissertation, University of Hamburg, 2022},
      abstract     = {Recent advances in laser technologies such as x-ray
                      free-electron lasers and high harmonic generation have led
                      to ever-shorter light pulses that enable us to probe
                      ultrafastnuclear and electronic dynamics in atoms and
                      molecules. Theoretical quantum dynamics simulations are
                      indispensable in gaining deeper insights into these
                      ultrafastprocesses. However, treating both electrons and
                      nuclei fully quantum mechanically is computationally not
                      feasible for large systems. Hence, due to their
                      computationalefficiency, mixed quantum-classical dynamics
                      methods such as Tully’s fewest switches surface hopping
                      (FSSH) have become popular, in spite of their limitations.
                      In thisthesis, I demonstrate how FSSH dynamics, combined
                      with advanced statistical analysis techniques, can be used
                      to understand ultrafast phenomena traced in
                      experimentalspectra such as time-resolved x-ray absorption
                      spectra (TRXAS). Furthermore, I introduce a new method
                      developed to improve FSSH to provide a better description
                      ofelectronic coherences relevant in attosecond science.With
                      the aim of understanding the first steps of radiation damage
                      in biomolecules, the first part of this thesis focuses on ab
                      initio FSSH dynamics simulations of valenceionized urea
                      monomer and dimer in vacuum as a prototypical example.
                      Investigating the carbon, nitrogen, and oxygen K-edges in
                      the simulated TRXAS reveals rich in-sights into the
                      ultrafast processes. Further information is gained by
                      applying machine learning techniques for statistical
                      analysis which unravel uncorrelated collective mo-tions that
                      most influence the spectra. Extending these simulations to
                      urea in aqueous solution, I show in the second part of this
                      thesis how the effect of proton transferbetween two
                      hydrogen-bonded ureas and the subsequent electronic
                      structure changes leave two distinct marks on the carbon
                      K-edge of the TRXAS. This enables us to sep-arate the effect
                      of nuclear and electronic motion on the spectra. These
                      liquid phase results are in good agreement with recent
                      pump-probe experiments on aqueous urea.In the last part, I
                      present a new method, named ring polymer surface hopping -
                      density matrix approach (RPSH-DM), developed to alleviate
                      one of the shortcomingsof FSSH, namely the so-called
                      overcoherence problem, which manifests as a poor description
                      of electronic coherence and decoherence phenomena. RPSH-DM
                      combinesFSSH with ring polymer molecular dynamics to
                      incorporate decoherence effects by utilizing the spatial
                      extent of the ring polymer, mimicking the width of the
                      nuclearwave packet. By applying this method to a
                      one-dimensional model system, I show how RPSH-DM can capture
                      crucial decoherence mechanisms that are not present in
                      FSSH.In future studies, employing RPSH-DM to investigate
                      polyatomic systems can provide vital insights into ultrafast
                      electronic processes occurring in attosecond experiments.},
      cin          = {CFEL-DESYT / FS-CFEL-3},
      cid          = {I:(DE-H253)CFEL-DESYT-20160930 /
                      I:(DE-H253)FS-CFEL-3-20120731},
      pnm          = {631 - Matter – Dynamics, Mechanisms and Control
                      (POF4-631)},
      pid          = {G:(DE-HGF)POF4-631},
      experiment   = {EXP:(DE-MLZ)NOSPEC-20140101},
      typ          = {PUB:(DE-HGF)11},
      urn          = {	urn:nbn:de:gbv:18-ediss-111527},
      url          = {https://bib-pubdb1.desy.de/record/646135},
}