@article{HantschmannFoehlisch2022,
  author    = {Hantschmann, Markus and F{\"o}hlisch, Alexander},
  title     = {A rate model approach for FEL pulse induced transmissions changes, saturable absorption, X-ray transparency and stimulated emission},
  series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy},
  volume    = {256},
  journal   = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0368-2048},
  doi       = {10.1016/j.elspec.2021.147139},
  pages     = {9},
  year      = {2022},
  abstract  = {As the use of free electron laser (FEL) sources increases, so do the findings mentioning non-linear phenomena occurring at these experiments, such as saturable absorption, induced transparency and scattering breakdowns. These are well known among the laser community, but are still rarely understood and expected among the X-ray community and to date lack tools and theories to accurately predict the respective experimental parameters and results. We present a simple theoretical framework to access short X-ray pulse induced light- matter interactions which occur at intense short X-ray pulses as available at FEL sources. Our approach allows to investigate effects such as saturable absorption, induced transparency and scattering suppression, stimulated emission, and transmission spectra, while including the density of state influence relevant to soft X-ray spectroscopy in, for example, transition metal complexes or functional materials. This computationally efficient rate model based approach is intuitively adaptable to most solid state sample systems in the soft X-ray spectrum with the potential to be extended for liquid and gas sample systems as well. The feasibility of the model to estimate the named effects and the influence of the density of state is demonstrated using the example of CoPd transition metal systems at the Co edge. We believe this work is an important contribution for the preparation, performance, and understanding of FEL based high intensity and short pulse experiments, especially on functional materials in the soft X-ray spectrum.},
  language  = {en}
}
@article{MartenssonFoehlischSvensson2022,
  author    = {Martensson, Nils and F{\"o}hlisch, Alexander and Svensson, Svante},
  title     = {Uppsala and Berkeley},
  series = {Journal of vacuum science \& technology : JVST ; an AVS journal / A},
  volume    = {40},
  journal   = {Journal of vacuum science \& technology : JVST ; an AVS journal / A},
  number    = {4},
  publisher = {American Institute of Physics},
  address   = {New York},
  issn      = {0734-2101},
  doi       = {10.1116/6.0001879},
  pages     = {11},
  year      = {2022},
  abstract  = {The development of modern photoelectron spectroscopy is reviewed with a special focus on the importance of research at Uppsala University and at Berkeley. The influence of two pioneers, Kai Siegbahn and Dave Shirley, is underlined. Early interaction between the two centers helped to kick-start the field. Both laboratories have continued to play an important role in the field, both in terms of creating new experimental capabilities and developing the theoretical understanding of the spectroscopic processes.},
  language  = {en}
}
@article{DeckerBornRuotsalainenetal.2021,
  author    = {Decker, Regis and Born, Artur and Ruotsalainen, Kari and Bauer, Karl and Haverkamp, Robert and B{\"u}chner, Robby and Pietzsch, Annette and F{\"o}hlisch, Alexander},
  title     = {Spin-lattice angular momentum transfer of localized and valence electrons in the demagnetization transient state of gadolinium},
  series = {Applied physics letters},
  volume    = {119},
  journal   = {Applied physics letters},
  number    = {15},
  publisher = {AIP Publishing},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/5.0063404},
  pages     = {5},
  year      = {2021},
  abstract  = {The electron-phonon scattering is one of the main microscopic mechanisms responsible for the spin-flip in the transient state of ultrafast demagnetization. Here, we present an experimental determination of the temperature-dependent electron-phonon scattering rate in Gd. Using a static x-ray emission spectroscopy method, where the reduction of the decay peak intensities when increasing the temperature is quantified, we measure independently the electron-phonon scattering rate for the 5d and the 4f electrons. We deduce the temperature dependence of scattering for the 5d electrons, while no effect on the phonon population is observed for the 4f electrons. Our results suggest that the ultrafast magnetization dynamics in Gd is triggered by the spin-flip in the 5d electrons. We also evidence the existence of a temperature threshold, above which spin-flip scattering of the 5d electrons takes place. We deduce that during the transient state of ultrafast demagnetization, the exchange energy between 5d electrons has to be overcome before the microscopic electron-phonon scattering process can occur.},
  language  = {en}
}
@article{LiuRuotsalainenBaueretal.2022,
  author    = {Liu, Chun-Yu and Ruotsalainen, Kari and Bauer, Karl and Decker, R{\´e}gis and Pietzsch, Annette and F{\"o}hlisch, Alexander},
  title     = {Excited-state exchange interaction in NiO determined by high-resolution resonant inelastic x-ray scattering at the Ni M2,3 edges},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {106},
  journal   = {Physical review : B, Condensed matter and materials physics},
  number    = {3},
  publisher = {American Physical Society},
  address   = {Ridge, NY},
  issn      = {2469-9950},
  doi       = {10.1103/PhysRevB.106.035104},
  pages     = {7},
  year      = {2022},
  abstract  = {The electronic and magnetic excitations of bulk NiO have been determined using the 3A2g to 3T2g crystal-field transition at the Ni M2,3 edges with resonant inelastic x-ray scattering at 66.3- and 67.9-eV photon energies and 33-meV spectral resolution. Unambiguous assignment of the high-energy side of this state to a spin-flip satellite is achieved. We extract an effective exchange field of 89±4 meV in the 3T2g excited final state from empirical two-peak spin-flip model. The experimental data is found consistent with crystal-field model calculations using exchange fields of 60-100 meV. Full agreement with crystal-field multiplet calculations is achieved for the incident photon energy dependence of line shapes. The lower exchange parameter in the excited state as compared to the ground-state value of 120 meV is discussed in terms of the modification of the orbital occupancy (electronic effects) and of the structural dynamics: (A) With pure electronic effects, the lower exchange energy is attributed to the reduction in effective hopping integral. (B) With no electronic effects, we use the S = 1 Heisenberg model of antiferromagnetism to derive a second-nearest-neighbor exchange constant J2 = 14.8±0.6 meV. Based on the linear correlation between J2 and the lattice parameter from pressure-dependent experiments, an upper limit of 2\% local Ni-O bond elongation during the femtosecond scattering duration is derived.},
  language  = {en}
}
@article{LiuVazdaCruzPolyutovetal.2019,
  author    = {Liu, Ji-Cai and Vaz da Cruz, Vinicius and Polyutov, Sergey and F{\"o}hlisch, Alexander},
  title     = {Recoil-induced dissociation in hard-x-ray photoionization},
  series = {Physical review : A, Atomic, molecular, and optical physics},
  volume    = {100},
  journal   = {Physical review : A, Atomic, molecular, and optical physics},
  number    = {5},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2469-9926},
  doi       = {10.1103/PhysRevA.100.053408},
  pages     = {7},
  year      = {2019},
  abstract  = {We predict the recoil-induced molecular dissociation in hard-x-ray photoionization. The recoil effect is caused by electronic and photon momentum exchange with the molecule. We show the strong role of relativistic effects for the studied molecular fragmentation. The recoil-induced fragmentation of the molecule is caused by elongation of the bond due to the vibrational recoil effect and because of the centrifugal force caused by the rotational recoil. The calculations of the x-ray photoelectron spectra of the H-2 and NO molecules show that the predicted effects can be observed in high-energy synchrotrons like SOLEIL, SPring-8, PETRA, and XFEL SACLA. The relativistic effect enhances the recoil momentum transfer and makes it strongly sensitive to the direction of ejection of the fast photoelectron with respect to the photon momentum.},
  language  = {en}
}
@article{BuechnerFondellMascarenhasetal.2021,
  author    = {B{\"u}chner, Robby and Fondell, Mattis and Mascarenhas, Eric Johnn and Pietzsch, Annette and Vaz da Cruz, Vin{\´i}cius and F{\"o}hlisch, Alexander},
  title     = {How hydrogen bonding amplifies isomeric differences in pyridones toward strong changes in acidity and tautomerism},
  series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry},
  volume    = {125},
  journal   = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry},
  number    = {9},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1520-6106},
  doi       = {10.1021/acs.jpcb.0c10873},
  pages     = {2372 -- 2379},
  year      = {2021},
  abstract  = {Steric hindrance of hydration and hydrogen bond enhancement by localized charges have been identified as key factors for the massive chemical differences between the hydroxypyridine/pyridone isomers in aqueous solution. While all isomers occur mainly in the hydroxypyridine form in the gas phase, they differ by more than 3 orders of magnitude both in their acidity and tautomeric equilibrium constants upon hydration. By monitoring the electronic and solvation structures as a function of the protonation state and the O- substitution position on the pyridine ring, the amplification of the isomeric differences in aqueous solution has been investigated. Near-edge X-ray absorption fine structure (NEXAFS) measurements at the N K-edge served as the probe of the chemical state. The combination of molecular dynamics simulations, complete active space self-consistent field (CASSCF), and time-dependent density functional theory (TD-DFT) spectral calculations contributes to unraveling the principles of tautomerism and acidity in multiple biochemical systems based on tautomerism.},
  language  = {en}
}
@article{BuechnerFondellHaverkampetal.2021,
  author    = {B{\"u}chner, Robby and Fondell, Mattis and Haverkamp, Robert and Pietzsch, Annette and Vaz da Cruz, Vin{\´i}cius and F{\"o}hlisch, Alexander},
  title     = {The porphyrin center as a regulator for metal-ligand covalency and pi hybridization in the entire molecule},
  series = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies},
  volume    = {23},
  journal   = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies},
  number    = {43},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1463-9076},
  doi       = {10.1039/d1cp03944j},
  pages     = {24765 -- 24772},
  year      = {2021},
  abstract  = {The central moiety of porphyrins is shown to control the charge state of the inner complex and links it by covalent interaction to the peripheral substituents. This link, which enables the versatile functions of porphyrins, is not picked up in the established, reduced four orbital picture [Gouterman, J. Mol. Spectrosc., 1961, 6, 138]. X-ray absorption spectroscopy at the N K-edge with density functional theory approaches gives access to the full electronic structure, in particular the pi* manifold beyond the Gouterman orbitals. Systematic variation of the central moiety highlights two linked, governing trends: The ionicity of the porphyrin center increases from the aminic N-H to N-Cu to N-Zn to N-Mg to the iminic N:. At the same time covalency with peripheral substituents increases and compensates the buildup of high charge density at the coordinated nitrogen sites.},
  language  = {en}
}
@article{EckertVazdaCruzOchmannetal.2021,
  author    = {Eckert, Sebastian and Vaz da Cruz, Vin{\´i}cius and Ochmann, Miguel and Ahnen, Inga von and F{\"o}hlisch, Alexander and Huse, Nils},
  title     = {Breaking the symmetry of pyrimidine},
  series = {The journal of physical chemistry letters},
  volume    = {12},
  journal   = {The journal of physical chemistry letters},
  number    = {35},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.1c01865},
  pages     = {8637 -- 8643},
  year      = {2021},
  abstract  = {Symmetry and its breaking crucially define the chemical properties of molecules and their functionality. Resonant inelastic X-ray scattering is a local electronic structure probe reporting on molecular symmetry and its dynamical breaking within the femtosecond scattering duration. Here, we study pyrimidine, a system from the C-2v point group, in an aqueous solution environment, using scattering though its 2a(2) resonance. Despite the absence of clean parity selection rules for decay transitions from in-plane orbitals, scattering channels including decay from the 7b(2) and 11a(1) orbitals with nitrogen lone pair character are a direct probe for molecular symmetry. Computed spectra of explicitly solvated molecules sampled from a molecular dynamics simulation are combined with the results of a quantum dynamical description of the X-ray scattering process. We observe dominant signatures of core-excited Jahn-Teller induced symmetry breaking for resonant excitation. Solvent contributions are separable by shortening of the effective scattering duration through excitation energy detuning.},
  language  = {en}
}
@article{JayNorellEckertetal.2018,
  author    = {Jay, Raphael M. and Norell, Jesper and Eckert, Sebastian and Hantschmann, Markus and Beye, Martin and Kennedy, Brian and Quevedo, Wilson and Schlotter, William F. and Dakovski, Georgi L. and Minitti, Michael P. and Hoffmann, Matthias C. and Mitra, Ankush and Moeller, Stefan P. and Nordlund, Dennis and Zhang, Wenkai and Liang, Huiyang W. and Kunnus, Kristian and Kubicek, Katharina and Techert, Simone A. and Lundberg, Marcus and Wernet, Philippe and Gaffney, Kelly and Odelius, Michael and F{\"o}hlisch, Alexander},
  title     = {Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering},
  series = {The journal of physical chemistry letters},
  volume    = {9},
  journal   = {The journal of physical chemistry letters},
  number    = {12},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.8b01429},
  pages     = {3538 -- 3543},
  year      = {2018},
  abstract  = {Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. pi-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences sigma-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.},
  language  = {en}
}
@article{NiskanenFondellSahleetal.2019,
  author    = {Niskanen, Johannes and Fondell, Mattis and Sahle, Christoph J. and Eckert, Sebastian and Jay, Raphael Martin and Gilmore, Keith and Pietzsch, Annette and Dantz, Marcus and Lu, Xingye and McNally, Daniel E. and Schmitt, Thorsten and Vaz da Cruz, Vinicius and Kimberg, Victor and F{\"o}hlisch, Alexander and Gel'mukhanov, Faris},
  title     = {Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions},
  series = {Proceedings of the National Academy of Sciences of the United States of America},
  volume    = {116},
  journal   = {Proceedings of the National Academy of Sciences of the United States of America},
  number    = {10},
  publisher = {National Acad. of Sciences},
  address   = {Washington},
  issn      = {0027-8424},
  doi       = {10.1073/pnas.1815701116},
  pages     = {4058 -- 4063},
  year      = {2019},
  abstract  = {The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 +/- 2.1\% donated and accepted hydrogen bonds per molecule, pointing to a continuous-distribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray-matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscale-length correlations within the continuous distribution remains open.},
  language  = {en}
}