@article{VazdaCruzIgnatovaCoutoetal.2019, author = {Vaz da Cruz, Vin{\´i}cius and Ignatova, Nina and Couto, Rafael and Fedotov, Daniil and Rehn, Dirk R. and Savchenko, Viktoriia and Norman, Patrick and {\AA}gren, Hans and Polyutov, Sergey and Niskanen, Johannes and Eckert, Sebastian and Jay, Raphael Martin and Fondell, Mattis and Schmitt, Thorsten and Pietzsch, Annette and F{\"o}hlisch, Alexander and Odelius, Michael and Kimberg, Victor and Gel'mukhanov, Faris}, title = {Nuclear dynamics in resonant inelastic X-ray scattering and X-ray absorption of methanol}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {150}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {23}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.5092174}, pages = {20}, year = {2019}, abstract = {We report on a combined theoretical and experimental study of core-excitation spectra of gas and liquid phase methanol as obtained with the use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). The electronic transitions are studied with computational methods that include strict and extended second-order algebraic diagrammatic construction [ADC(2) and ADC(2)-x], restricted active space second-order perturbation theory, and time-dependent density functional theory-providing a complete assignment of the near oxygen K-edge XAS. We show that multimode nuclear dynamics is of crucial importance for explaining the available experimental XAS and RIXS spectra. The multimode nuclear motion was considered in a recently developed "mixed representation" where dissociative states and highly excited vibrational modes are accurately treated with a time-dependent wave packet technique, while the remaining active vibrational modes are described using Franck-Condon amplitudes. Particular attention is paid to the polarization dependence of RIXS and the effects of the isotopic substitution on the RIXS profile in the case of dissociative core-excited states. Our approach predicts the splitting of the 2a RIXS peak to be due to an interplay between molecular and pseudo-atomic features arising in the course of transitions between dissociative core- and valence-excited states. The dynamical nature of the splitting of the 2a peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical results are in good agreement with our liquid phase measurements and gas phase experimental data available from the literature. (C) 2019 Author(s).}, language = {en} } @article{VazdaCruzEckertIannuzzietal.2019, author = {Vaz da Cruz, Vinicius and Eckert, Sebastian and Iannuzzi, Marcella and Ertan, Emelie and Pietzsch, Annette and Couto, Rafael C. and Niskanen, Johannes and Fondell, Mattis and Dantz, Marcus and Schmitt, Thorsten and Lu, Xingye and McNally, Daniel and Jay, Raphael Martin and Kimberg, Victor and F{\"o}hlisch, Alexander and Odelius, Michael}, title = {Probing hydrogen bond strength in liquid water by resonant inelastic X-ray scattering}, series = {Nature Communications}, volume = {10}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/s41467-019-08979-4}, pages = {9}, year = {2019}, abstract = {Local probes of the electronic ground state are essential for understanding hydrogen bonding in aqueous environments. When tuned to the dissociative core-excited state at the O1s pre-edge of water, resonant inelastic X-ray scattering back to the electronic ground state exhibits a long vibrational progression due to ultrafast nuclear dynamics. We show how the coherent evolution of the OH bonds around the core-excited oxygen provides access to high vibrational levels in liquid water. The OH bonds stretch into the long-range part of the potential energy curve, which makes the X-ray probe more sensitive than infra-red spectroscopy to the local environment. We exploit this property to effectively probe hydrogen bond strength via the distribution of intramolecular OH potentials derived from measurements. In contrast, the dynamical splitting in the spectral feature of the lowest valence-excited state arises from the short-range part of the OH potential curve and is rather insensitive to hydrogen bonding.}, language = {en} } @article{VazdaCruzBuechnerFondelletal.2022, author = {Vaz da Cruz, Vinicius and B{\"u}chner, Robby and Fondell, Mattis and Pietzsch, Annette and Eckert, Sebastian and F{\"o}hlisch, Alexander}, title = {Targeting individual tautomers in equilibrium by resonant inelastic X-ray scattering}, series = {The journal of physical chemistry letters}, volume = {13}, journal = {The journal of physical chemistry letters}, number = {10}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.1c03453}, pages = {2459 -- 2466}, year = {2022}, abstract = {Tautomerism is one of the most important forms of isomerism, owing to the facile interconversion between species and the large differences in chemical properties introduced by the proton transfer connecting the tautomers. Spectroscopic techniques are often used for the characterization of tautomers. In this context, separating the overlapping spectral response of coexisting tautomers is a long-standing challenge in chemistry. Here, we demonstrate that by using resonant inelastic X-ray scattering tuned to the core excited states at the site of proton exchange between tautomers one is able to experimentally disentangle the manifold of valence excited states of each tautomer in a mixture. The technique is applied to the prototypical keto-enol equilibrium of 3-hydroxypyridine in aqueous solution. We detect transitions from the occupied orbitals into the LUMO for each tautomer in solution, which report on intrinsic and hydrogen-bond-induced orbital polarization within the pi and sigma manifolds at the proton-transfer site.}, language = {en} } @article{SchickEckertPontiusetal.2016, author = {Schick, Daniel and Eckert, Sebastian and Pontius, Niko and Mitzner, Rolf and F{\"o}hlisch, Alexander and Holldack, Karsten and Sorgenfrei, Florian}, title = {Versatile soft X-ray-optical cross-correlator for ultrafast applications}, series = {Structural dynamics}, volume = {3}, journal = {Structural dynamics}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.4964296}, pages = {054304-1 -- 054304-8}, year = {2016}, abstract = {We present an X-ray-optical cross-correlator for the soft (> 150 eV) up to the hard X-ray regime based on a molybdenum-silicon superlattice. The cross-correlation is done by probing intensity and position changes of superlattice Bragg peaks caused by photoexcitation of coherent phonons. This approach is applicable for a wide range of X-ray photon energies as well as for a broad range of excitation wavelengths and requires no external fields or changes of temperature. Moreover, the cross-correlator can be employed on a 10 ps or 100 fs time scale featuring up to 50\% total X-ray reflectivity and transient signal changes of more than 20\%. (C) 2016 Author(s).}, language = {en} } @misc{SchickEckertPontiusetal.2016, author = {Schick, Daniel and Eckert, Sebastian and Pontius, Niko and Mitzner, Rolf and F{\"o}hlisch, Alexander and Holldack, Karsten and Sorgenfrei, Florian}, title = {Versatile soft X-ray-optical cross-correlator for ultrafast applications}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1331}, issn = {1866-8372}, doi = {10.25932/publishup-43696}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-436962}, pages = {054304-1 -- 054304-8}, year = {2016}, abstract = {We present an X-ray-optical cross-correlator for the soft (> 150 eV) up to the hard X-ray regime based on a molybdenum-silicon superlattice. The cross-correlation is done by probing intensity and position changes of superlattice Bragg peaks caused by photoexcitation of coherent phonons. This approach is applicable for a wide range of X-ray photon energies as well as for a broad range of excitation wavelengths and requires no external fields or changes of temperature. Moreover, the cross-correlator can be employed on a 10 ps or 100 fs time scale featuring up to 50\% total X-ray reflectivity and transient signal changes of more than 20\%. (C) 2016 Author(s).}, language = {en} } @article{PontiusBeyeTrabantetal.2018, author = {Pontius, Niko and Beye, Martin and Trabant, Christoph and Mitzner, Rolf and Sorgenfrei, Florian and Kachel, Torsten and Woestmann, Michael and Roling, Sebastian and Zacharias, Helmut and Ivanov, Rosen and Treusch, Rolf and Buchholz, Marcel and Metcalf, Pete and Schuessler-Langeheine, Christian and F{\"o}hlisch, Alexander}, title = {Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment}, series = {Structural dynamics}, volume = {5}, journal = {Structural dynamics}, number = {5}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.5042847}, pages = {8}, year = {2018}, abstract = {We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 +/- 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices. (C) 2018 Author(s).}, language = {en} } @article{PietzschNiskanenVazdaCruzetal.2022, author = {Pietzsch, Annette and Niskanen, Johannes and Vaz da Cruz, Vinicius and B{\"u}chner, Robby and Eckert, Sebastian and Fondell, Mattis and Jay, Raphael Martin and Lu, Xingye and McNally, Daniel and Schmitt, Thorsten and F{\"o}hlisch, Alexander}, title = {Cuts through the manifold of molecular H2O potential energy surfaces in liquid water at ambient conditions}, series = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {119}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, number = {28}, publisher = {National Acad. of Sciences}, address = {Washington, DC}, issn = {1091-6490}, doi = {10.1073/pnas.2118101119}, pages = {6}, year = {2022}, abstract = {The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H2O molecular moieties. This is mirrored in a manifold of the H2O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H2O symmetric normal mode and the local asymmetric O-H bond coordinate up to 1 and 1.5 angstrom, respectively. The comparison of the single H2O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H2O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O-H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen ls to 4a(1) resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1 s to 2b(2) resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H2O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O-H, H-H, and O-O correlation lengths from neutron scattering.}, language = {en} } @article{OchmannVazdaCruzEckertetal.2022, author = {Ochmann, Miguel and Vaz da Cruz, Vinicius and Eckert, Sebastian and Huse, Nils and F{\"o}hlisch, Alexander}, title = {R-Group stabilization in methylated formamides observed by resonant inelastic X-ray scattering}, series = {Chemical communications: ChemComm}, volume = {58}, journal = {Chemical communications: ChemComm}, number = {63}, publisher = {The Royal Society of Chemistry}, address = {Cambridge}, issn = {1359-7345}, doi = {10.1039/d2cc00053a}, pages = {8834 -- 8837}, year = {2022}, abstract = {The inherent stability of methylated formamides is traced to a stabilization of the deep-lying sigma-framework by resonant inelastic X-ray scattering at the nitrogen K-edge. Charge transfer from the amide nitrogen to the methyl groups underlie this stabilization mechanism that leaves the aldehyde group essentially unaltered and explains the stability of secondary and tertiary amides.}, language = {en} } @misc{NorellJayHantschmannetal.2018, author = {Norell, Jesper and Jay, Raphael and Hantschmann, Markus and Eckert, Sebastian and Guo, Meiyuan and Gaffney, Kelly and Wernet, Philippe and Lundberg, Marcus and F{\"o}hlisch, Alexander and Odelius, Michael}, title = {Fingerprints of electronic, spin and structural dynamics from resonant inelastic soft x-ray scattering in transient photo-chemical species}, series = {Physical chemistry, chemical physics}, journal = {Physical chemistry, chemical physics}, number = {20}, publisher = {RSC Publ.}, address = {Cambridge}, issn = {1463-9084}, doi = {10.1039/c7cp08326b}, pages = {7243 -- 7253}, year = {2018}, abstract = {We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L3-edge RIXS in the ferricyanide complex Fe(CN)63-, we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics.}, language = {en} } @article{NorellEckertVanKuikenetal.2019, author = {Norell, Jesper and Eckert, Sebastian and Van Kuiken, Benjamin E. and F{\"o}hlisch, Alexander and Odelius, Michael}, title = {Ab initio simulations of complementary K-edges and solvatization effects for detection of proton transfer in aqueous 2-thiopyridone}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {151}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {11}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.5109840}, pages = {12}, year = {2019}, abstract = {The nitrogen and sulfur K-edge X-ray absorption spectra of aqueous 2-thiopyridone, a model system for excited-state proton transfer in several recent time-resolved measurements, have been simulated from ab initio molecular dynamics. Spectral signatures of the local intra- and inter-molecular structure are identified and rationalized, which facilitates experimental interpretation and optimization. In particular, comparison of aqueous and gas phase spectrum simulations assesses the previously unquantified solvatization effects, where hydrogen bonding is found to yield solvatochromatic shifts up to nearly 1 eV of the main peak positions. Thereby, while each K-edge can still decisively determine the local protonation of its core-excited site, only their combined, complementary fingerprints allow separating all of the three relevant molecular forms, giving a complete picture of the proton transfer.}, 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} } @misc{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}, title = {Reply to Pettersson et al.: Why X-ray spectral features are compatible to continuous distribution models in ambient water}, 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 = {35}, publisher = {National Acad. of Sciences}, address = {Washington}, issn = {0027-8424}, doi = {10.1073/pnas.1909551116}, pages = {17158 -- 17159}, year = {2019}, language = {en} } @article{MascarenhasFondellBuechneretal.2022, author = {Mascarenhas, Eric Johnn and Fondell, Mattis and B{\"u}chner, Robby and Eckert, Sebastian and Vaz da Cruz, Vin{\´i}cius and F{\"o}hlisch, Alexander}, title = {Photo-induced ligand substitution of Cr(CO)(6) in 1-pentanol probed by time resolved X-ray absorption spectroscopy}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {24}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {30}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/d1cp05834g}, pages = {17979 -- 17985}, year = {2022}, abstract = {Cr(CO)(6) was investigated by X-ray absorption spectroscopy. The spectral signature at the metal edge provides information about the back-bonding of the metal in this class of complexes. Among the processes it participates in is ligand substitution in which a carbonyl ligand is ejected through excitation to a metal to ligand charge transfer (MLCT) band. The unsaturated carbonyl Cr(CO)(5) is stabilized by solution media in square pyramidal geometry and further reacts with the solvent. Multi-site-specific probing after photoexcitation was used to investigate the ligand substitution photoreaction process which is a common first step in catalytic processes involving metal carbonyls. The data were analysed with the aid of TD-DFT computations for different models of photoproducts and signatures for ligand rearrangement after substitution were found. The rearrangement was found to occur in about 790 ps in agreement with former studies of the photoreaction.}, 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{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{LeitnerJosefssonMazzaetal.2018, author = {Leitner, T. and Josefsson, Ida and Mazza, T. and Miedema, Piter S. and Schr{\"o}der, H. and Beye, Martin and Kunnus, Kristjan and Schreck, S. and D{\"u}sterer, Stefan and F{\"o}hlisch, Alexander and Meyer, M. and Odelius, Michael and Wernet, Philippe}, title = {Time-resolved electron spectroscopy for chemical analysis of photodissociation}, series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, volume = {149}, journal = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr}, number = {4}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-9606}, doi = {10.1063/1.5035149}, pages = {12}, year = {2018}, abstract = {The prototypical photoinduced dissociation of Fe(CO)(5) in the gas phase is used to test time-resolved x-ray photoelectron spectroscopy for studying photochemical reactions. Upon one-photon excitation at 266 nm, Fe(CO)(5) successively dissociates to Fe(CO)(4) and Fe(CO)(3) along a pathway where both fragments retain the singlet multiplicity of Fe(CO)(5). The x-ray free-electron laser FLASH is used to probe the reaction intermediates Fe(CO)(4) and Fe(CO)(3) with time-resolved valence and core-level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valenceorbital interactions upon Fe-CO dissociation, thereby validating fundamental theoretical concepts in Fe-CO bonding. Chemical shifts of CO 3 sigma inner-valence and Fe 3 sigma core-level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination-dependent charge localization and core-hole screening based on calculated changes in electron densities upon core-hole creation in the final ionic states. This extends the established capabilities of steady-state electron spectroscopy for chemical analysis to time-resolved investigations. It could also serve as a benchmark for howcharge and spin density changes in molecular dissociation and excited-state dynamics are expressed in valence and core-level photoelectron spectroscopy. Published by AIP Publishing.}, language = {en} } @article{KuehnMuellerSorgenfreietal.2019, author = {K{\"u}hn, Danilo and M{\"u}ller, Moritz and Sorgenfrei, Florian and Giangrisostomi, Erika and Jay, Raphael Martin and Ovsyannikov, Ruslan and Martensson, Nils and Sanchez-Portal, Daniel and F{\"o}hlisch, Alexander}, title = {Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2}, series = {Scientific reports}, volume = {9}, journal = {Scientific reports}, number = {488}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-018-36637-0}, pages = {9}, year = {2019}, abstract = {For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.}, language = {en} } @article{KuehnGiangrisostomiJayetal.2019, author = {K{\"u}hn, Danilo and Giangrisostomi, Erika and Jay, Raphael Martin and Sorgenfrei, Florian and F{\"o}hlisch, Alexander}, title = {The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission}, series = {New journal of physics : the open-access journal for physics}, volume = {21}, journal = {New journal of physics : the open-access journal for physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {1367-2630}, doi = {10.1088/1367-2630/ab2f5c}, pages = {12}, year = {2019}, abstract = {Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources.}, language = {en} } @article{KunnusJosefssonSchrecketal.2017, author = {Kunnus, Kristjan and Josefsson, I. and Schreck, Simon Frederik and Quevedo, W. and Miedema, P. S. and Techert, S. and de Groot, F. M. F. and F{\"o}hlisch, Alexander and Odelius, M. and Wernet, Ph.}, title = {Quantifying covalent interactions with resonant inelastic soft X-ray scattering}, series = {Chemical physics letters}, volume = {669}, journal = {Chemical physics letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0009-2614}, doi = {10.1016/j.cplett.2016.12.046}, pages = {196 -- 201}, year = {2017}, abstract = {We analyze the effects of covalent interactions in Ni 2p3d resonant inelastic X-ray scattering (RIXS) spectra from aqueous Ni2+ ions and find that the relative RIXS intensities of ligand-to-metal charge-transfer final states with respect to the ligand-field final states reflect the covalent mixing between Ni 3d and water orbitals. Specifically, the experimental intensity ratio at the Ni L-3-edge allows to determine that the Ni 3d orbitals have on average 5.5\% of water character. We propose that 2p3d RIXS at the Ni L-3-edge can be utilized to quantify covalency in Ni complexes without the use of external references or simulations.}, language = {en} } @article{JayEckertVazdaCruzetal.2019, author = {Jay, Raphael Martin and Eckert, Sebastian and Vaz da Cruz, Vinicius and Fondell, Mattis and Mitzner, Rolf and F{\"o}hlisch, Alexander}, title = {Covalency-driven preservation of local charge densities in a metal-to-ligand charge-transfer excited iron photosensitizer}, series = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition}, volume = {58}, journal = {Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition}, number = {31}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1433-7851}, doi = {10.1002/anie.201904761}, pages = {10742 -- 10746}, year = {2019}, abstract = {Charge-density rearrangements after metal-to-ligand charge-transfer excitation in an iron photosensitizer are investigated by R. M Jay, A. Fohlisch et al. in their Communication (DOI: 10.1002/anie.201904761). By using time-resolved X-ray absorption spectroscopy, surprising covalency-effects are revealed that inhibit charge-separation at the intra-molecular level. Furthermore, the underlying mechanism is proposed to be generally in effect for all commonly used photosensitizers in light-harvesting applications, which challenges the common perception of electronic charge-transfer.}, language = {en} } @article{JayEckertFondelletal.2018, author = {Jay, Raphael Martin and Eckert, Sebastian and Fondell, Mattis and Miedema, Piter S. and Norell, Jesper and Pietzsch, Annette and Quevedo, Wilson and Niskanen, Johannes and Kunnus, Kristjan and F{\"o}hlisch, Alexander}, title = {The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes}, series = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, volume = {20}, journal = {Physical chemistry, chemical physics : a journal of European Chemical Societies}, number = {44}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1463-9076}, doi = {10.1039/c8cp04341h}, pages = {27745 -- 27751}, year = {2018}, abstract = {Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X-ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron(II) center. Exchanging cyanide with 2-2′-bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal-ligand covalency results in lower metal-centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent-effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution-phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light-harvesting applications.}, language = {en} } @article{JayVazdaCruzEckertetal.2020, author = {Jay, Raphael M. and Vaz da Cruz, Vinicius and Eckert, Sebastian and Fondell, Mattis and Mitzner, Rolf and F{\"o}hlisch, Alexander}, title = {Probing solute-solvent interactions of transition metal complexes using L-edge absorption spectroscopy}, series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, volume = {124}, journal = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, number = {27}, publisher = {American Chemical Society}, address = {Washington}, issn = {1520-6106}, doi = {10.1021/acs.jpcb.0c00638}, pages = {5636 -- 5645}, year = {2020}, abstract = {In order to tailor solution-phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute-solvent interactions of a solvatochromic mixed-ligand iron complex were investigated using X-ray absorption spectroscopy at the transition metal L-2,L-3-edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L-2,L-3-edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal-ligand-bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time-dependent density functional theory. The simulations reproduce the spectral trends observed in the X-ray but also optical absorption experiments. Our results underscore the importance of solute-solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L-2,L-3-edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distribution.}, 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{JayEckertMitzneretal.2020, author = {Jay, Raphael M. and Eckert, Sebastian and Mitzner, Rolf and Fondell, Mattis and F{\"o}hlisch, Alexander}, title = {Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective}, series = {Chemical physics letters}, volume = {754}, journal = {Chemical physics letters}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0009-2614}, doi = {10.1016/j.cplett.2020.137681}, pages = {5}, year = {2020}, abstract = {It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L-3-edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range.}, language = {en} } @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{GiangrisostomiOvsyannikovSorgenfreietal.2018, author = {Giangrisostomi, Erika and Ovsyannikov, Ruslan and Sorgenfrei, Florian and Zhang, Teng and Lindblad, Andreas and Sassa, Yasmine and Cappel, Ute B. and Leitner, Torsten and Mitzner, Rolf and Svensson, Svante and Martensson, Nils and F{\"o}hlisch, Alexander}, title = {Low Dose Photoelectron Spectroscopy at BESSY II}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {224}, 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.2017.05.011}, pages = {68 -- 78}, year = {2018}, abstract = {The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances.}, language = {en} } @misc{FondellEckertJayetal.2017, author = {Fondell, Mattis and Eckert, Sebastian and Jay, Raphael Martin and Weniger, Christian and Quevedo, Wilson and Niskanen, Johannes and Kennedy, Brian and Sorgenfrei, Florian and Schick, Daniel and Giangrisostomi, Erika and Ovsyannikov, Ruslan and Adamczyk, Katrin and Huse, Nils and Wernet, Philippe and Mitzner, Rolf and F{\"o}hlisch, Alexander}, title = {Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates}, series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe}, number = {780}, issn = {1866-8372}, doi = {10.25932/publishup-43752}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-437529}, pages = {12}, year = {2017}, abstract = {We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology.}, 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{EckertVazdaCruzErtanetal.2018, author = {Eckert, Sebastian and Vaz da Cruz, Vinicius and Ertan, Emelie and Ignatova, Nina and Polyutov, Sergey and Couto, Rafael C. and Fondell, Mattis and Dantz, Marcus and Kennedy, Brian and Schmitt, Thorsten and Pietzsch, Annette and Odelius, Michael and F{\"o}hlisch, Alexander}, title = {One-dimensional cuts through multidimensional potential-energy surfaces by tunable x rays}, series = {Physical review : A, Atomic, molecular, and optical physics}, volume = {97}, 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.97.053410}, pages = {7}, year = {2018}, abstract = {The concept of the potential-energy surface (PES) and directional reaction coordinates is the backbone of our description of chemical reaction mechanisms. Although the eigenenergies of the nuclear Hamiltonian uniquely link a PES to its spectrum, this information is in general experimentally inaccessible in large polyatomic systems. This is due to (near) degenerate rovibrational levels across the parameter space of all degrees of freedom, which effectively forms a pseudospectrum given by the centers of gravity of groups of close-lying vibrational levels. We show here that resonant inelastic x-ray scattering (RIXS) constitutes an ideal probe for revealing one-dimensional cuts through the ground-state PES of molecular systems, even far away from the equilibrium geometry, where the independent-mode picture is broken. We strictly link the center of gravity of close-lying vibrational peaks in RIXS to a pseudospectrum which is shown to coincide with the eigenvalues of an effective one-dimensional Hamiltonian along the propagation coordinate of the core-excited wave packet. This concept, combined with directional and site selectivity of the core-excited states, allows us to experimentally extract cuts through the ground-state PES along three complementary directions for the showcase H2O molecule.}, language = {en} } @article{EckertNorellJayetal.2019, author = {Eckert, Sebastian and Norell, Jesper and Jay, Raphael Martin and Fondell, Mattis and Mitzner, Rolf and Odelius, Michael and F{\"o}hlisch, Alexander}, title = {T-1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone}, series = {Chemistry - a European journal}, volume = {25}, journal = {Chemistry - a European journal}, number = {7}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.201804166}, pages = {1733 -- 1739}, year = {2019}, abstract = {Excited-state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2-thiopyridone, by employing transient N1s X-ray absorption spectroscopy and multi-configurational spectrum simulations. Photoexcitations to the singlet S-2 and S-4 states both relax promptly through intersystem crossing to the triplet T-1 state. The T-1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S-0 potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved.}, language = {en} } @article{EckertMascarenhasMitzneretal.2022, author = {Eckert, Sebastian and Mascarenhas, Eric Johnn and Mitzner, Rolf and Jay, Raphael Martin and Pietzsch, Annette and Fondell, Mattis and Vaz da Cruz, Vinicius and F{\"o}hlisch, Alexander}, title = {From the free ligand to the transition metal complex}, series = {Inorganic chemistry}, volume = {61}, journal = {Inorganic chemistry}, number = {27}, publisher = {American Chemical Society}, address = {Washington}, issn = {0020-1669}, doi = {10.1021/acs.inorgchem.2c00789}, pages = {10321 -- 10328}, year = {2022}, abstract = {Chelating agents are an integral part of transition metal complex chemistry with broad biological and industrial relevance. The hexadentate chelating agent ethylenediaminetetraacetic acid (EDTA) has the capability to bind to metal ions at its two nitrogen and four of its carboxylate oxygen sites. We use resonant inelastic X-ray scattering at the 1s absorption edge of the aforementioned elements in EDTA and the iron(III)-EDTA complex to investigate the impact of the metal-ligand bond formation on the electronic structure of EDTA. Frontier orbital distortions, occupation changes, and energy shifts through metal- ligand bond formation are probed through distinct spectroscopic signatures.}, language = {en} } @article{DeckerBornBuechneretal.2019, author = {Decker, R{\´e}gis and Born, Artur and B{\"u}chner, Robby and Ruotsalainen, Kari and Str{\aa}hlman, Christian and Neppl, Stefan and Haverkamp, Robert and Pietzsch, Annette and F{\"o}hlisch, Alexander}, title = {Measuring the atomic spin-flip scattering rate by x-ray emission spectroscopy}, series = {Scientific reports}, volume = {9}, journal = {Scientific reports}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-019-45242-8}, pages = {6}, year = {2019}, abstract = {While extensive work has been dedicated to the measurement of the demagnetization time following an ultra-short laser pulse, experimental studies of its underlying microscopic mechanisms are still scarce. In transition metal ferromagnets, one of the main mechanism is the spin-flip of conduction electrons driven by electron-phonon scattering. Here, we present an original experimental method to monitor the electron-phonon mediated spin-flip scattering rate in nickel through the stringent atomic symmetry selection rules of x-ray emission spectroscopy. Increasing the phonon population leads to a waning of the 3d -> 2p(3/2) decay peak intensity, which reflects an increase of the angular momentum transfer scattering rate attributed to spin-flip. We find a spin relaxation time scale in the order of 50 fs in the 3d-band of nickel at room temperature, while consistantly, no such peak evolution is observed for the diamagnetic counterexample copper, using the same method.}, 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{CoutoCruzErtanetal.2017, author = {Couto, Rafael C. and Cruz, Vinicius V. and Ertan, Emelie and Eckert, Sebastian and Fondell, Mattis and Dantz, Marcus and Kennedy, Brian and Schmitt, Thorsten and Pietzsch, Annette and Guimaraes, Freddy F. and Agren, Hans and Odelius, Michael and Kimberg, Victor and F{\"o}hlisch, Alexander}, title = {Selective gating to vibrational modes through resonant X-ray scattering}, series = {Nature Communications}, volume = {8}, journal = {Nature Communications}, publisher = {Nature Publ. Group}, address = {London}, issn = {2041-1723}, doi = {10.1038/ncomms14165}, pages = {7}, year = {2017}, abstract = {The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. Here we present a resonant inelastic X-ray scattering (RIXS) study to show how it is possible to break down such a complex multidimensional problem into elementary components. Local multimode nuclear wave packets created by X-ray excitation to different core-excited potential energy surfaces (PESs) will act as spatial gates to selectively probe the particular ground-state vibrational modes and, hence, the PES along these modes. We demonstrate this principle by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations.}, 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{BornDeckerHaverkampetal.2021, author = {Born, Artur and Decker, Regis and Haverkamp, Robert and Ruotsalainen, Kari and Bauer, Karl and Pietzsch, Annette and F{\"o}hlisch, Alexander and B{\"u}chner, Robby}, title = {Thresholding of the Elliott-Yafet spin-flip scattering in multi-sublattice magnets by the respective exchange energies}, series = {Scientific reports}, volume = {11}, journal = {Scientific reports}, number = {1}, publisher = {Springer Nature}, address = {Berlin}, issn = {2045-2322}, doi = {10.1038/s41598-021-81177-9}, pages = {7}, year = {2021}, abstract = {How different microscopic mechanisms of ultrafast spin dynamics coexist and interplay is not only relevant for the development of spintronics but also for the thorough description of physical systems out-of-equilibrium. In pure crystalline ferromagnets, one of the main microscopic mechanism of spin relaxation is the electron-phonon (el-ph) driven spin-flip, or Elliott-Yafet, scattering. Unexpectedly, recent experiments with ferro- and ferrimagnetic alloys have shown different dynamics for the different sublattices. These distinct sublattice dynamics are contradictory to the Elliott-Yafet scenario. In order to rationalize this discrepancy, it has been proposed that the intra- and intersublattice exchange interaction energies must be considered in the microscopic demagnetization mechanism, too. Here, using a temperature-dependent x-ray emission spectroscopy (XES) method, we address experimentally the element specific el-ph angular momentum transfer rates, responsible for the spin-flips in the respective (sub)lattices of Fe20Ni80, Fe50Ni50 and pure nickel single crystals. We establish how the deduced rate evolution with the temperature is linked to the exchange coupling constants reported for different alloy stoichiometries and how sublattice exchange energies threshold the related el-ph spin-flip channels. Thus, these results evidence that the Elliott-Yafet spin-flip scattering, thresholded by sublattice exchange energies, is the relevant microscopic process to describe sublattice dynamics in alloys and elemental magnetic systems.}, language = {en} }