TY - JOUR A1 - Kühn, Danilo A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Sorgenfrei, Nomi A1 - Föhlisch, Alexander T1 - The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission JF - New journal of physics : the open-access journal for physics N2 - 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. KW - space-charge effects KW - mean-field model KW - x-ray photoemission KW - electron spectroscopy KW - pump-probe KW - ARTOF Y1 - 2019 U6 - https://doi.org/10.1088/1367-2630/ab2f5c SN - 1367-2630 VL - 21 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Jay, Raphael M. A1 - Vaz da Cruz, Vinicius A1 - Eckert, Sebastian A1 - Fondell, Mattis A1 - Mitzner, Rolf A1 - Föhlisch, Alexander T1 - Probing solute-solvent interactions of transition metal complexes using L-edge absorption spectroscopy JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - 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. KW - basis-sets KW - charge-transfer KW - density KW - dynamics KW - electron localization KW - iron KW - solvation KW - spin-crossover KW - tranfer excited-state KW - x-ray-absorption Y1 - 2020 U6 - https://doi.org/10.1021/acs.jpcb.0c00638 SN - 1520-6106 SN - 1520-5207 VL - 124 IS - 27 SP - 5636 EP - 5645 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Jay, Raphael M. A1 - Eckert, Sebastian A1 - Mitzner, Rolf A1 - Fondell, Mattis A1 - Föhlisch, Alexander T1 - Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective JF - Chemical physics letters N2 - 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. KW - iron cyanides KW - photochemistry KW - soft X-ray absorption Y1 - 2020 U6 - https://doi.org/10.1016/j.cplett.2020.137681 SN - 0009-2614 SN - 1873-4448 VL - 754 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Decker, Regis A1 - Born, Artur A1 - Ruotsalainen, Kari A1 - Bauer, Karl A1 - Haverkamp, Robert A1 - Büchner, Robby A1 - Pietzsch, Annette A1 - Föhlisch, Alexander T1 - Spin-lattice angular momentum transfer of localized and valence electrons in the demagnetization transient state of gadolinium JF - Applied physics letters N2 - 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. Y1 - 2021 U6 - https://doi.org/10.1063/5.0063404 SN - 0003-6951 SN - 1077-3118 VL - 119 IS - 15 PB - AIP Publishing CY - Melville ER - TY - JOUR A1 - Born, Artur A1 - Decker, Regis A1 - Haverkamp, Robert A1 - Ruotsalainen, Kari A1 - Bauer, Karl A1 - Pietzsch, Annette A1 - Föhlisch, Alexander A1 - Büchner, Robby T1 - Thresholding of the Elliott-Yafet spin-flip scattering in multi-sublattice magnets by the respective exchange energies JF - Scientific reports N2 - 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. Y1 - 2021 U6 - https://doi.org/10.1038/s41598-021-81177-9 SN - 2045-2322 VL - 11 IS - 1 PB - Springer Nature CY - Berlin ER - TY - JOUR A1 - Büchner, Robby A1 - Fondell, Mattis A1 - Mascarenhas, Eric Johnn A1 - Pietzsch, Annette A1 - Vaz da Cruz, Vinícius A1 - Föhlisch, Alexander T1 - How hydrogen bonding amplifies isomeric differences in pyridones toward strong changes in acidity and tautomerism JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - 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. Y1 - 2021 U6 - https://doi.org/10.1021/acs.jpcb.0c10873 SN - 1520-6106 SN - 1520-5207 VL - 125 IS - 9 SP - 2372 EP - 2379 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Büchner, Robby A1 - Fondell, Mattis A1 - Haverkamp, Robert A1 - Pietzsch, Annette A1 - Vaz da Cruz, Vinícius A1 - Föhlisch, Alexander T1 - The porphyrin center as a regulator for metal-ligand covalency and pi hybridization in the entire molecule JF - Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies N2 - 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. Y1 - 2021 U6 - https://doi.org/10.1039/d1cp03944j SN - 1463-9076 SN - 1463-9084 VL - 23 IS - 43 SP - 24765 EP - 24772 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Eckert, Sebastian A1 - Vaz da Cruz, Vinícius A1 - Ochmann, Miguel A1 - Ahnen, Inga von A1 - Föhlisch, Alexander A1 - Huse, Nils T1 - Breaking the symmetry of pyrimidine BT - solvent effects and core-excited state dynamics JF - The journal of physical chemistry letters N2 - 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. Y1 - 2021 U6 - https://doi.org/10.1021/acs.jpclett.1c01865 SN - 1948-7185 VL - 12 IS - 35 SP - 8637 EP - 8643 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Vaz da Cruz, Vinícius A1 - Eckert, Sebastian A1 - Föhlisch, Alexander T1 - TD-DFT simulations of K-edge resonant inelastic X-ray scattering within the restricted subspace approximation JF - Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies N2 - A scheme for simulations of resonant inelastic X-ray scattering (RIXS) cross-sections within time-dependent density functional theory (TD-DFT) applying the restricted subspace approximation (RSA) is presented. Therein both occupied core and valence Kohn-Sham orbitals are included in the donor-space, while the accepting virtual orbital space in the linear response TD-DFT equations is restricted to efficiently compute both the valence- and core-excited states of the many electron system. This yields a consistent description of all states contributing to the RIXS scattering process within a single calculation. The introduced orbital truncation allows to automatize the method and facilitates RIXS simulations for systems considerably larger than ones accessible with wave-function based methods. Using the nitrogen K-edge RIXS spectra of 2-thiopyridone and its deprotonated anion as a showcase, the method is benchmarked for different exchange-correlation functionals, the impact of the RSA is evaluated, and the effects of explicit solvation are discussed. Improvements compared to simulations in the frozen orbital approximation are also assessed. The general applicability of the framework is further tested by comparison to experimental data from the literature. The use of TD-DFT core-excited states to the calculation of vibrationally resolved RIXS spectra is also investigated by combining potential energy scans along relevant coordinates with wave packet simulations. Y1 - 2020 U6 - https://doi.org/10.1039/d0cp04726k SN - 1463-9076 SN - 1463-9084 VL - 23 IS - 3 SP - 1835 EP - 1848 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Kühn, Danilo A1 - Neppl, Stefan A1 - Ovsyannikov, Ruslan A1 - Sezen, Hikmet A1 - Svensson, Svante A1 - Föhlisch, Alexander T1 - Photodriven transient picosecond top-layer semiconductor to metal phase-transition in p-doped molybdenum disulfide JF - Advanced materials N2 - Visible light is shown to create a transient metallic S-Mo-S surface layer on bulk semiconducting p-doped indirect-bandgap 2H-MoS2. Optically created electron-hole pairs separate in the surface band bending region of the p-doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H-semiconductor to 1T-metal phase-transition of the surface layer. Electron-phonon coupling of the indirect-bandgap p-doped 2H-MoS2 enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p-doped 2H-MoS2. In particular, light-induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H-MoS2. Generally, it opens up for a way to control not only the surface of p-doped 2H-MoS2 but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time-resolved electron spectroscopy for chemical analysis with photon-energy-tuneable synchrotron radiation. KW - catalysis KW - dichalcogenides KW - hydrogen evolution reaction KW - phase transitions KW - photoelectron spectroscopy Y1 - 2021 U6 - https://doi.org/10.1002/adma.202006957 SN - 0935-9648 SN - 1521-4095 VL - 33 IS - 14 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Haverkamp, Robert A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Neppl, Stefan A1 - Kühn, Danilo A1 - Föhlisch, Alexander T1 - Directional charge delocalization dynamics in semiconducting 2H-MoS2 and metallic 1T-LixMoS2 JF - Scientific reports N2 - The layered dichalcogenide MoS2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS2 to metallic 1T-LixMoS2. With the core hole clock approach at the S L1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane () and out-of-plane (perpendicular to) for 2H-MoS2 as tau 2H,=0.38 +/- 0.08 fs and tau 2H,perpendicular to =0.33 +/- 0.06 fs and for 1T-LixMoS2 as tau 1T,=0.32 +/- 0.12 fs and tau 1T,perpendicular to =0.09 +/- 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-LixMoS2 by lithiation accelerates the in-plane charge transfer by a factor of similar to 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of similar to 3.7 upon lithiation occurs due to S-Li coupling. Y1 - 2021 U6 - https://doi.org/10.1038/s41598-021-86364-2 SN - 2045-2322 VL - 11 IS - 1 PB - Macmillan Publishers Limited, part of Springer Nature CY - London ER - TY - JOUR A1 - Hantschmann, Markus A1 - Föhlisch, Alexander T1 - A rate model approach for FEL pulse induced transmissions changes, saturable absorption, X-ray transparency and stimulated emission JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - 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. KW - Free-electron-laser science KW - RIXS at FELs KW - Stimulated scattering KW - Pulse induced transparency KW - Scattering breakdown Y1 - 2022 U6 - https://doi.org/10.1016/j.elspec.2021.147139 SN - 0368-2048 VL - 256 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Martensson, Nils A1 - Föhlisch, Alexander A1 - Svensson, Svante T1 - Uppsala and Berkeley BT - two essential laboratories in the development of modern photoelectron spectroscopy JF - Journal of vacuum science & technology : JVST ; an AVS journal / A N2 - 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. KW - Electronic structure KW - Condensed matter physics KW - X-ray emission spectroscopy KW - Electron spectroscopy KW - Photoelectron spectroscopy KW - Nuclear physics KW - Storage rings KW - Synchrotron radiation KW - Gas phase KW - Surface science Y1 - 2022 U6 - https://doi.org/10.1116/6.0001879 SN - 0734-2101 SN - 1520-8559 VL - 40 IS - 4 PB - American Institute of Physics CY - New York ER - TY - JOUR A1 - Liu, Chun-Yu A1 - Ruotsalainen, Kari A1 - Bauer, Karl A1 - Decker, Régis A1 - Pietzsch, Annette A1 - Föhlisch, Alexander T1 - Excited-state exchange interaction in NiO determined by high-resolution resonant inelastic x-ray scattering at the Ni M2,3 edges JF - Physical review : B, Condensed matter and materials physics N2 - 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. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevB.106.035104 SN - 2469-9950 SN - 2469-9969 VL - 106 IS - 3 PB - American Physical Society CY - Ridge, NY ER - TY - JOUR A1 - Pietzsch, Annette A1 - Niskanen, Johannes A1 - Vaz da Cruz, Vinicius A1 - Büchner, Robby A1 - Eckert, Sebastian A1 - Fondell, Mattis A1 - Jay, Raphael Martin A1 - Lu, Xingye A1 - McNally, Daniel A1 - Schmitt, Thorsten A1 - Föhlisch, Alexander T1 - Cuts through the manifold of molecular H2O potential energy surfaces in liquid water at ambient conditions JF - Proceedings of the National Academy of Sciences of the United States of America N2 - 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. KW - water KW - potential ene rgy surface KW - RIXS Y1 - 2022 U6 - https://doi.org/10.1073/pnas.2118101119 SN - 1091-6490 VL - 119 IS - 28 PB - National Acad. of Sciences CY - Washington, DC ER - TY - JOUR A1 - Mascarenhas, Eric Johnn A1 - Fondell, Mattis A1 - Büchner, Robby A1 - Eckert, Sebastian A1 - Vaz da Cruz, Vinícius A1 - Föhlisch, Alexander T1 - Photo-induced ligand substitution of Cr(CO)(6) in 1-pentanol probed by time resolved X-ray absorption spectroscopy JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - 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. Y1 - 2022 U6 - https://doi.org/10.1039/d1cp05834g SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 30 SP - 17979 EP - 17985 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Vaz da Cruz, Vinicius A1 - Büchner, Robby A1 - Fondell, Mattis A1 - Pietzsch, Annette A1 - Eckert, Sebastian A1 - Föhlisch, Alexander T1 - Targeting individual tautomers in equilibrium by resonant inelastic X-ray scattering JF - The journal of physical chemistry letters N2 - 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. KW - Equilibrium KW - Molecular structure KW - Molecules KW - Nitrogen KW - Solvents Y1 - 2022 U6 - https://doi.org/10.1021/acs.jpclett.1c03453 SN - 1948-7185 VL - 13 IS - 10 SP - 2459 EP - 2466 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Eckert, Sebastian A1 - Mascarenhas, Eric Johnn A1 - Mitzner, Rolf A1 - Jay, Raphael Martin A1 - Pietzsch, Annette A1 - Fondell, Mattis A1 - Vaz da Cruz, Vinicius A1 - Föhlisch, Alexander T1 - From the free ligand to the transition metal complex BT - FeEDTA(-) formation seen at ligand K-Edges JF - Inorganic chemistry N2 - 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. KW - Energy KW - Ligands KW - Metals KW - Nitrogen KW - Oxygen Y1 - 2022 U6 - https://doi.org/10.1021/acs.inorgchem.2c00789 SN - 0020-1669 SN - 1520-510X VL - 61 IS - 27 SP - 10321 EP - 10328 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Ochmann, Miguel A1 - Vaz da Cruz, Vinicius A1 - Eckert, Sebastian A1 - Huse, Nils A1 - Föhlisch, Alexander T1 - R-Group stabilization in methylated formamides observed by resonant inelastic X-ray scattering JF - Chemical communications: ChemComm N2 - 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. Y1 - 2022 U6 - https://doi.org/10.1039/d2cc00053a SN - 1359-7345 SN - 1364-548X VL - 58 IS - 63 SP - 8834 EP - 8837 PB - The Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Büchner, Robby A1 - da Cruz, Vinicius Vaz A1 - Grover, Nitika A1 - Charisiadis, Asterios A1 - Fondell, Mattis A1 - Haverkamp, Robert A1 - Senge, Mathias O. A1 - Föhlisch, Alexander T1 - Fundamental electronic changes upon intersystem crossing in large aromatic photosensitizers: free base 5,10,15,20-tetrakis(4-carboxylatophenyl)porphyrin JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Free base 5,10,15,20-tetrakis(4-carboxylatophenyl)porphyrin stands for the class of powerful porphyrin photosensitizers for singlet oxygen generation and light-harvesting. The atomic level selectivity of dynamic UV pump - N K-edge probe X-ray absorption spectroscopy in combination with time-dependent density functional theory (TD-DFT) gives direct access to the crucial excited molecular states within the unusual relaxation pathway. The efficient intersystem crossing, that is El-Sayed forbidden and not facilitated by a heavy atom is confirmed to be the result of the long singlet excited state lifetime (Q(x) 4.9 ns) and thermal effects. Overall, the interplay of stabilization by conservation of angular momenta and vibronic relaxation drive the de-excitation in these chromophores. Y1 - 2022 U6 - https://doi.org/10.1039/d1cp05420a SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 12 SP - 7505 EP - 7511 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Johansson, Fredrik O. L. A1 - Leitner, Torsten A1 - Bidermane, Ieva A1 - Born, Artur A1 - Föhlisch, Alexander A1 - Svensson, Svante A1 - Mårtensson, Nils A1 - Lindblad, Andreas T1 - Auger- and photoelectron coincidences of molecular O2 adsorbed on Ag(111) JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - The oxygen on Ag(111) system has been investigated with Auger electron-photoelectron coincidence spectroscopy (APECS). The coincidence spectra between O 1s core level photoelectrons and O KLL Auger electrons have been studied together with Ag(3)d/AgM4,5NN coincidences. We also describe the electron-electron coincidence spectrometer setup, CoESCA, consisting of two angle resolved time-of-flight spectrometers at a synchrotron light source. Contributions from molecular oxygen and chemisorbed oxygen are assigned using the coincidence data, conclusions are drawn primarily from the O 1s/O KLL data. The data acquisition and treatment procedure are also outlined. The chemisorbed oxygen species observed are relevant for the catalytic ethylene oxidation. KW - oxygen/Ag(111) KW - Auger electron KW - photoelectron KW - coincidence KW - APECS KW - spectroscopy Y1 - 2022 U6 - https://doi.org/10.1016/j.elspec.2022.147174 SN - 0368-2048 SN - 1873-2526 VL - 256 PB - Elsevier CY - New York, NY [u.a.] ER - TY - JOUR A1 - Liu, Ji-Cai A1 - Ignatova, Nina A1 - Kimberg, Victor A1 - Krasnov, Pavel A1 - Föhlisch, Alexander A1 - Simon, Marc A1 - Gel'mukhanov, Faris T1 - Time-resolved study of recoil-induced rotation by X-ray pump - X-ray probe spectroscopy JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Modern stationary X-ray spectroscopy is unable to resolve rotational structure. In the present paper, we propose to use time-resolved two color X-ray pump-probe spectroscopy with picosecond resolution for real-time monitoring of the rotational dynamics induced by the recoil effect. The proposed technique consists of two steps. The first short pump X-ray pulse ionizes the valence electron, which transfers angular momentum to the molecule. The second time-delayed short probe X-ray pulse resonantly excites a 1s electron to the created valence hole. Due to the recoil-induced angular momentum the molecule rotates and changes the orientation of transition dipole moment of core-excitation with respect to the transition dipole moment of the valence ionization, which results in a temporal modulation of the probe X-ray absorption as a function of the delay time between the pulses. We developed an accurate theory of the X-ray pump-probe spectroscopy of the recoil-induced rotation and study how the energy of the photoelectron and thermal dephasing affect the structure of the time-dependent X-ray absorption using the CO molecule as a case-study. We also discuss the feasibility of experimental observation of our theoretical findings, opening new perspectives in studies of molecular rotational dynamics. Y1 - 2022 U6 - https://doi.org/10.1039/d1cp05000a SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 11 SP - 6627 EP - 6638 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Born, Artur A1 - Johansson, Fredrik O. L. A1 - Leitner, Torsten A1 - Bidermane, Ieva A1 - Kuehn, Danilo A1 - Martensson, Nils A1 - Föhlisch, Alexander T1 - The degree of electron itinerancy and shell closing in the core-ionized state of transition metals probed by Auger-photoelectron coincidence spectroscopy JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Auger-photoelectron coincidence spectroscopy (APECS) has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d-shell is localized (atomic-like) and with a self-convoluted valence band for band-like (itinerant) materials as explained using the Cini-Sawatzky model. For transition metals, the 3d band changes from band-like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self-convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini-Sawatzky model. KW - spectra KW - Ni Y1 - 2022 U6 - https://doi.org/10.1039/d2cp02477b SN - 1463-9076 SN - 1463-9084 VL - 24 IS - 32 SP - 19218 EP - 19222 PB - Royal Society of Chemistry CY - Cambridge ER -