@article{KrollKernKubinetal.2016, author = {Kroll, Thomas and Kern, Jan and Kubin, Markus and Ratner, Daniel and Gul, Sheraz and Fuller, Franklin D. and L{\"o}chel, Heike and Krzywinski, Jacek and Lutman, Alberto and Ding, Yuantao and Dakovski, Georgi L. and Moeller, Stefan and Turner, Joshua J. and Alonso-Mori, Roberto and Nordlund, Dennis L. and Rehanek, Jens and Weniger, Christian and Firsov, Alexander and Brzhezinskaya, Maria and Chatterjee, Ruchira and Lassalle-Kaiser, Benedikt and Sierra, Raymond G. and Laksmono, Hartawan and Hill, Ethan and Borovik, Andrew S. and Erko, Alexei and F{\"o}hlisch, Alexander and Mitzner, Rolf and Yachandra, Vittal K. and Yano, Junko and Wernet, Philippe and Bergmann, Uwe}, title = {X-ray absorption spectroscopy using a self-seeded soft X-ray free-electron laser}, series = {Optics express : the international electronic journal of optics}, volume = {24}, journal = {Optics express : the international electronic journal of optics}, publisher = {Optical Society of America}, address = {Washington}, issn = {1094-4087}, doi = {10.1364/OE.24.022469}, pages = {22469 -- 22480}, year = {2016}, abstract = {X-ray free electron lasers (XFELs) enable unprecedented new ways to study the electronic structure and dynamics of transition metal systems. L-edge absorption spectroscopy is a powerful technique for such studies and the feasibility of this method at XFELs for solutions and solids has been demonstrated. However, the required x-ray bandwidth is an order of magnitude narrower than that of self-amplified spontaneous emission (SASE), and additional monochromatization is needed. Here we compare L-edge x-ray absorption spectroscopy (XAS) of a prototypical transition metal system based on monochromatizing the SASE radiation of the linac coherent light source (LCLS) with a new technique based on self-seeding of LCLS. We demonstrate how L-edge XAS can be performed using the self-seeding scheme without the need of an additional beam line monochromator. We show how the spectral shape and pulse energy depend on the undulator setup and how this affects the x-ray spectroscopy measurements. (C) 2016 Optical Society of America}, language = {en} } @article{MitznerRehanekKernetal.2013, author = {Mitzner, Rolf and Rehanek, Jens and Kern, Jan and Gul, Sheraz and Hattne, Johan and Taguchi, Taketo and Alonso-Mori, Roberto and Tran, Rosalie and Weniger, Christian and Schr{\"o}der, Henning and Quevedo, Wilson and Laksmono, Hartawan and Sierra, Raymond G. and Han, Guangye and Lassalle-Kaiser, Benedikt and Koroidov, Sergey and Kubicek, Katharina and Schreck, Simon and Kunnus, Kristjan and Brzhezinskaya, Maria and Firsov, Alexander and Minitti, Michael P. and Turner, Joshua J. and M{\"o}ller, Stefan and Sauter, Nicholas K. and Bogan, Michael J. and Nordlund, Dennis and Schlotter, William F. and Messinger, Johannes and Borovik, Andrew S. and Techert, Simone and de Groot, Frank M. F. and F{\"o}hlisch, Alexander and Erko, Alexei and Bergmann, Uwe and Yachandra, Vittal K. and Wernet, Philippe and Yano, Junko}, title = {L-edge x-ray absorption spectroscopy of dilute systems relevant to metalloproteins using an X-ray free-electron laser}, series = {The journal of physical chemistry letters}, volume = {4}, journal = {The journal of physical chemistry letters}, number = {21}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz401837f}, pages = {3641 -- 3647}, year = {2013}, abstract = {L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming 0 K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.}, 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{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} } @misc{JayNorellKunnusetal.2018, author = {Jay, Raphael J. and Norell, Jesper and Kunnus, Kristjan and Lundberg, Marcus and Gaffney, Kelly and Wernet, Philippe and Odelius, Michael and F{\"o}hlisch, Alexander}, title = {Dynamcis of local charge densities and metal-ligand covalency in iron complexes from femtosecond resonant inelastic soft X-ray scattering}, series = {Abstracts of Papers of the American Chemical Society}, volume = {256}, journal = {Abstracts of Papers of the American Chemical Society}, publisher = {American Chemical Society}, address = {Washington}, issn = {0065-7727}, url = {http://nbn-resolving.de/urn:nbn:se:uu:diva-370051}, pages = {2}, year = {2018}, language = {en} } @article{BeyeOebergXinetal.2016, author = {Beye, Martin and {\"O}berg, Henrik and Xin, Hongliang and Dakovski, Georgi L. and F{\"o}hlisch, Alexander and Gladh, Jorgen and Hantschmann, Markus and Hieke, Florian and Kaya, Sarp and K{\"u}hn, Danilo and LaRue, Jerry and Mercurio, Giuseppe and Minitti, Michael P. and Mitra, Ankush and Moeller, Stefan P. and Ng, May Ling and Nilsson, Anders and Nordlund, Dennis and Norskov, Jens and {\"O}str{\"o}m, Henrik and Ogasawara, Hirohito and Persson, Mats and Schlotter, William F. and Sellberg, Jonas A. and Wolf, Martin and Abild-Pedersen, Frank and Pettersson, Lars G. M. and Wurth, Wilfried}, title = {Chemical Bond Activation Observed with an X-ray Laser}, series = {The journal of physical chemistry letters}, volume = {7}, journal = {The journal of physical chemistry letters}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/acs.jpclett.6b01543}, pages = {3647 -- 3651}, year = {2016}, abstract = {The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.}, language = {en} } @article{OstromObergXinetal.2015, author = {Ostrom, H. and Oberg, H. and Xin, H. and Larue, J. and Beye, Martin and Gladh, J. and Ng, M. L. and Sellberg, J. A. and Kaya, S. and Mercurio, G. and Nordlund, D. and Hantschmann, Markus and Hieke, F. and Kuehn, D. and Schlotter, W. F. and Dakovski, G. L. and Turner, J. J. and Minitti, M. P. and Mitra, A. and Moeller, S. P. and F{\"o}hlisch, Alexander and Wolf, M. and Wurth, W. and Persson, Mats and Norskov, J. K. and Abild-Pedersen, Frank and Ogasawara, Hirohito and Pettersson, Lars G. M. and Nilsson, A.}, title = {Probing the transition state region in catalytic CO oxidation on Ru}, series = {Science}, volume = {347}, journal = {Science}, number = {6225}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {0036-8075}, doi = {10.1126/science.1261747}, pages = {978 -- 982}, year = {2015}, abstract = {Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10\% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.}, language = {en} } @article{OvsyannikovKarlssonLundqvistetal.2013, author = {Ovsyannikov, Ruslan and Karlsson, P. and Lundqvist, M. and Lupulescu, C. and Eberhardt, W. and F{\"o}hlisch, Alexander and Svensson, S. and Martensson, N.}, title = {Principles and operation of a new type of electron spectrometer - ArTOF}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {191}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, number = {12}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2013.08.005}, pages = {92 -- 103}, year = {2013}, abstract = {A new energy and angular electron analyzer ArTOF (Angular Resolved Time of Flight) is described. The analyzer is based on simultaneous measurement of flight times and angles in an advanced electron lens system. In angular modes the new analyzer combines an increase in transmission by almost three orders of magnitude with improved resolution, in comparison to standard state-of-the-art electron spectrometers. In this report we describe some design principles and we give a review of calibration and alignment procedures necessary for the use of the ArTOF on a synchrotron radiation facility. Our program scripts to handle the large datasets are also discussed. Furthermore we give a broad description of the new research fields that benefit from the use of the ArTOF and give a short summary of the first results of angle resolved photoemission measurement with ArTOF using the single-bunch X-ray pulses from the BESSY II storage ring facility. (C) 2013 Published by Elsevier B.V.}, language = {en} } @article{KunnusJosefssonSchrecketal.2013, author = {Kunnus, Kristjan and Josefsson, Ida and Schreck, Simon and Quevedo, Wilson and Miedema, Piter S. and Techert, Simone and de Groot, Frank M. F. and Odelius, Michael and Wernet, Philippe and F{\"o}hlisch, Alexander}, title = {From Ligand Fields to Molecular Orbitals: Probing the Local Valence Electronic Structure of Ni2+ in Aqueous Solution with Resonant Inelastic X-ray Scattering}, series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, volume = {117}, journal = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry}, number = {51}, publisher = {American Chemical Society}, address = {Washington}, issn = {1520-6106}, doi = {10.1021/jp4100813}, pages = {16512 -- 16521}, year = {2013}, abstract = {Bonding of the Ni2+(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d-3d electron correlation interactions in the Ni2+(aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d-3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV-vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra.}, language = {en} } @article{BeyeFoehlisch2013, author = {Beye, Martin and F{\"o}hlisch, Alexander}, title = {Soft X-ray probes of ultrafast dynamics for heterogeneous catalysis}, series = {Chemical physics : a journal devoted to experimental and theoretical research involving problems of both a chemical and physical nature}, volume = {414}, journal = {Chemical physics : a journal devoted to experimental and theoretical research involving problems of both a chemical and physical nature}, number = {5}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0301-0104}, doi = {10.1016/j.chemphys.2012.03.023}, pages = {130 -- 138}, year = {2013}, abstract = {Soft X-ray spectroscopy is one of the best tools to directly address the electronic structure, the driving force of chemical reactions. It enables selective studies on sample surfaces to single out reaction centers in heterogeneous catalytic reactions. With core-hole clock methods, specific dynamics are related to the femtosecond life time of a core-hole. Typically, this method is used with photoemission spectroscopy, but advancements in soft X-ray emission techniques render more specific studies possible. With the advent of bright femtosecond pulsed soft X-ray sources, highly selective pump-probe X-ray emission studies are enabled with temporal resolutions down to tens of femtoseconds. This finally allows to study dynamics in the electronic structure of adsorbed reaction centers on the whole range of relevant time scales - closing the gap between kinetic soft X-ray studies and the atto- to femtosecond core-hole clock techniques.}, language = {en} } @article{KoenneckeFollathPontiusetal.2013, author = {K{\"o}nnecke, Rene and Follath, R. and Pontius, N. and Schlappa, J. and Eggenstein, F. and Zeschke, T. and Bischoff, P. and Schmidt, J. -S. and Noll, T. and Trabant, C. and Schreck, S. and Wernet, Ph. and Eisebitt, S. and Senf, F. and Schuessler-Langeheine, Christian and Erko, A. and F{\"o}hlisch, Alexander}, title = {The confocal plane grating spectrometer at BESSY II}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {188}, 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.2012.11.003}, pages = {133 -- 139}, year = {2013}, abstract = {At BESSY II a confocal plane grating spectrometer for resonant inelastic X-ray scattering (RIXS) is currently under commissioning. The new endstation operates with a source size of 4 x 1 mu m(2) provided by its dedicated beamline. The RIXS-spectrometer covers an energy range from 50 eV to 1000 eV, providing a resolving power E/Delta E of 5000-15,000. The beamline allows full polarization control and gives a photon flux of up to 7 x 10(14) photons/s/0.1 A/0.1\%bandwidth by offering a resolving power E/Delta E of 4000-12,000.}, language = {en} } @article{BeyeWernetSchuesslerLangeheineetal.2013, author = {Beye, Martin and Wernet, Ph. and Sch{\"u}ßler-Langeheine, Christian and F{\"o}hlisch, Alexander}, title = {Time resolved resonant inelastic X-ray scattering: a supreme tool to understand dynamics in solids and molecules}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {188}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, number = {3}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2013.04.013}, pages = {172 -- 182}, year = {2013}, abstract = {Dynamics in materials typically involve different degrees of freedom, like charge, lattice, orbital and spin in a complex interplay. Time-resolved resonant inelastic X-ray scattering (RIXS) as a highly selective tool can provide unique insight and follow the details of dynamical processes while resolving symmetries, chemical and charge states, momenta, spin configurations, etc. In this paper, we review examples where the intrinsic scattering duration time is used to study femtosecond phenomena. Free-electron lasers access timescales starting in the sub-ps range through pump-probe methods and synchrotrons study the time scales longer than tens of ps. In these examples, time-resolved resonant inelastic X-ray scattering is applied to solids as well as molecular systems.}, language = {en} } @article{MiedemaBeyeKoenneckeetal.2014, author = {Miedema, P. S. and Beye, Martin and Koennecke, R. and Schiwietz, G. and F{\"o}hlisch, Alexander}, title = {The angular- and crystal-momentum transfer through electron-phonon coupling in silicon and silicon-carbide: similarities and differences}, series = {New journal of physics : the open-access journal for physics}, volume = {16}, journal = {New journal of physics : the open-access journal for physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {1367-2630}, doi = {10.1088/1367-2630/16/9/093056}, pages = {17}, year = {2014}, abstract = {Electron-phonon scattering has been studied for silicon carbide (6H-SiC) with resonant inelastic x-ray scattering at the silicon 2p edge. The observed electron-phonon scattering yields a crystal momentum transfer rate per average phonon in 6H-SiC of 1.8 fs(-1) while it is 0.2 fs(-1) in crystalline silicon. The angular momentum transfer rate per average phonon for 6H-SiC is 0.1 fs(-1), which is much higher than 0.0035 fs(-1) obtained for crystalline silicon in a previous study. The higher electron-phonon scattering rates in 6H-SiC are a result of the larger electron localization at the silicon atoms in 6H-SiC as compared to crystalline silicon. While delocalized valence electrons can screen effectively (part of) the electron-phonon interaction, this effect is suppressed for 6H-SiC in comparison to crystalline silicon. Smaller contributions to the difference in electron-phonon scattering rates between 6H-SiC and silicon arise from the lower atomic mass of carbon versus silicon and the difference in local symmetry.}, language = {en} } @article{WernetKunnusJosefssonetal.2015, author = {Wernet, Philippe and Kunnus, Kristjan and Josefsson, Ida and Rajkovic, Ivan and Quevedo, Wilson and Beye, Martin and Schreck, Simon and Gruebel, S. and Scholz, Mirko and Nordlund, Dennis and Zhang, Wenkai and Hartsock, Robert W. and Schlotter, William F. and Turner, Joshua J. and Kennedy, Brian and Hennies, Franz and de Groot, Frank M. F. and Gaffney, Kelly J. and Techert, Simone and Odelius, Michael and F{\"o}hlisch, Alexander}, title = {Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)(5) in solution}, series = {Nature : the international weekly journal of science}, volume = {520}, journal = {Nature : the international weekly journal of science}, number = {7545}, publisher = {Nature Publ. Group}, address = {London}, issn = {0028-0836}, doi = {10.1038/nature14296}, pages = {78 -- 81}, year = {2015}, abstract = {Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion(1,2). Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site(3-11) that need to be controlled to optimize complexes for photocatalytic hydrogen production(8) and selective carbon-hydrogen bond activation(9-11). An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)(5) in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)(4) species, a homogeneous catalyst(12,13) with an electron deficiency at the Fe centre(14,15), in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)(5) (refs 4, 16-20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.}, language = {en} } @article{XinLaRueObergetal.2015, author = {Xin, Hong and LaRue, Jerry and Oberg, Henrik and Beye, Martin and Turner, J. J. and Gladh, J{\"o}rgen and Ng, May L. and Sellberg, Jonas A. and Kaya, Sarp and Mercurio, G. and Hieke, F. and Nordlund, Dennis and Schlotter, William F. and Dakovski, Georgi L. and Minitti, Michael P. and F{\"o}hlisch, Alexander and Wolf, Martin and Wurth, Wilfried and Ogasawara, Hirohito and Norskov, Jens K. and Ostrom, Henrik and Pettersson, Lars G. M. and Nilsson, Anders and Abild-Pedersen, Frank}, title = {Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations}, series = {Physical review letters}, volume = {114}, journal = {Physical review letters}, number = {15}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.114.156101}, pages = {6}, year = {2015}, abstract = {We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5 sigma and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.}, language = {en} } @article{FoersterLindenauLeyendeckeretal.2015, author = {F{\"o}rster, Daniel F. and Lindenau, Bernd and Leyendecker, Marko and Janssen, Franz and Winkler, Carsten and Schumann, Frank O. and Kirschner, Juergen and Holldack, Karsten and F{\"o}hlisch, Alexander}, title = {Phase-locked MHz pulse selector for x-ray sources}, series = {Optics letters : a publication of the Optical Society of America}, volume = {40}, journal = {Optics letters : a publication of the Optical Society of America}, number = {10}, publisher = {Optical Society of America}, address = {Washington}, issn = {0146-9592}, doi = {10.1364/OL.40.002265}, pages = {2265 -- 2268}, year = {2015}, abstract = {Picosecond x-ray pulses are extracted with a phase-locked x-ray pulse selector at 1.25 MHz repetition rate from the pulse trains of the accelerator-driven multiuser x-ray source BESSY II preserving the peak brilliance at high pulse purity. The system consists of a specially designed in-vacuum chopper wheel rotating with approximate to 1 kHz angular frequency. The wheel is driven in an ultrahigh vacuum and is levitated on magnetic bearings being capable of withstanding high centrifugal forces. Pulses are picked by 1252 high-precision slits of 70 mu m width on the outer rim of the wheel corresponding to a temporal opening window of the chopper of 70 ns. We demonstrate how the electronic phase stabilization of +/- 2 ns together with an arrival time jitter of the individual slits of the same order of magnitude allows us to pick short single bunch x-ray pulses out of a 200 ns ion clearing gap in a multibunch pulse train as emitted from a synchrotron facility at 1.25 MHz repetition rate with a pulse purity below the shot noise detection limit. The approach is applicable to any high-repetition pulsed radiation source, in particular in the x-ray spectral range up to 10 keV. The opening window in a real x-ray beamline, its stability, as well as the limits of mechanical pulse picking techniques in the MHz range are discussed. (C) 2015 Optical Society of America}, language = {en} } @article{BeyeFoehlisch2011, author = {Beye, Martin and F{\"o}hlisch, Alexander}, title = {A soft X-ray approach to electron-phonon interactions beyond the Born-Oppenheimer approximation}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {184}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, number = {3-6}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2010.12.032}, pages = {313 -- 317}, year = {2011}, abstract = {With modern soft X-ray methods, the whole field of electron-phonon interactions becomes accessible directly in the ultrafast time domain with ultrashort pulsed X-ray sources, as well as in the energy domain through modern highly resolving spectrometers. The well-known core-hole clock approach plays an intermediate role, resolving energetic and temporal features at the same time. In this perspective paper, we review several experiments to illustrate the modern advances in the selective study of electron-phonon interactions as fundamentally determining ingredients for materials properties. We present the different complementary approaches that can be taken with soft X-ray methods to conquer this field beyond the Born-Oppenheimer approximation.}, language = {en} } @unpublished{FoehlischdeGrootOdeliusetal.2014, author = {F{\"o}hlisch, Alexander and de Groot, F. M. F. and Odelius, Michael and Techert, Simone and Wernet, P.}, title = {Comment on "state-dependent electron delocalization dynamics at the solute-solvent interface: soft-x-ray absorption spectroscopy and lambda b initio calculations"}, series = {Physical review letters}, volume = {112}, journal = {Physical review letters}, number = {12}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.112.129302}, pages = {2}, year = {2014}, language = {en} } @article{KunnusSchreckFoehlisch2015, author = {Kunnus, Kristjan and Schreck, Simon and F{\"o}hlisch, Alexander}, title = {Free-electron laser based resonant inelastic X-ray scattering on molecules and liquids}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {204}, 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.2015.08.012}, pages = {345 -- 355}, year = {2015}, abstract = {The unprecedented beam properties of free-electron laser based X-ray sources enable novel resonant inelastic X-ray scattering (RIXS) experiments. Femtosecond time-resolved RIXS can be used to follow charge, spin and structural dynamics of dilute solute molecules in solution. Ultrashort X-ray pulses allow probing of highly radiation sensitive states of matter such as the metastable phase of supercooled liquid water. Nonlinear X-ray probes like amplified spontaneous emission and stimulated resonant X-ray scattering provide an enhanced selectivity and sensitivity as well as a path to control radiation damage and increase the photon yields in RIXS experiments. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @article{WernetKunnusSchrecketal.2012, author = {Wernet, Philippe and Kunnus, Kristjan and Schreck, Simon and Quevedo, Wilson and Kurian, Reshmi and Techert, Simone and de Groot, Frank M. F. and Odelius, Michael and F{\"o}hlisch, Alexander}, title = {Dissecting local atomic and intermolecular interactions of transition-metal ions in solution with selective X-ray spectroscopy}, series = {The journal of physical chemistry letters}, volume = {3}, journal = {The journal of physical chemistry letters}, number = {23}, publisher = {American Chemical Society}, address = {Washington}, issn = {1948-7185}, doi = {10.1021/jz301486u}, pages = {3448 -- 3453}, year = {2012}, abstract = {Determining covalent and charge-transfer contributions to bonding in solution has remained an experimental challenge. Here, the quenching of fluorescence decay channels as expressed in dips in the L-edge X-ray spectra of solvated 3d transition-metal ions and complexes was reported as a probe. With a full set of experimental and theoretical ab initio L-edge X-ray spectra of aqueous Cr3+, including resonant inelastic X-ray scattering, we address covalency and charge transfer for this prototypical transition-metal ion in solution. We dissect local atomic effects from intermolecular interactions and quantify X-ray optical effects. We find no evidence for the asserted ultrafast charge transfer to the solvent and show that the dips are readily explained by X-ray optical effects and local atomic state dependence of the fluorescence yield. Instead, we find, besides ionic interactions, a covalent contribution to the bonding in the aqueous complex of ligand-to-metal charge-transfer character.}, language = {en} }