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 - Kühn, Danilo A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Jay, Raphael A1 - Musazay, Abdurrahman A1 - Ovsyannikov, Ruslan A1 - Strahlman, Christian A1 - Svensson, Svante A1 - Mårtensson, Nils A1 - Föhlisch, Alexander T1 - Capabilities of angle resolved time of flight electron spectroscopy with the 60 degrees wide angle acceptance lens JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - The simultaneous detection of energy, momentum and temporal information in electron spectroscopy is the key aspect to enhance the detection efficiency in order to broaden the range of scientific applications. Employing a novel 60 degrees wide angle acceptance lens system, based on an additional accelerating electron optical element, leads to a significant enhancement in transmission over the previously employed 30 degrees electron lenses. Due to the performance gain, optimized capabilities for time resolved electron spectroscopy and other high transmission applications with pulsed ionizing radiation have been obtained. The energy resolution and transmission have been determined experimentally utilizing BESSY II as a photon source. Four different and complementary lens modes have been characterized. (C) 2017 The Authors. Published by Elsevier B.V. KW - Artof KW - Electron spectroscopy KW - Wide angle KW - Time of flight KW - Energy resolution KW - Synchrotron Y1 - 2018 U6 - https://doi.org/10.1016/j.elspec.2017.06.008 SN - 0368-2048 SN - 1873-2526 VL - 224 SP - 45 EP - 50 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - de Jong, S. A1 - Kukreja, R. A1 - Trabant, C. A1 - Pontius, N. A1 - Chang, C. F. A1 - Kachel, T. A1 - Beye, Martin A1 - Sorgenfrei, Nomi A1 - Back, C. H. A1 - Braeuer, B. A1 - Schlotter, W. F. A1 - Turner, J. J. A1 - Krupin, O. A1 - Doehler, M. A1 - Zhu, D. A1 - Hossain, M. A. A1 - Scherz, A. O. A1 - Fausti, D. A1 - Novelli, F. A1 - Esposito, M. A1 - Lee, W. S. A1 - Chuang, Y. D. A1 - Lu, D. H. A1 - Moore, R. G. A1 - Yi, M. A1 - Trigo, M. A1 - Kirchmann, P. A1 - Pathey, L. A1 - Golden, M. S. A1 - Buchholz, Marcel A1 - Metcalf, P. A1 - Parmigiani, F. A1 - Wurth, W. A1 - Föhlisch, Alexander A1 - Schuessler-Langeheine, Christian A1 - Duerr, H. A. T1 - Speed limit of the insulator-metal transition in magnetite JF - Nature materials N2 - As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown(1), magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible(2-8). Recently, three- Fe- site lattice distortions called trimeronswere identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase(9). Here we investigate the Verwey transition with pump- probe X- ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two- step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5 +/- 0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics(10). Y1 - 2013 U6 - https://doi.org/10.1038/NMAT3718 SN - 1476-1122 SN - 1476-4660 VL - 12 IS - 10 SP - 882 EP - 886 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Oberg, H. A1 - Gladh, Jörgen A1 - Anniyev, Toyli A1 - Beye, Martin A1 - Coffee, Ryan A1 - Föhlisch, Alexander A1 - Katayama, T. A1 - Kaya, Sarp A1 - LaRue, Jerry A1 - Mogelhoj, Andreas A1 - Nordlund, Dennis A1 - Ogasawara, Hirohito A1 - Schlotter, William F. A1 - Sellberg, Jonas A. A1 - Sorgenfrei, Nomi A1 - Turner, Joshua J. A1 - Wolf, Martin A1 - Wurth, W. A1 - Ostrom, Henrik A1 - Nilsson, Anders A1 - Norskov, Jens K. A1 - Pettersson, Lars G. M. T1 - Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state JF - Surface science N2 - We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate. (C) 2015 Elsevier B.V. All rights reserved. KW - CO desorption KW - Potential of mean force KW - Two-temperature model KW - Pump-probe KW - X-ray spectroscopy KW - Density functional theory Y1 - 2015 U6 - https://doi.org/10.1016/j.susc.2015.03.011 SN - 0039-6028 SN - 1879-2758 VL - 640 SP - 80 EP - 88 PB - Elsevier CY - Amsterdam ER - TY - GEN A1 - Schick, Daniel A1 - Eckert, Sebastian A1 - Pontius, Niko A1 - Mitzner, Rolf A1 - Föhlisch, Alexander A1 - Holldack, Karsten A1 - Sorgenfrei, Nomi T1 - Versatile soft X-ray-optical cross-correlator for ultrafast applications T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - 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). T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1331 Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-436962 SN - 1866-8372 IS - 1331 SP - 054304-1 EP - 054304-8 ER - TY - JOUR A1 - Schick, Daniel A1 - Eckert, Sebastian A1 - Pontius, Niko A1 - Mitzner, Rolf A1 - Föhlisch, Alexander A1 - Holldack, Karsten A1 - Sorgenfrei, Nomi T1 - Versatile soft X-ray-optical cross-correlator for ultrafast applications JF - Structural dynamics N2 - 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). Y1 - 2016 U6 - https://doi.org/10.1063/1.4964296 SN - 2329-7778 VL - 3 SP - 054304-1 EP - 054304-8 PB - American Institute of Physics CY - Melville ER - TY - GEN A1 - Fondell, Mattis A1 - Eckert, Sebastian A1 - Jay, Raphael Martin A1 - Weniger, Christian A1 - Quevedo, Wilson A1 - Niskanen, Johannes A1 - Kennedy, Brian A1 - Sorgenfrei, Nomi A1 - Schick, Daniel A1 - Giangrisostomi, Erika A1 - Ovsyannikov, Ruslan A1 - Adamczyk, Katrin A1 - Huse, Nils A1 - Wernet, Philippe A1 - Mitzner, Rolf A1 - Föhlisch, Alexander T1 - Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - 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. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 780 KW - l-edge xas KW - electronic-structure KW - molecular-structure KW - spin-state KW - dynamics KW - complexes KW - probe KW - water KW - iron(II) KW - spectra Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-437529 SN - 1866-8372 IS - 780 ER - TY - GEN A1 - Kühn, Danilo A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Musazayb, Abdurrahman A1 - Ovsyannikov, Ruslan A1 - Stråhlman, Christian A1 - Svensson, Svante A1 - Mårtensson, Nils A1 - Föhlisch, Alexander T1 - Capabilities of angle resolved time of flight electron spectroscopy with the 60 degrees wide angle acceptance lens T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - The simultaneous detection of energy, momentum and temporal information in electron spectroscopy is the key aspect to enhance the detection efficiency in order to broaden the range of scientific applications. Employing a novel 60 degrees wide angle acceptance lens system, based on an additional accelerating electron optical element, leads to a significant enhancement in transmission over the previously employed 30 degrees electron lenses. Due to the performance gain, optimized capabilities for time resolved electron spectroscopy and other high transmission applications with pulsed ionizing radiation have been obtained. The energy resolution and transmission have been determined experimentally utilizing BESSY II as a photon source. Four different and complementary lens modes have been characterized. (C) 2017 The Authors. Published by Elsevier B.V. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 782 KW - Artof KW - electron spectroscopy KW - wide angle KW - time of flight KW - energy resolution KW - synchrotron Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-436629 SN - 1866-8372 IS - 782 SP - 45 EP - 50 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 - Katayama, T. A1 - Anniyev, Toyli A1 - Beye, Martin A1 - Coffee, Ryan A1 - Dell'Angela, M. A1 - Föhlisch, Alexander A1 - Gladh, J. A1 - Kaya, S. A1 - Krupin, O. A1 - Nilsson, A. A1 - Nordlund, D. A1 - Schlotter, W. F. A1 - Sellberg, J. A. A1 - Sorgenfrei, Nomi A1 - Turner, J. J. A1 - Wurth, W. A1 - Öström, H. A1 - Ogasawara, H. T1 - Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface. KW - X-ray emission spectroscopy KW - Surface science KW - Free electron laser KW - Ultrafast Y1 - 2013 U6 - https://doi.org/10.1016/j.elspec.2013.03.006 SN - 0368-2048 VL - 187 IS - 1 SP - 9 EP - 14 PB - Elsevier CY - Amsterdam ER -