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  • 1. Ali-Löytty, Harri
    et al.
    Hannula, Markku
    Valden, Mika
    Eilert, André
    Ogasawara, Hirohito
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States.
    Chemical Dissolution of Pt(111) during Potential Cycling under Negative pH Conditions Studied by Operando X-ray Photoelectron Spectroscopy2019In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 41, p. 25128-25134Article in journal (Refereed)
    Abstract [en]

    Dissolution of a platinum catalyst is a major degradation mechanism of fuel cells, but the exact reaction mechanism has remained unclear. Here, electrochemical ambient pressure X-ray photoelectron spectroscopy (EC-APXPS) was utilized to provide direct information on chemical species on a single-crystal Pt(111) electrode under extremely low pH conditions. Measurements were conducted using a novel condensed electrolyte film electrochemical cell applying work function measurement as a loss-free probe for electrochemical potential. We show that platinum can dissolve chemically as Pt2+ ion during potential cycling and redeposit as Pt2+ at the onset potential for cathodic reactions. The dissolution of Pt does not require electrochemical oxidation via oxide place exchange. In contrast, the adsorption of oxygenated species (OH* or O*) at the onset potential for anodic reactions is a sufficient prerequisite to the dissolution. These results provide new insight into the degradation mechanism of Pt under extremely low pH conditions, predicted by the Pourbaix diagram, having practical applications to the durability of Pt-based catalysts in electrochemical energy conversion devices.

  • 2.
    Amann, Peter
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Degerman, David
    Stockholm University, Faculty of Science, Department of Physics.
    Lee, Ming-Tao
    Alexander, John D.
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Cavalca, Filippo
    Weston, Matthew
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Blom, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Björkhage, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Löfgren, Patrik
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Drube, Wofgang
    Lömker, Patrick
    Ederer, Katrin
    Noei, Heshmat
    Zehetner, Johann
    Wentzel, Henrik
    Åhlund, John
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    A dedicated photoelectron spectroscopy instrument for studies of catalytic reactions at pressures exceeding 1 barManuscript (preprint) (Other academic)
    Abstract [en]

    Here, we present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditions at pressures exceeding 1 bar. The instrument builds around the concept of a “virtual cell” in which a gasflow is directed onto the sample surface creating a local high pressure on top of the sample. This allows the instrument to maintain a low pressure of a few mbars in the main chamber, while simultaneously keeping a local pressure of around 1 bar. Synchrotron radiation based grazing incidence photoemission within ± 5° is used to enhance the surface sensitivity in the experiment. The aperture, separating the high-pressure region from the differential pumping of the electron spectrometer, consists of multiple, evenly spaced, mm sized holes matching the footprint of the x-ray beam on the sample surface. As the photo-emitted electrons are subject to strong scattering in the gas phase and the resulting signal is therefore highly dependent on the sample to aperture distance, the latter is controlled with high precision using a fully integrated manipulator that allows for sample movement with step sizes of 10 nm between 0 and –5 mm with very low vibrational amplitude. The instrumental features allows acquisition of metallic bulk spectra at He pressures up to 2.5 bar and also allows for following C1s spectra under realistic gas mixtures of CO + H2with various temperatures up to 500°C. This capability opens for studies of catalytic reactions in operandi.

  • 3.
    Amann, Peter
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Degerman, David
    Stockholm University, Faculty of Science, Department of Physics.
    Lee, Ming-Tao
    Stockholm University, Faculty of Science, Department of Physics.
    Alexander, John D.
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Cavalca, Filippo
    Stockholm University, Faculty of Science, Department of Physics.
    Weston, Matthew
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Blom, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Björkhage, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Löfgren, Patrik
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Loemker, Patrick
    Ederer, Katrin
    Drube, Wolfgang
    Noei, Heshmat
    Zehetner, Johann
    Wentzel, Henrik
    Ahlund, John
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    A high-pressure x-ray photoelectron spectroscopy instrument for studies of industrially relevant catalytic reactions at pressures of several bars2019In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 90, no 10, article id 103102Article in journal (Refereed)
    Abstract [en]

    We present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditions at pressures of multiple bars. The instrument builds around the novel concept of a virtual cell in which a gas flow onto the sample surface creates a localized high-pressure pillow. This allows the instrument to be operated with a low pressure of a few millibar in the main chamber, while simultaneously a local pressure exceeding 1 bar can be supplied at the sample surface. Synchrotron based hard x-ray excitation is used to increase the electron mean free path in the gas region between sample and analyzer while grazing incidence <5 degrees close to total external refection conditions enhances surface sensitivity. The aperture separating the high-pressure region from the differential pumping of the electron spectrometer consists of multiple, evenly spaced, micrometer sized holes matching the footprint of the x-ray beam on the sample. The resulting signal is highly dependent on the sample-to-aperture distance because photoemitted electrons are subject to strong scattering in the gas phase. Therefore, high precision control of the sample-to-aperture distance is crucial. A fully integrated manipulator allows for sample movement with step sizes of 10 nm between 0 and -5 mm with very low vibrational amplitude and also for sample heating up to 500 degrees C under reaction conditions. We demonstrate the performance of this novel instrument with bulk 2p spectra of a copper single crystal at He pressures of up to 2.5 bars and C1s spectra measured in gas mixtures of CO + H-2 at pressures of up to 790 mbar. The capability to detect emitted photoelectrons at several bars opens the prospect for studies of catalytic reactions under industrially relevant operando conditions.

  • 4.
    Amann, Peter
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Klötzer, Bernhard
    Degerman, David
    Stockholm University, Faculty of Science, Department of Physics.
    Köpfle, Norbert
    Götsch, Thomas
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics. Deutsches Elektronen-Synchrotron DESY, Germany .
    Rameshan, Christoph
    Ploner, Kevin
    Bikaljevic, Djuro
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Soldemo, Markus
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher M.
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Halldin Stenlid, Joakim
    Stockholm University, Faculty of Science, Department of Physics.
    Börner, Mia
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst2022In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 376, no 6593, p. 603-608Article in journal (Refereed)
    Abstract [en]

    The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO2/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO2/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO2 methanol synthesis.  

  • 5.
    Amann-Winkel, Katrin
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Bellissent-Funel, Marie-Claire
    Bove, Livia E.
    Loerting, Thomas
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Paciaroni, Alessandro
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Skinner, Lawrie
    X-ray and Neutron Scattering of Water2016In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 116, no 13, p. 7570-7589Article, review/survey (Refereed)
    Abstract [en]

    This review article focuses on the most recent advances in X-ray and neutron scattering studies of water structure, from ambient temperature to the deeply supercooled and amorphous states, and of water diffusive and collective dynamics, in disparate thermodynamic conditions and environments. In particular, the ability to measure X-ray and neutron diffraction of water with unprecedented high accuracy in an extended range of momentum transfers has allowed the derivation of detailed O-O pair correlation functions. A panorama of the diffusive dynamics of water in a wide range of temperatures (from 400 K down to supercooled water) and pressures (from ambient up to multiple gigapascals) is presented. The recent results obtained by quasi-elastic neutron scattering under high pressure are compared with the existing data from nuclear magnetic resonance, dielectric and infrared measurements, and modeling. A detailed description of the vibrational dynamics of water as measured by inelastic neutron scattering is presented. The dependence of the water vibrational density of states on temperature and pressure, and in the presence of biological molecules, is discussed. Results about the collective dynamics of water and its dispersion curves as measured by coherent inelastic neutron scattering and inelastic X-ray scattering in different thermodynamic conditions are reported.

  • 6.
    Amann-Winkel, Katrin
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Max Planck Institute for Polymer Research and Johannes Gutenberg University, Germany.
    Kim, Kyung Hwan
    Giovambattista, Nicolas
    Ladd-Parada, Marjorie
    Stockholm University, Faculty of Science, Department of Physics.
    Späh, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Perakis, Fivos
    Stockholm University, Faculty of Science, Department of Physics.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Yang, Cheolhee
    Eklund, Tobias
    Stockholm University, Faculty of Science, Department of Physics.
    Lane, Thomas J.
    You, Seonju
    Jeong, Sangmin
    Lee, Jae Hyuk
    Eom, Intae
    Kim, Minseok
    Park, Jaeku
    Chun, Sae Hwan
    Poole, Peter H.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Liquid-liquid phase separation in supercooled water from ultrafast heating of low-density amorphous ice2023In: Nature Communications, E-ISSN 2041-1723, Vol. 14, no 1, article id 442Article in journal (Refereed)
    Abstract [en]

    Recent experiments continue to find evidence for a liquid-liquid phase transition (LLPT) in supercooled water, which would unify our understanding of the anomalous properties of liquid water and amorphous ice. These experiments are challenging because the proposed LLPT occurs under extreme metastable conditions where the liquid freezes to a crystal on a very short time scale. Here, we analyze models for the LLPT to show that coexistence of distinct high-density and low-density liquid phases may be observed by subjecting low-density amorphous (LDA) ice to ultrafast heating. We then describe experiments in which we heat LDA ice to near the predicted critical point of the LLPT by an ultrafast infrared laser pulse, following which we measure the structure factor using femtosecond x-ray laser pulses. Consistent with our predictions, we observe a LLPT occurring on a time scale < 100 ns and widely separated from ice formation, which begins at times >1 μs.

  • 7. Amaya, Andrew J.
    et al.
    Pathak, Harshad
    Modak, Viraj P.
    Laksmono, Hartawan
    Loh, N. Duane
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; KTH Royal Institute of Technology, Sweden.
    Sierra, Raymond G.
    McQueen, Trevor A.
    Hayes, Matt J.
    Williams, Garth J.
    Messerschmidt, Marc
    Boutet, Sebastien
    Bogan, Michael J.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Acceleratory Laboratory, United States.
    Stan, Claudiu A.
    Wyslouzil, Barbara E.
    How Cubic Can Ice Be?2017In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, no 14, p. 3216-3222Article in journal (Refereed)
    Abstract [en]

    Using an X-ray laser, we investigated the crystal structure of ice formed by homogeneous ice nucleation in deeply supercooled water nanodrops (r approximate to 10 nm) at similar to 225 K The nanodrops were formed by condensation of vapor in a supersonic nozzle, and the ice was probed within 100 mu s of freezing using femtosecond wide-angle X-ray scattering at the Linac Coherent Light Source free-electron X-ray laser. The X-ray diffraction spectra indicate that this ice has a metastable, predominantly cubic structure; the shape of the first ice diffraction peak suggests stacking-disordered ice with a cubicity value, chi, in the range of 0.78 +/- 0.05. The cubicity value determined here is higher than those determined in experiments with micron-sized drops but comparable to those found in molecular dynamics simulations. The high cubicity is most likely caused by the extremely low freezing temperatures and by the rapid freezing, which occurs on a similar to 1 mu s time scale in single nanodroplets.

  • 8.
    Andersson, Klas J.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Ogasawara, Hirohito
    Nordlund, Dennis
    Brown, Gordon E., Jr.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Preparation, Structure, and Orientation of Pyrite FeS2{100} Surfaces: Anisotropy, Sulfur Monomers, Dimer Vacancies, and a Possible FeS Surface Phase2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 38, p. 21896-21903Article in journal (Refereed)
    Abstract [en]

    Sulfur dimer (S-2(2-)) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (LEED) pattern of 2 x 1 symmetry are reported. The 2 X 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne+ sputter-cleaning procedure followed by a 570 K anneal of the sample in a 10(-7) Ton S-2(g) atmosphere. Controlled amounts of surface sulfur monomers (S2-) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 x 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/ root 2 x 1/ root 2)R45 degrees LEED pattern emerges, and soft X-ray photoelectron spectroscopy (XPS) results show a sulfur dinner deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of local coordination environments where sulfur monomers are coordinated by four Fe ions compared to three as in the pyrite structure. The plausible formation of a defective FeS-like surface phase where monomeric sulfurs are coordinated by four Fe ions, and bond counting energetics favoring surface sulfur monomer recombination around Fe vacancy sites on pyrite FeS2{100}, both imply surface sulfur dimer vacancy sites with unique adsorption and reactivity properties. Taken together, our results suggest a very rich and dynamic defect structural landscape at pyrite FeS2{100} surfaces with direct implications for its surface chemical activity.

  • 9. Anniyev, Toyli
    et al.
    Ogasawara, Hirohito
    Ljungberg, Mathias
    Stockholm University, Faculty of Science, Department of Physics.
    Wikfeldt, Kjartan T.
    Stockholm University, Faculty of Science, Department of Physics.
    MacNaughton, Janay B.
    Näslund, Lars-Åke
    Bergmann, Uwe
    Koh, Shirlaine
    Strasser, Peter
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Complementarity between high-energy photoelectron and L-edge spectroscopy for probing the electronic structure of 5d transition metal catalysts2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 21, p. 5694-5700Article in journal (Refereed)
    Abstract [en]

    We demonstrate the successful use of hard X-ray photoelectron spectroscopy (HAXPES) for selectively probing the platinum partial d-density of states (DOS) in a Pt-Cu nanoparticle catalyst which shows activity superior to pure Pt towards the oxygen-reduction reaction (ORR). The information about occupied Pt d-band states was complemented by Pt L-2-edge X-ray absorption near-edge spectroscopy (XANES), which probes unoccupied valence states. We found a significant electronic perturbation of the Pt projected d-DOS which was narrowed and shifted to higher binding energy compared to pure platinum. The effect of this electronic structure perturbation on the chemical properties of the nanoparticle surface is discussed in terms of the d-band model. We have thereby demonstrated that the combination of L-edge spectroscopy and HAXPES allows for an experimental derivation of the valence electronic structure in an element-specific way for 5d metal catalysts.

  • 10. Beye, M.
    et al.
    Anniyev, T.
    Coffee, R.
    Dell'Angela, M.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Katayama, T.
    Kaya, S.
    Krupin, O.
    Mogelhoj, A.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Norskov, J. K.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Schlotter, W. F.
    Sellberg, Jonas
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wolf, M.
    Wurth, W.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 18, article id 186101Article in journal (Refereed)
    Abstract [en]

    We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

  • 11. Beye, Martin
    et al.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Xin, Hongliang
    Dakovski, Georgi L.
    Dell'Angela, Martina
    Föhlisch, Alexander
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Hantschmann, Markus
    Hieke, Florian
    Kaya, Sarp
    Kuehn, Danilo
    LaRue, Jerry
    Mercurio, Giuseppe
    Minitti, Michael P.
    Mitra, Ankush
    Stockholm University, Faculty of Science, Department of Physics.
    Moeller, Stefan P.
    Ng, May Ling
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States.
    Nordlund, Dennis
    Norskov, Jens
    Östrom, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, Hirohito
    Persson, Mats
    Schlotter, William F.
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. KTH Royal Institute of Technology, Sweden; SLAC National Accelerator Laboratory, United States.
    Wolf, Martin
    Abild-Pedersen, Frank
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Wurth, Wilfried
    Chemical Bond Activation Observed with an X-ray Laser2016In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, no 18, p. 3647-3651Article in journal (Refereed)
    Abstract [en]

    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.

  • 12. Blomberg, Sara
    et al.
    Hejral, Uta
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Albertin, Stefano
    Karlsson, Hanna
    Hulteberg, Christian
    Lömker, Patrick
    Goodwin, Christopher
    Stockholm University, Faculty of Science, Department of Physics.
    Degerman, David
    Stockholm University, Faculty of Science, Department of Physics.
    Gustafson, Johan
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Lundgren, Edvin
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Bridging the Pressure Gap in CO Oxidation2021In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 11, no 15, p. 9128-9135Article in journal (Refereed)
    Abstract [en]

    Performing fundamental operando catalysis studies under realistic conditions is a key to further develop and increase the efficiency of industrial catalysts. Operando X-ray photoelectron spectroscopy (XPS) experiments have been limited to pressures, and the relevance for industrial applications has been questioned. Herein, we report on the CO oxidation experiment on Pd(100) performed at a total pressure of 1 bar using XPS. We investigate the light-off regime and the surface chemical composition at the atomistic level in the highly active phase. Furthermore, the observed gas-phase photoemission peaks of CO2, CO, and O-2 indicate that the kinetics of the reaction during the light-off regime can be followed operando, and by studying the reaction rate of the reaction, the activation energy is calculated. The reaction was preceded by an in situ oxidation study in 7% O-2 in He and a total pressure of 70 mbar to confirm the surface sensitivity and assignment of the oxygen-induced photoemission peaks. However, oxygen-induced photoemission peaks were not observed during the reaction studies, but instead, a metallic Pd phase is present in the highly active regime under the conditions applied. The novel XPS setup utilizes hard X-rays to enable high-pressure studies, combined with a grazing incident angle to increase the surface sensitivity of the measurement. Our findings demonstrate the possibilities of achieving chemical information of the catalyst, operando, on an atomistic level, under industrially relevant conditions.

  • 13.
    Brena, B.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nordlund, D.
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, M.
    Ogasawara, H.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, A.
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, L.G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Ultrafast Molecular Dissociation of Water in Ice2004In: Physical Review Letters, ISSN 1079-7114, Vol. 93, p. 148302-148305Article in journal (Refereed)
    Abstract [en]

    Using x-ray emission and photoemission spectroscopies to measure the occupied valence levels in a thin crystalline ice film, we resolve the ionization-induced dissociation of water in ice on a femtosecond time scale. Isotope substitution confirms proton transfer during the core-hole lifetime in spite of the nonresonant excitation. Through ab initio molecular dynamics on the core-ionized state, the dissociation and spectrum evolution are followed at femtosecond intervals. The theoretical simulations confirm the experimental analysis and allow for a detailed study of the dissociative reaction path.

  • 14.
    Cavalca, Filippo
    et al.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States.
    Ferragut, Rafael
    Aghion, Stefano
    Eilert, André
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Diaz-Morales, Oscar
    Stockholm University, Faculty of Science, Department of Physics.
    Liu, Chang
    Stockholm University, Faculty of Science, Department of Physics.
    Koh, Ai Leen
    Hansen, Thomas W.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Nature and Distribution of Stable Subsurface Oxygen in Copper Electrodes During Electrochemical CO2 Reduction2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 45, p. 25003-25009Article in journal (Refereed)
    Abstract [en]

    Oxide-derived copper (OD-Cu) electrodes exhibit higher activity than pristine copper during the carbon dioxide reduction reaction (CO2RR) and higher selectivity toward ethylene. The presence of residual subsurface oxygen in OD-Cu has been proposed to be responsible for such improvements, although its stability under the reductive CO2RR conditions remains unclear. This work sheds light on the nature and stability of subsurface oxygen. Our spectroscopic results show that oxygen is primarily concentrated in an amorphous 1-2 nm thick layer within the Cu subsurface, confirming that subsurface oxygen is stable during CO2RR for up to 1 h at -1.15 V vs RHE. Besides, it is associated with a high density of defects in the OD-Cu structure. We propose that both low coordination of the amorphous OD-Cu surface and the presence of subsurface oxygen that withdraws charge from the copper sp- and d-bands might selectively enhance the binding energy of CO.

  • 15.
    Cavalleri, M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, L.G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    X-absorption spectra of water within a plane-wave Car-Parrinello molecular dynamics framework2004In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 121, no 20, p. 10065-10075Article in journal (Refereed)
    Abstract [en]

    We describe the implementation of a simple technique to simulate core-level spectra within the Car-Parrinello plane-waves molecular dynamics framework. The x-ray absorption (XA) spectra are generated using the transition potential technique with the effect of the core hole included through a specifically developed pseudopotential for the core-excited atom. Despite the lack of 1s core orbitals in the pseudopotential treatment, the required transition moments are accurately calculated without reconstruction of the all-electron orbitals. The method is applied to the oxygen XA spectra of water in its various aggregation states, but it is transferable to any first-row element. The computed spectra are compared favorably with the results from all-electron cluster calculations, as well as with experimental data. The periodicity of the plane-wave technique improves the description of condensed phases. The molecular dynamics simulation enables in principle a proper treatment of thermal effects and dynamical averaging in complex systems.

  • 16. Chen, Chen
    et al.
    Huang, Congcong
    Waluyo, Iradwikanari
    Weiss, Thomas
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Long-range ion-water and ion-ion interactions in aqueous solutions2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 13, p. 8427-8430Article in journal (Refereed)
    Abstract [en]

    Using small-angle X-ray scattering (SAXS), we obtained direct experimental evidence on the structure of hydrated polyatomic anions, with hydration effects starkly different from those of cations (J. Chem. Phys., 2011, 134, 064513). We propose that the size and charge density of the naked ions do not sufficiently account for the differences in the SAXS curves. For cations, the ion-ion contribution gives a prominent first-order diffraction peak, whereas for anions, the low-Q enhancement in the SAXS curves indicates density inhomogeneities as a result of ion-water interactions.

  • 17.
    Degerman, David
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics. Scienta Omicron AB, Sweden.
    Goodwin, Christopher M.
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics. Deutches Elektronen Synchrotron DESY, Germany.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics. Enerpoly AB, Sweden.
    Soldemo, Markus
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Operando X-ray Photoelectron Spectroscopy for High-Pressure Catalysis Research Using the POLARIS Endstation2022In: Synchrotron Radiation News, ISSN 0894-0886, E-ISSN 1931-7344, Vol. 35, no 3, p. 11-18Article in journal (Refereed)
  • 18.
    Degerman, David
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Shilpilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Heshmat, Noei
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Surface adsorbates during CO2 Hydrogenation on Rh(111) probed in-situ by x-ray photoelectron spectroscopy at 150 mbarManuscript (preprint) (Other academic)
    Abstract [en]

    The catalytic CO2 hydrogenation reaction was examined in situ by High Pressure X-ray Photoelectron Spectroscopy (HP-XPS) at 150 mbar and between 150 and 350°C. The results indicate two temperature regimes; the first one with temperature dependent desorption of carbon species between 150°C and 200°C. The second temperature regime is between 250 and 350 °C. In this interval, the carbon species are formed and immediately reacted away, resulting in a lower temperature dependence on surface coverage. The XPS coverage calculations and the component analysis indicate that water is the most abundant surface adsorbate, and that CHx fragments and CO are the most abundant carbon species. The hydrogenation state of the CHx species varies with temperature, where higher temperatures result in a larger population of more hydrogenated species.

  • 19.
    Degerman, David
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics.
    Garcia-Martinez, Fernando
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Gloskovskii, Andrei
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Demonstrating Pressure Jumping as a Tool to Address the Pressure Gap in High Pressure Photoelectron Spectroscopy of CO and CO2 Hydrogenation on Rh(211)2024In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 25, no 1, article id e202300523Article in journal (Refereed)
    Abstract [en]

    Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a remnant of the high pressure surface as well as the time dynamics during the re-equilibration to the new pressure. This methodology is demonstrated using the CO2 and the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows an adsorbate distribution which favors chemisorbed CHx adsorbates over chemisorbed CO. This contrasts against previous static operando spectra acquired at lower pressures. Furthermore, the pressure jumping method yields a faster acquisition and more detailed spectra than static operando measurements above 1 bar. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, and different hypotheses are presented regarding this observation.

  • 20.
    Degerman, David
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    García-Martínez, Fernando
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy2023In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 127, no 8, p. 4021-4032Article in journal (Refereed)
    Abstract [en]

    The state of the surface near-region during CO hydro- genation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C.

  • 21.
    Degerman, David
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Lömker, Patrick
    Stockholm University, Faculty of Science, Department of Physics.
    Goodwin, Christopher M.
    Stockholm University, Faculty of Science, Department of Physics.
    Gericke, Sabrina M.
    Hejral, Uta
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    Operando Observation of Oxygenated Intermediates during CO Hydrogenation on Rh Single Crystals2022In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 144, no 16, p. 7038-7042Article in journal (Refereed)
    Abstract [en]

    The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.

  • 22. Dell'Angela, M.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Katayama, T.
    Kaya, S.
    Krupin, O.
    LaRue, J.
    Mogelhoj, A.
    Nordlund, D.
    Norskov, J. K.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wolf, M.
    Wurth, W.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Real-Time Observation of Surface Bond Breaking with an X-ray Laser2013In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, no 6125, p. 1302-1305Article in journal (Refereed)
    Abstract [en]

    We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.

  • 23. Dell'Angela, M.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Kaya, S.
    Katayama, T.
    Krupin, O.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SUNCAT Center for Interface Science and Catalysis, USA; SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. SUNCAT Center for Interface Science and Catalysis, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Wolf, M.
    Wurth, W.
    Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer2015In: Structural Dynamics, E-ISSN 2329-7778, Vol. 2, no 2, article id 025101Article in journal (Refereed)
    Abstract [en]

    Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.

  • 24. Diesen, Elias
    et al.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Schreck, Simon
    Stockholm University, Faculty of Science, Department of Physics.
    Weston, Matthew
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, Hirohito
    LaRue, Jerry
    Perakis, Foivos
    Stockholm University, Faculty of Science, Department of Physics.
    Dell'Angela, Martina
    Capotondi, Flavio
    Giannessi, Luca
    Pedersoli, Emanuele
    Naumenko, Denys
    Nikolov, Ivaylo
    Raimondi, Lorenzo
    Spezzani, Carlo
    Beye, Martin
    Cavalca, Filippo
    Stockholm University, Faculty of Science, Department of Physics.
    Liu, Boyang
    Stockholm University, Faculty of Science, Department of Physics.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Koroidov, Sergey
    Stockholm University, Faculty of Science, Department of Physics.
    Miedema, Piter S.
    Costantini, Roberto
    Heinz, Tony F.
    Abild-Pedersen, Frank
    Voss, Johannes
    Luntz, Alan C.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy2021In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 127, no 1, article id 016802Article in journal (Refereed)
    Abstract [en]

    We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100  fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.

  • 25.
    Eilert, André
    et al.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Cavalca, Filippo
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Roberts, F. Sloan
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Osterwalder, Jürg
    Liu, Chang
    Stockholm University, Faculty of Science, Department of Physics.
    Favaro, Marco
    Crumlin, Ethan J.
    Ogasawara, Hirohito
    Friebel, Daniel
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction2017In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, no 1, p. 285-290Article in journal (Refereed)
    Abstract [en]

    Copper electrocatalysts derived from an oxide have shown extraordinary electrochemical properties for the carbon dioxide reduction reaction (CO2RR). Using in situ ambient pressure X-ray photoelectron spectroscopy and quasi in situ electron energy loss spectroscopy in a transmission electron microscope, we show that there is a substantial amount of residual oxygen in nanostructured, oxide-derived copper electrocatalysts but no residual copper oxide. On the basis of these findings in combination with density functional theory simulations, we propose that residual subsurface oxygen changes the electronic structure of the catalyst and creates sites with higher carbon monoxide binding energy. If such sites are stable under the strongly reducing conditions found in CO2RR, these findings would explain the high efficiencies of oxide-derived copper in reducing carbon dioxide to multicarbon compounds such as ethylene.

  • 26. Eilert, André
    et al.
    Roberts, F. Sloan
    Friebel, Daniel
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States; Stanford University, United States.
    Formation of Copper Catalysts for CO2 Reduction with High Ethylene/Methane Product Ratio Investigated with In Situ X-ray Absorption Spectroscopy2016In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, no 8, p. 1466-1470Article in journal (Refereed)
    Abstract [en]

    Nanostructured copper cathodes are among the most efficient and selective catalysts to date for making multicarbon products from the electrochemical carbon dioxide reduction reaction (CO2RR). We report an in situ X-ray absorption spectroscopy investigation of the formation of a copper nanocube CO2RR catalyst with high activity that highly favors ethylene over methane production. The results show that the precursor for the copper nanocube formation is copper(I)-oxide, not copper(I)-chloride as previously assumed. A second route to an electrochemically similar material via a copper(II)-carbonate/hydroxide is also reported. This study highlights the importance of using oxidized copper precursors for constructing selective CO2 reduction catalysts and shows the precursor oxidation state does not affect the electrocatalyst selectivity toward ethylene formation.

  • 27. Esmaeildoost, Niloofar
    et al.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Späh, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Lane, Thomas J.
    Kim, Kyung Hwan
    Yang, Cheolhee
    Amann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics.
    Ladd-Parada, Marjorie
    Stockholm University, Faculty of Science, Department of Physics.
    Perakis, Fivos
    Stockholm University, Faculty of Science, Department of Physics.
    Koliyadu, Jayanath
    Oggenfuss, Alexander R.
    Johnson, Philip J. M.
    Deng, Yunpei
    Zerdane, Serhane
    Mankowsky, Roman
    Beaud, Paul
    Lemke, Henrik T.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Sellberg, Jonas A.
    Anomalous temperature dependence of the experimental x-ray structure factor of supercooled water2021In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 155, no 21, article id 214501Article in journal (Refereed)
    Abstract [en]

    The structural changes of water upon deep supercooling were studied through wide-angle x-ray scattering at SwissFEL. The experimental setup had a momentum transfer range of 4.5 Å−1, which covered the principal doublet of the x-ray structure factor of water. The oxygen–oxygen structure factor was obtained for temperatures down to 228.5 ± 0.6 K. Similar to previous studies, the second diffraction peak increased strongly in amplitude as the structural change accelerated toward a local tetrahedral structure upon deep supercooling. We also observed an anomalous trend for the second peak position of the oxygen–oxygen structure factor (q2). We found that q2 exhibits an unprecedented positive partial derivative with respect to temperature for temperatures below 236 K. Based on Fourier inversion of our experimental data combined with reference data, we propose that the anomalous q2 shift originates from that a repeat spacing in the tetrahedral network, associated with all peaks in the oxygen–oxygen pair-correlation function, gives rise to a less dense local ordering that resembles that of low-density amorphous ice. The findings are consistent with that liquid water consists of a pentamer-based hydrogen-bonded network with low density upon deep supercooling. 

  • 28. Fransson, Thomas
    et al.
    Harada, Yoshihisa
    Kosugi, Nobuhiro
    Besley, Nicholas A.
    Winter, Bernd
    Rehr, John J.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray and Electron Spectroscopy of Water2016In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 116, no 13, p. 7551-7569Article, review/survey (Refereed)
    Abstract [en]

    Here we present an overview of recent developments of X-ray and electron spectroscopy to probe water at different temperatures. Photon-induced ionization followed by detection of electrons from either the 0 is level or the valence band is the basis of photoelectron spectroscopy. Excitation between the 0 is and the unoccupied states or occupied states is utilized in X-ray absorption and X-ray emission spectroscopies. These techniques probe the electronic structure of the liquid phase and show sensitivity to the local hydrogen-bonding structure. Both experimental aspects related to the measurements and theoretical simulations to assist in the interpretation are discussed in detail. Different model systems are presented such as the different bulk phases of ice and various adsorbed monolayer structures on metal surfaces.

  • 29. Friebel, Daniel
    et al.
    Miller, Daniel J.
    O'Grady, Christopher P.
    Anniyev, Toyli
    Bargar, John
    Bergmann, Uwe
    Ogasawara, Hirohito
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    In situ x-ray probing reveals the importance of surface platinum oxide formation in fuel cell catalysis2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 1, p. 262-266Article in journal (Refereed)
    Abstract [en]

    In situ X-ray absorption spectroscopy (XAS) at the Pt L3 edge is a useful probe for Pt–O interactions at polymer electrolyte membrane fuel cell (PEMFC) cathodes. We show that XAS using the high energy resolution fluorescence detection (HERFD) mode, applied to a well-defined monolayer Pt/Rh(111) sample where the bulk penetrating hard X-rays probe only surface Pt atoms, provides a unique sensitivity to structure and chemical bonding at the Pt-electrolyte interface. Ab initio multiple-scattering calculations using the FEFF code and complementary extended X-ray absorption fine structure (EXAFS) results indicate that the commonly observed large increase of the white-line at high electrochemical potentials on PEMFC cathodes originates from platinum oxide formation, whereas previously proposed chemisorbed oxygen-containing species merely give rise to subtle spectral changes.

  • 30. Furukawa, Masashi
    et al.
    Yamada, Taro
    Katano, Satoshi
    Kawai, Maki
    Ogasawara, Hirohito
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Geometrical characterization of adenine and guanine on Cu(110) by NEXAFS, XPS, and DFT calculation2007In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 601, no 23, p. 5433-5440Article in journal (Refereed)
    Abstract [en]

    Adsorption of purine DNA bases (guanine and adenine) on Cu(110) was studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), and density-functional theory (DFT) calculation. At coverages near 0.2 monolayers, Angular-resolved NEXAFS analysis revealed that adenine adsorbates lie almost flat and that guanine adsorbates are tilted up on the surface with the purine ring parallel to the atom rows of Cu(110). Referring to the previous studies on pyrimidine DNA bases [M. Furukawa, H. Fujisawa, S. Katano, H. Ogasawara, Y. Kim, T. Komeda, A. Nilsson, M. Kawai, Surf. Sci. 532-535 (2003) 261], the isomerization of DNA bases on Cu(110) was found to play an important role in the adsorption geometry. Guanine, thymine and cytosine adsorption have an amine-type nitrogen next to a carbonyl group, which is dehydrogenated into imine nitrogen on Cu(110). These bases are bonded by the inherent portion of -NH-CO- altered by conversion into enolic form and dehydrogenation. Adenine contains no CO group and is bonded to Cu(110) by participation of the inherent amine parts, resulting in nearly flatly-lying position.

  • 31. Gimpel, Thomas
    et al.
    Börner, Mia
    Stockholm University, Faculty of Science, Department of Physics.
    Hoffmann, Viktor
    Lederle-Flamm, Madita
    Hedin, Niklas
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Schade, Wolfgang
    Turek, Thomas
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Diaz-Morales, Oscar
    Stockholm University, Faculty of Science, Department of Physics.
    Electrochemical Carbon Dioxide Reduction on Femtosecond Laser-Processed Copper Electrodes: Effect on the Liquid Products by Structuring and Doping2021In: ACS Applied Energy Materials, E-ISSN 2574-0962, Vol. 4, no 6, p. 5927-5934Article in journal (Refereed)
    Abstract [en]

    A femtosecond laser process is presented increasing the surface area of copper electrocatalysts for an electrochemical CO2 reduction reaction (CO2RR). The laser treatment allows us to tune the surface morphology and the chemical composition of the copper electrocatalysts. This tunability is used to correlate the role of the surface area and catalyst dopants with the selectivity of the CO2RR. The liquid products of the CO2RR are monitored through ex situ nuclear magnetic resonance spectroscopy. The products’ distribution shows that the electrode surface area plays a key role in the electrochemical conversion of CO2 into multicarbon liquid products. We show that sulfur dopants boost the production of formate. Remarkably, by co-doping sulfur and fluoride, we show that the chalcogenide dopant counteracts the known boosting effect of fluoride to convert CO2 into multicarbon products. Oxygen doping in the range of 2–19 atom % does not significantly affect the distribution of liquid products from CO2 electroreduction. In a broad perspective, this work highlights the potential of the femtosecond laser process to fine-tune surfaces to produce photo- and electrocatalyst materials.

  • 32.
    Gladh, Jörgen
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Li, Jibiao
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, M. P.
    Stockholm University, Faculty of Science, Department of Physics.
    Matsuda, A.
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. Stanford Synchrotron Radiation Lightsource, USA.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray emission spectroscopy and density functional study of CO/Fe(100)2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 3, article id 034702Article in journal (Refereed)
    Abstract [en]

    We report x-ray emission and absorption spectroscopy studies of the electronic structure of the pre-dissociative alpha(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage alpha(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of lying down CO in the hollow site is well described in terms of pi to pi* charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage alpha(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of vertical on-top adsorbed CO similar to other transition-metal surfaces.

  • 33.
    Goodwin, Christopher M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Albertin, Stefano
    Hejral, Uta
    Lömker, Patrick
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Blomberg, Sara
    Degerman, David
    Stockholm University, Faculty of Science, Department of Physics.
    Schlueter, Christoph
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Lundgren, Edvin
    Amann, Peter
    Stockholm University, Faculty of Science, Department of Physics.
    The Structure of the Active Pd State During Catalytic Carbon Monoxide Oxidization2021In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 12, no 18, p. 4461-4465Article in journal (Refereed)
    Abstract [en]

    Using grazing incidence X-rays and X-ray photoelectron spectroscopy during the mass transfer limited catalytic oxidation of CO, the long-range surface structure of Pd(100) was investigated. Under the reaction conditions of 50:4 O-2 to CO, 300 mbar pressure, and temperatures between 200 and 450 degrees C, the surface structure resulting from oxidation and the subsequent oxide reduction was elucidated. The reduction cycle was halted, and while under reaction conditions, angle-dependent X-ray photoelectron spectroscopy close to the critical angle of Pd and modeling of the data was performed. Two proposed models for the system were compared. The suggestion with the metallic islands formed on top of the oxide island was shown to be consistent with the data.

  • 34.
    Görlin, Mikaela
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Uppsala University, Sweden.
    Halldin Stenlid, Joakim
    Stockholm University, Faculty of Science, Department of Physics.
    Koroidov, Sergey
    Stockholm University, Faculty of Science, Department of Physics.
    Wang, Hsin-Yi
    Stockholm University, Faculty of Science, Department of Physics.
    Börner, Mia
    Stockholm University, Faculty of Science, Department of Physics.
    Shipilin, Mikhail
    Stockholm University, Faculty of Science, Department of Physics.
    Kalinko, Aleksandr
    Murzin, Vadim
    Safonova, Olga V.
    Nachtegaal, Maarten
    Uheida, Abdusalam
    Dutta, Joydeep
    Bauer, Matthias
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Diaz-Morales, Oscar
    Stockholm University, Faculty of Science, Department of Physics. KTH Royal Institute of Technology, Sweden.
    Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations2020In: Nature Communications, E-ISSN 2041-1723, Vol. 11, no 1, article id 6181Article in journal (Refereed)
    Abstract [en]

    Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+delta)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH>NaOH approximate to KOH>RbOH>LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect. It is commonly accepted that electrolyte alkali metal cations modify the catalytic activity for oxygen evolution reaction. Here the authors challenge this assumption, showing that the activity is actually affected by a change in the electrolyte pH rather than a specific alkali cation.

  • 35. Hansen, Martin Hangaard
    et al.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Rossmeisl, Jan
    Modelling pH and potential in dynamic structures of the water/Pt(111) interface on the atomic scale2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 34, p. 23505-23514Article in journal (Refereed)
    Abstract [en]

    We present atomic-scale structures of the Pt(111)/water interface, by calculating distributions of atomic distances as functions of pH. The structure of the Pt(111)/water interface is a particularly interesting model system in electro-catalysis for proton exchange reactions, especially the oxygen reduction reaction in polymer electrolyte membrane fuel cells. Further insight into such reactions requires accurate simulations of the electrolyte structure in the interface. The study displays many interesting details in the behaviour of the electrolyte structure, e.g. that the electrolyte structure average responds to the presence of protons by a H-down water orientation and that hexagonal adsorbed water layers are present only when they are anchored at the surface by HO*. New adsorbate configurations were also found at 5/12 ML coverage of HO*, suggesting an explanation for reported cyclic voltammetry experiments. The present study is a step towards a more complete understanding of the structure of the electrochemical interface on the atomic scale.

  • 36. Harada, Y.
    et al.
    Miyawaki, J.
    Niwa, H.
    Yamazoe, K.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Probing the OH Stretch in Different Local Environments in Liquid Water2017In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, no 22, p. 5487-5491Article in journal (Refereed)
    Abstract [en]

    We use resonant inelastic X-ray scattering (RIXS) to resolve vibrational losses corresponding to the OH stretch where the X-ray absorption process allows us to selectively probe different structural subensembles in liquid water. The results point to a unified interpretation of X-ray and vibrational spectroscopic data in line with a picture of two classes of structural environments in the liquid at ambient conditions with predominantly close-packed high-density liquid (HDL) and occasional local fluctuations into strongly tetrahedral low-density liquid (LDL).

  • 37. Harada, Yoshihisa
    et al.
    Tokushima, Takashi
    Horikawa, Yuka
    Takahashi, Osamu
    Niwa, Hideharu
    Kobayashi, Masaki
    Oshima, Masaharu
    Senba, Yasunori
    Ohashi, Haruhiko
    Wikfeldt, Kjartan Thor
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SUNCAT Center for Interface Science and Catalysis, USA.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Shin, Shik
    Selective Probing of the OH or OD Stretch Vibration in Liquid Water Using Resonant Inelastic Soft-X-Ray Scattering2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 19, p. 193001-Article in journal (Refereed)
    Abstract [en]

    High-resolution O 1s resonant inelastic x-ray scattering spectra of liquid H2O, D2O, and HDO, obtained by excitation near the preedge resonance show, in the elastic line region, well-separated multiple vibrational structures corresponding to the internal OH stretch vibration in the ground state of water. The energy of the first-order vibrational excitation is strongly blueshifted with respect to the main band in the infrared or Raman spectra of water, indicating that water molecules with a highly weakened or broken donating hydrogen bond are correlated with the preedge structure in the x-ray absorption spectrum. The vibrational profile of preedge excited HDO water is well fitted with 50% +/- 20% greater OH-stretch contribution compared to OD, which strongly supports a preference for OH being the weakened or broken H-bond in agreement with the well-known picture that D2O makes stronger H-bonds than H2O. Accompanying path-integral molecular dynamics simulations show that this is particularly the case for strongly asymmetrically H-bonded molecules, i.e., those that are selected by preedge excitation.

  • 38. Huang, C.
    et al.
    Wikfeldt, K. T.
    Stockholm University, Faculty of Science, Department of Physics.
    Nordlund, D.
    Bergmann, U.
    McQueen, T.
    Sellberg, J.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray diffraction study of temperature dependent structure of liquid waterArticle in journal (Refereed)
    Abstract [en]

    We have developed x-ray diffraction measurements with high energy-resolution and accuracy to study water structure at three different temperatures (7, 25 and 66 °C) under normal pressure. Using a spherically curved Ge crystal an energy resolution better than 15 eV has been achieved which eliminates influence from Compton scattering. The high quality of the data allows a precise oxygen-oxygen pair correlation function (PCF) to be directly derived from the Fourier transform of the experimental data resolving shell structure out to ~12 Å, i.e. 5 hydration shells. Large-scale molecular dynamics (MD) simulations using the TIP4P/2005 force-field reproduce excellently the experimental shell structure in the range 4-12 Å although less agreement is seen for the first peak in the PCF. The Local Structure Index (LSI) [J. Chem. Phys. 104, 7671 (1996)] identifies a tetrahedral minority giving the long-range oscillations in the PCF and a disordered majority providing a more featureless background in this range. The current study supports the proposal that the structure of liquid water even under ambient conditions can be described in terms of a two-state fluctuation model involving local structures related to the high-density and low-density forms of liquid water as postulated in the liquid-liquid phase transition hypothesis.

  • 39. Huang, Congcong
    et al.
    Weiss, T. M.
    Nordlund, Dennis
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Increasing correlation length in bulk supercooled H2O, D2O and NaCl solution determined from small angle x-ray scattering2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 13, p. 134504-Article in journal (Refereed)
    Abstract [en]

    Using small angle x-ray scattering, we find that the correlation length of bulk liq. water shows a steep increase as temp. decreases at subzero temps. (supercooling) and that it can, similar to the thermodn. response functions, be fitted to a power law.  This indicates that the anomalous properties of water are attributable to fluctuations between low- and high-d. regions with rapidly growing av. size upon supercooling.  The substitution of H2O with D2O, as well as the addn. of NaCl salt, leads to substantial changes of the power law behavior of the correlation length.  Our results are consistent with the proposed existence of a liq.-liq. crit. point in the deeply supercooled region but do not exclude a singularity-free model

  • 40. Huang, Congcong
    et al.
    Wikfeldt, K. Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Tokushima, Takashi
    Nordlund, Dennis
    Harada, Yoshi
    Bergmann, Uwe
    Niebuhr, Marc
    Weiss, T. M.
    Horikawa, Yoshi
    Leetmaa, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Takahashi, Osamu
    Lenz, Annika
    Ojamäe, Lars
    Lyubartsev, Alexander
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry, Physical Chemistry.
    Shin, Shik
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    The Inhomogeneous Structure of Water at Ambient Conditions2009In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 106, p. 15214-15218Article in journal (Refereed)
    Abstract [en]

    Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of ≈1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (nondirectional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.

  • 41. Huang, Congcong
    et al.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Tokushima, Takashi
    Nordlund, Dennis
    Harada, Yoshi
    Bergmann, Uwe
    Niebuhr, M.
    Weiss, T. M.
    Horikawa, Y.
    Leetmaa, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Takahashi, Osamu
    Lentz, Annika
    Ojamäe, Lars
    Lyubartsev, Alexander P.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Shin, Shik
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Reply to Soper "Fluctuations in water around a bimodal distribution of local hydrogen bonded structural motifs"2010In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, no 12, article id E45Article in journal (Refereed)
  • 42. Huang, Congcong
    et al.
    Wikfeldt, Thor Kjartan
    Stockholm University, Faculty of Science, Department of Physics.
    Nordlund, D.
    Bergmann, U.
    McQueen, T.
    Sellberg, Jonas
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Wide-angle X-ray diffraction and molecular dynamics study of medium-range order in ambient and hot water2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 44, p. 19997-20007Article in journal (Refereed)
    Abstract [en]

    We have developed wide-angle X-ray diffraction measurements with high energy-resolution and accuracy to study water structure at three different temperatures (7, 25 and 66 degrees C) under normal pressure. Using a spherically curved Ge crystal an energy resolution better than 15 eV has been achieved which eliminates influence from Compton scattering. The high quality of the data allows for a reliable Fourier transform of the experimental data resolving shell structure out to similar to 12 angstrom, i.e. 5 hydration shells. Large-scale molecular dynamics (MD) simulations using the TIP4P/2005 force-field reproduce excellently the experimental shell-structure in the range 4-12 angstrom although less agreement is seen for the first peak in the intermolecular pair-correlation function (PCF). The Shiratani-Sasai Local Structure Index [J. Chem. Phys. 104, 7671 (1996)] identifies a tetrahedral minority giving the intermediate-range oscillations in the O-O PCF and a disordered majority providing a more featureless background in this range. The current study supports the proposal that the structure of liquid water, even at high temperatures, can be described in terms of a two-state fluctuation model involving local structures related to the high-density and low-density forms of liquid water postulated in the liquid-liquid phase transition hypothesis.

  • 43. Huang, Ningdong
    et al.
    Nordlund, Dennis
    Huang, Congcong
    Bergmann, Uwe
    Weiss, Thomas M.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray Raman scattering provides evidence for interfacial acetonitrile-water dipole interactions in aqueous solutions2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, no 16, p. 164509-Article in journal (Refereed)
    Abstract [en]

    Aqueous solutions of acetonitrile (MeCN) have been studied with oxygen K-edge x-ray Raman scattering (XRS) which is found to be sensitive to the interaction between water and MeCN. The changes in the XRS spectra can be attributed to water directly interacting with MeCN and are reproduced by density functional theory calculations on small clusters of water and MeCN. The dominant structural arrangement features dipole interaction instead of H-bonds between the two species as revealed by the XRS spectra combined with spectrum calculations. Small-angle x-ray scattering shows the largest heterogeneity for a MeCN to water ratio of 0.4 in agreement with earlier small-angle neutron scattering data.

  • 44.
    Huang, Ningdong
    et al.
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Nordlund, Dennis
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Huang, Congcong
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Tyliszczak, Tolek
    LBL, Adv Light Source, Berkeley, CA 94720 USA .
    Weiss, Thomas M.
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA .
    Acremann, Yves
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA .
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Microscopic Probing of the Size Dependence in Hydrophobic Solvation2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 7, p. 074507-Article in journal (Refereed)
    Abstract [en]

    A dependence on solute size of the hydrophobic effect has been proposed based on theory and simulations, such that small apolar solutes leave the hydrogen (H-) bonding network in water intact or even strengthened, whereas hydration of larger, nanometer-sized apolar solutes breaks hydrogen bonds and creates a liquid-vapor-like interface around the solutes. Here we report the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in aqueous solutions of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N + (TAA) cations with increased side chain length by probing the H-bonding network in water through O K-edge x-ray absorption spectroscopy and the solute-solute interaction using small angle x-ray scattering. These results open for unique experimental opportunities to investigate hydrophobic effects for a range of important processes in chemistry and biology.

    We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N+ (R4N+) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.

  • 45. Huse, Nils
    et al.
    Wen, Haidan
    Nordlund, Dennis
    Szilagyi, Erzsi
    Daranciang, Dan
    Miller, Timothy A.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Schoenlein, Robert W.
    Lindenberg, Aaron M.
    Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy2009In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 11, no 20, p. 3951-3957Article in journal (Refereed)
    Abstract [en]

    We report time-resolved studies of hydrogen bonding in liquid H2O, in response to direct excitation of the O-H stretch mode at 3 mm, probed via soft X-ray absorption spectroscopy at the oxygen K-edge. This approach employs a newly developed nanofluidic cell for transient soft X-ray spectroscopy in the liquid phase. Distinct changes in the near-edge spectral region (XANES) are observed, and are indicative of a transient temperature rise of 10 K following transient laser excitation and rapid thermalization of vibrational energy. The rapid heating occurs at constant volume and the associated increase in internal pressure, estimated to be 8 MPa, is manifested by distinct spectral changes that differ from those induced by temperature alone. We conclude that the near-edge spectral shape of the oxygen K-edge is a sensitive probe of internal pressure, opening new possibilities for testing the validity of water models and providing new insight into the nature of hydrogen bonding in water.

  • 46. Katayama, T.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Dell'Angela, M.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholm University, Faculty of Science, Department of Physics.
    Kaya, S.
    Krupin, O.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wurth, W.
    Östrom, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser2013In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 187, p. 9-14Article in journal (Refereed)
    Abstract [en]

    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.

  • 47. Kaya, Sarp
    et al.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Yamamoto, Susumu
    Newberg, John T.
    Bluhm, Hendrik
    Ogasawara, Hirohito
    Kendelewicz, Tom
    Brown, Gordon E., Jr.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Highly Compressed Two-Dimensional Form of Water at Ambient Conditions2013In: Scientific Reports, E-ISSN 2045-2322, Vol. 3, article id 1074Article in journal (Refereed)
    Abstract [en]

    The structure of thin-film water on a BaF2(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF2 exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF2(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.

  • 48.
    Kim, Kyung Hwan
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Pohang University of Science and Technology (POSTECH), Republic of Korea.
    Amann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics.
    Giovambattista, Nicolas
    Späh, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Perakis, Fivos
    Stockholm University, Faculty of Science, Department of Physics.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Ladd Parada, Marjorie
    Stockholm University, Faculty of Science, Department of Physics.
    Yang, Cheolhee
    Mariedahl, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Eklund, Tobias
    Stockholm University, Faculty of Science, Department of Physics.
    Lane, Thomas J.
    You, Seonju
    Jeong, Sangmin
    Weston, Matthew
    Stockholm University, Faculty of Science, Department of Physics.
    Lee, Jae Hyuk
    Eom, Intae
    Kim, Minseok
    Park, Jaeku
    Chun, Sae Hwan
    Poole, Peter H.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure2020In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 370, no 6519, p. 978-982Article in journal (Refereed)
    Abstract [en]

    We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

  • 49.
    Kim, Kyung Hwan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Späh, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Perakis, Fivos
    Stockholm University, Faculty of Science, Department of Physics.
    Mariedahl, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Sellberg, Jonas A.
    Katayama, Tetsuo
    Harada, Yoshihisa
    Ogasawara, Hirohito
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Temperature-Independent Nuclear Quantum Effects on the Structure of Water2017In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 7, article id 075502Article in journal (Refereed)
    Abstract [en]

    Nuclear quantum effects (NQEs) have a significant influence on the hydrogen bonds in water and aqueous solutions and have thus been the topic of extensive studies. However, the microscopic origin and the corresponding temperature dependence of NQEs have been elusive and still remain the subject of ongoing discussion. Previous x-ray scattering investigations indicate that NQEs on the structure of water exhibit significant temperature dependence [Phys. Rev. Lett. 94, 047801 (2005)]. Here, by performing wide-angle x-ray scattering of H2O and D2O droplets at temperatures from 275 K down to 240 K, we determine the temperature dependence of NQEs on the structure of water down to the deeply supercooled regime. The data reveal that the magnitude of NQEs on the structure of water is temperature independent, as the structure factor of D2O is similar to H2O if the temperature is shifted by a constant 5 K, valid from ambient conditions to the deeply supercooled regime. Analysis of the accelerated growth of tetrahedral structures in supercooled H2O and D2O also shows similar behavior with a clear 5 K shift. The results indicate a constant compensation between NQEs delocalizing the proton in the librational motion away from the bond and in the OH stretch vibrational modes along the bond. This is consistent with the fact that only the vibrational ground state is populated at ambient and supercooled conditions.

  • 50.
    Kim, Kyung Hwan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Späh, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Perakis, Fivos
    Stockholm University, Faculty of Science, Department of Physics.
    Mariedahl, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Amann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics.
    Sellberg, Jonas A.
    Lee, Jae Hyuk
    Kim, Sangsoo
    Park, Jaehyun
    Nam, Ki Hyun
    Katayama, Tetsuo
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Maxima in the thermodynamic response and correlation functions of deeply supercooled water2017In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 358, no 6370, p. 1589-1593Article in journal (Refereed)
    Abstract [en]

    Femtosecond x-ray laser pulses were used to probe micrometer-sized water droplets that were cooled down to 227 kelvin in vacuum. Isothermal compressibility and correlation length were extracted from x-ray scattering at the low-momentum transfer region. The temperature dependence of these thermodynamic response and correlation functions shows maxima at 229 kelvin for water and 233 kelvin for heavy water. In addition, we observed that the liquids undergo the fastest growth of tetrahedral structures at similar temperatures. These observations point to the existence of a Widom line, defined as the locus of maximum correlation length emanating from a critical point at positive pressures in the deeply supercooled regime. The difference in the maximum value of the isothermal compressibility between the two isotopes shows the importance of nuclear quantum effects.

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