Change search
Refine search result
123 1 - 50 of 103
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    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.

  • 2. 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.

  • 3.
    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.

  • 4. 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.

  • 5. 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.

  • 6. 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.

  • 7.
    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.

  • 8.
    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.

  • 9.
    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.

  • 10. 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.

  • 11. 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.

  • 12. 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 us, 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.

  • 13.
    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.

  • 14. 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.

  • 15. 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.

  • 16. 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.

  • 17. 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.

  • 18.
    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.

  • 19. 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.

  • 20. 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).

  • 21. 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.

  • 22. 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.

  • 23. 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

  • 24. 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.

  • 25.
    Huang, Congcong
    et al.
    Stanford.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Tokushima, Takashi
    SPring-8.
    Nordlund, Dennis
    Stanford.
    Harada, Yoshi
    Bergmann, Uwe
    Stanford.
    Niebuhr, M.
    Stanford.
    Weiss, T.M.
    Stanford.
    Horikawa, Y.
    SPring-8.
    Leetmaa, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Takahashi, Osamu
    University of Hiroshima.
    Lentz, Annika
    Linköpings Universitet.
    Ojamäe, Lars
    Linköpings Universitet.
    Lyubartsev, Alexander P.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Shin, Shik
    Tokyo University.
    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, p. E45-Article in journal (Refereed)
  • 26. 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.

  • 27. 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.

  • 28.
    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.

  • 29. 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.

  • 30. 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.

  • 31. 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, ISSN 2045-2322, 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.

  • 32.
    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.

  • 33.
    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.

  • 34. Laksmono, Hartawan
    et al.
    McQueen, Trevor A.
    Sellberg, Jonas A.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Loh, N. Duane
    Huang, Congcong
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Sierra, Raymond G.
    Hampton, Christina Y.
    Nordlund, Dennis
    Beye, Martin
    Martin, Andrew V.
    Barty, Anton
    Seibert, M. Marvin
    Messerschmidt, Marc
    Williams, Garth J.
    Boutet, Sebastien
    Arnann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics. University of Innsbruck, Austria.
    Loerting, Thomas
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Bogan, Michael J.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA; .
    Anomalous Behavior of the Homogeneous Ice Nucleation Rate in No-Man's Land2015In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 6, no 14, p. 2826-2832Article in journal (Refereed)
    Abstract [en]

    We present an analysis of ice nucleation kinetics from near-ambient pressure water as temperature decreases below the homogeneous limit T-H by cooling micrometer-sized droplets (microdroplets) evaporatively at 10(3)-10(4) K/s and probing the structure ultrafast using femtosecond pulses from the Linac Coherent Light Source (LCLS) free-electron X-ray laser. Below 232 K, we observed a slower nucleation rate increase with decreasing temperature than anticipated from previous measurements, which we suggest is due to the rapid decrease in water's diffusivity. This is consistent with earlier findings that microdroplets do not crystallize at <227 K, but vitrify at cooling rates of 10(6)-10(7) K/s. We also hypothesize that the slower increase in the nucleation rate is connected with the proposed fragile-to-strong transition anomaly in water.

  • 35.
    LaRue, Jerry
    et al.
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA; Chapman University, USA; Fritz-Haber Institute of the Max-Planck-Society, Germany.
    Krejčí, O.
    Stockholm University, Faculty of Science, Department of Physics. Charles University in Prague, Czech Republic; Institute of Physics of the Czech Academy of Sciences, Czech Republic.
    Yu, L.
    Beye, M.
    Ng, M. L.
    Öberg, H.
    Stockholm University, Faculty of Science, Department of Physics.
    Xin, H.
    Mercurio, G.
    Moeller, S.
    Turner, J. J.
    Nordlund, D.
    Coffee, R.
    Minitti, M. P.
    Wurth, W.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Abild-Pedersen, F.
    Ogasawara, H.
    Real-Time Elucidation of Catalytic Pathways in CO Hydrogenation on Ru2017In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, no 16, p. 3820-3825Article in journal (Refereed)
    Abstract [en]

    The direct elucidation of the reaction pathways in heterogeneous catalysis has been challenging due to the short-lived nature of reaction intermediates. Here, we directly measured on ultrafast time scales the initial hydrogenation steps of adsorbed CO on a Ru catalyst surface, which is known as the bottleneck reaction in syngas and CO2 reforming processes. We initiated the hydrogenation of CO with an ultrafast laser temperature jump and probed transient changes in the electronic structure using real-time X-ray spectroscopy. In combination with theoretical simulations, we verified the formation of CHO during CO hydrogenation.

  • 36. LaRue, Jerry L.
    et al.
    Katayama, Tetsuo
    Lindenberg, Aaron
    Fisher, Alan S.
    Östrom, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
    Ogasawara, Hirohito
    THz-Pulse-Induced Selective Catalytic CO Oxidation on Ru2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 3, article id 036103Article in journal (Refereed)
    Abstract [en]

    We demonstrate the use of intense, quasi-half-cycle THz pulses, with an associated electric field component comparable to intramolecular electric fields, to direct the reaction coordinate of a chemical reaction by stimulating the nuclear motions of the reactants. Using a strong electric field from a THz pulse generated via coherent transition radiation from an ultrashort electron bunch, we present evidence that CO oxidation on Ru(0001) is selectively induced, while not promoting the thermally induced CO desorption process. The reaction is initiated by the motion of the O atoms on the surface driven by the electric field component of the THz pulse, rather than thermal heating of the surface.

  • 37.
    Leetmaa, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias
    Stockholm University, Faculty of Science, Department of Physics.
    Lyubartsev, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Theoretical Approximations to X-ray Absorption Spectroscopy of Liquid Water and Ice2010In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 177, no 2-3, p. 135-157Article in journal (Refereed)
    Abstract [en]

    We review methods to compute x-ray absorption spectra (XAS) with special focus on the transition potential approach of Triguero et al. [Phys. Rev. B 58, 8097 (1998)] and its application to calculations on water in condensed phase. We discuss the absolute energy scale, functional dependence, broadening versus sampling of intra- and intermolecular vibrational modes, treatment of the continuum, cluster size convergence as well as compare with periodic calculations and with experiment; periodic and cluster model calculations are found to agree very closely in the relevant near-edge region although neither reproduces the pre-edge and main-edge features in the experimental spectra of thin ice films. The real space grid representation of the wave function in the periodic calculations allows a more extended energy range to be described and we find satisfactory agreement with experiment for higher energy continuum resonances. Two proposed alternative approaches using either the potential from a full core-hole (FCH) or the full core-hole with an excited electron in the lowest state (XCH) are shown to lead to spectra that deviate significantly from experiment.

  • 38.
    Leetmaa, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Näslund, L.Å.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Are Recent Water Models Obtained by Fitting Diffraction Data Consistent with IR/Raman and X-ray Absorption Spectra?2006In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, p. 244510-Article in journal (Refereed)
    Abstract [en]

    X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data. ©2006 American Institute of Physics

  • 39.
    Leetmaa, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray Spectroscopy Calculations within Kohn-Sham DFT: Theory and Applications, in Computational methods in Catalysis and Materials Science2009In: Computational Methods in Catalysis and Materials Science / [ed] R.A. van Santen and P. Sautet, Weinheim: Wiley-VCH , 2009, p. 221-264Chapter in book (Other academic)
  • 40.
    Leetmaa, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Swenson, Jan
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Diffraction and IR/Raman Data do not Prove Tetrahedral Water2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 8, p. 084502-Article in journal (Refereed)
    Abstract [en]

    We use the reverse Monte Carlo modeling technique to fit two extreme structure models for water to available x-ray and neutron diffraction data in q space as well as to the electric field distribution as a representation of the OH stretch Raman spectrum of dilue HOD in D2O; the internal geometries were fitted to a quantum distribution. Forcing the fit to maximize the number of hydrogen (H) bonds results in a tetrahedral model with 74% double H-bond donors (DD) and 21% single donors (SD). Maximizing instead the number of SD species gives 81% SD and 18% DD, while still reproducing the experimental data and losing only 0.7–1.8 kJ/mole interaction energy. By decomposing the simulated Raman spectrum we can relate the models to the observed ultrafast frequency shifts in recent pump-probe measurements. Within the tetrahedral DD structure model the assumed connection between spectrum position and H-bonding indicates ultrafast dynamics in terms of breaking and reforming H bonds while in the strongly distorted model the observed frequency shifts do not necessarily imply H-bond changes. Both pictures are equally valid based on present diffraction and vibrational experimental data. There is thus no strict proof of tetrahedral water based on these data. We also note that the tetrahedral structure model must, to fit diffraction data, be less structured than most models obtained from molecular dynamics simulations. ©2008 American Institute of Physics

  • 41.
    Liu, Chang
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lourenco, Maicon P.
    Hedström, Svante
    Stockholm University, Faculty of Science, Department of Physics.
    Cavalca, Filippo
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, United States.
    Diaz-Morales, Oscar
    Stockholm University, Faculty of Science, Department of Physics.
    Duarte, Hélio A.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO2 Reduction2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 45, p. 25010-25017Article in journal (Refereed)
    Abstract [en]

    Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO2 reduction reaction (CO2RR) due to the enhanced selectivity toward ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (O-sb). In this work, O-sb is investigated with theoretical methods. Although O-sb is unstable in slab models, it becomes stabilized within a manually reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The. higher stability of O-sb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by O-sb withdrawing electron density from the Cu atom, resulting in reduction of the sigma-repulsion. Hence, the coverage of CO may be increased, facilitating its dimerization.

  • 42.
    Ljungberg, Mathias P.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Semi-classical description of nuclear dynamics in x-ray emission of water2010In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 24, p. 245115-Article in journal (Refereed)
    Abstract [en]

    In this article we present a semi-classical approximation to the Kramers-Heisenberg formula for calculating x-ray emission (XES) spectra, including vibrational effects. We compare the method to the quantum Kramers-Heisenberg formula for a test system consisting of a model water dimer where the hydrogen-bond donor is core-ionized and obtain excellent agreement. In the semi-classical approach we average spectra from classical trajectories where the core-hole-induced dynamics is performed with initial conditions sampling the quantum zero-point position and momentum probability distributions in the O-H vibration. We find very similar time-evolution of the squared quantum wave packet compared to the probability distribution under classical dynamics until the proton interacts with the next water. We compare our semi-classical approach with other methods to compute the XES spectra of water that have been used in the past and conclude that our approach gives superior results while requiring the same computational effort.

  • 43.
    Ljungberg, Mathias P.
    et al.
    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.
    A simple picture of x-ray emission of condensed phase waterManuscript (preprint) (Other academic)
    Abstract [en]

    We apply the Kramers-Heisenberg formula to a one-dimensional model of the water dimer to discuss vibrational interference in the x-ray emission spectrum of the donor molecule for which the core-ionized potential energy surface is known to be dissociative. The isotope effect is discussed and connections are made to recent experimental data on liquid water in terms of asymmetrical broadening of the 1b1 spectral features. A long core-hole life time leads to decay from Zundel-like, fully delocalized vibrational states in the intermediate potential without involvement of a specific dissociated component.

  • 44.
    Ljungberg, Mathias P.
    et al.
    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.
    Vibrational interference effects in x-ray emission of a model water dimer: implications for the interpretation of the liquid spectrum2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 4, p. 044513-Article in journal (Refereed)
    Abstract [en]

    We apply the Kramers-Heisenberg formula to a model water dimer to discuss vibrational interference in the x-ray emission spectrum of the donor molecule for which the core-ionized potential energy surface is dissociative but bounded by the accepting molecule. A long core-hole life time leads to decay from Zundel-like, fully delocalized vibrational states in the intermediate potential without involvement of a specific dissociated component. Comparison is made to a model with an unbound intermediate state allowing dissociation to infinity which gives a sharp, fully dissociated feature and a broad molecular peak at long core-hole life time. The implications of the vibrational interference effect on the liquid water spectrum are discussed and it is proposed that this mainly gives rise to an isotope-dependent asymmetrical broadening of the lone pair peak.

  • 45. Miller, D. J.
    et al.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Kaya, S.
    Casalongue, H. Sanchez
    Friebel, D.
    Anniyev, T.
    Ogasawara, H.
    Bluhm, H.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Oxidation of Pt(111) under Near-Ambient Conditions2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 19, p. 195502-Article in journal (Refereed)
    Abstract [en]

    The oxidation of Pt(111) at near-ambient O(2) pressures has been followed in situ using x-ray photoelectron spectroscopy (XPS) and ex situ using x-ray absorption spectroscopy (XAS). Polarization-dependent XAS signatures at the O K edge reveal significant temperature-and pressure-dependent changes of the Pt-O interaction. Oxide growth commences via a PtO-like surface oxide that coexists with chemisorbed oxygen, while an ultrathin alpha-PtO(2) trilayer is identified as the precursor to bulk oxidation. These results have important implications for understanding the chemical state of Pt in catalysis.

  • 46.
    Miller, Daniel J.
    et al.
    Stanford.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Näslund, Lars-Åke
    Stanford.
    Anniyev, Toyli
    Stanford.
    Ogasawara, Hirohito
    Stanford.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Low O-2 dissociation barrier on Pt(111) due to adsorbate-adsorbate interactions2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 22, p. 224701-Article in journal (Refereed)
    Abstract [en]

    O2 dissociation on Pt(111) has been followed at low and saturation coverage using temperature-programmed x-ray photoelectron spectroscopy and simulated with mean-field kinetic modeling, yielding dissociation (Ea) and desorption (Ed) barriers of 0.32 and 0.36 eV, respectively. DFT calculations show that Ea is strongly influenced by the O–O interatomic potential in the atomic final state: of the supercells considered, that which maximizes attractive third-nearest-neighbor interactions in the atomic final state yields both the lowest computed dissociation barrier (0.24 eV) and the best agreement with experiment. It is proposed that the effect of adsorbate-adsorbate interactions must be considered when modeling catalytic processes involving dissociative steps.

  • 47. Nikitin, Anton
    et al.
    Naeslund, Lars-Ake
    Zhang, Zhiyong
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    C-H bond formation at the graphite surface studied with core level spectroscopy2008In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 602, no 14, p. 2575-2580Article in journal (Refereed)
    Abstract [en]

    We studied the formation of C-H bonds at the surface of graphite under atomic hydrogen treatment using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). We found that the amount of hydrogenation of the graphite surface at saturation coverage measured by C1s XPS coincides with the amount of hydrogen measured by temperature programmed desorption. This directly indicates that C1s XPS measurements provide reliable information about the amount of hydrogen stored in carboneous materials through the formation of C-H bonds. From angle resolved XAS measurements we found that the C-H bonds are not perpendicular to the surface of the graphite and that the hydrogenated carbon atoms are buckled up from the highly oriented pyrolitic graphite surface.

  • 48. Nikitin, Anton
    et al.
    Zhang, Zhiyong
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Energetics of C-H Bonds Formed at Single-Walled Carbon Nanotubes2009In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 4, p. 1301-1306Article in journal (Refereed)
    Abstract [en]

    We have calculated the adsorption energies for different hydrogen dimers adsorbed at the surface of zigzag single-walled carbon nanotubes (SWCNs) (n,0) (for n = 6-28) to identify the range of nanotube structural parameters that provide optimal adsorption energetics. We determined that, for H-2 gas in equilibrium with adsorbed hydrogen dimers, under normal conditions, carbon nanotubes with diameters in the range of 0.77 +/- 0.19 nm have a minimum energy overhead for hydrogen release and uptake in the hydrogen storage process. Most interestingly, we also found that the adsorption energies of hydrogen are correlated to the modulations of the nanotube density of states, because of the quantum confinement of electrons along the circumference of the SWCN. This effect is discussed from the perspective of chemical bond formation and is related to the valence to conduction band excitation energy.

  • 49.
    Nilsson, A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Stanford Synchrotron Radiation Laboratory.
    Cavalleri, M.
    Nordlund, D.
    Stockholm University, Faculty of Science, Department of Physics.
    Nyberg, M.
    Pettersson, L.G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    The Hydrogen Bond in Ice Probed by Soft X-ray Spectroscopy and Density Functional Theory2005In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, ISSN 0021-9606, Vol. 122, no 15, p. 154505-Article in journal (Refereed)
    Abstract [en]

    We combine photoelectron and x-ray absorption spectroscopy with density functional theory to derive a molecular orbital picture of the hydrogen bond in ice. We find that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O–H antibonding orbitals on neighboring molecules. Together with internal s-p rehybridization this minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms, which is essential for a strong attractive electrostatic interaction. Our joint experimental and theoretical results demonstrate that an electrostatic model based on only charge induction from the surrounding medium fails to properly describe the internal charge redistributions upon hydrogen bonding.

  • 50.
    Nilsson, A.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ogasawara, H.
    Stanford Synchrotron Radiation Laboratory.
    Cavalleri, M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nordlund, D.
    Stockholm University, Faculty of Science, Department of Physics.
    Nyberg, M.
    Stockholm University, Faculty of Science, Department of Physics.
    Wernet, Ph.
    Pettersson, L.G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    The hydrogen bond in ice probed by soft x-ray spectroscopy and density functional theory2005In: Journal of Chemical Physics, ISSN 1089-7690, Vol. 122, p. 154505-Article in journal (Refereed)
    Abstract [en]

    We combine photoelectron and x-ray absorption spectroscopy with density functional theory to derive a molecular orbital picture of the hydrogen bond in ice. We find that the hydrogen bond involves donation and back-donation of charge between the oxygen lone pair and the O–H antibonding orbitals on neighboring molecules. Together with internal s-p rehybridization this minimizes the repulsive charge overlap of the connecting oxygen and hydrogen atoms, which is essential for a strong attractive electrostatic interaction. Our joint experimental and theoretical results demonstrate that an electrostatic model based on only charge induction from the surrounding medium fails to properly describe the internal charge redistributions upon hydrogen bonding.

123 1 - 50 of 103
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf