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  • 1.
    Alonso Mori, Roberto
    et al.
    Universita di Camerino, Italy.
    Paris, E.
    Universita di Camerino, Italy.
    Giuli, G.
    Universita di Camerino, Italy.
    Eeckhout, S.G.
    European Synchrotron Radiation Facility (ESRF).
    Kavčič, M.
    J. Stefan Institute, Slovenia.
    Žitnik, M.
    J. Stefan Institute, Slovenia.
    Bučar, K.
    J. Stefan Institute, Slovenia.
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Glatzel, Pieter
    European Synchrotron Radiation Facility (ESRF).
    Sulfur-Metal Orbital Hybridization in Sulfur-Bearing Compounds Studied by X-ray Emission Spectroscopy2010In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 49, p. 6468-6473Article in journal (Refereed)
    Abstract [en]

     

     

    The electronic structure and ligand environment of sulfur was investigated in various sulfur-containing compounds with different structures and chemical states by using X-ray emission spectroscopy (XES). Calculations were performed using density functional theory (DFT) as implemented in the StoBe code. The sulfur chemical state and atomic environment is discussed in terms of the molecular orbitals and partial charges that are obtained from the calculations. The main spectral features can be modeled using our calculational approach. The sensitivity of the K β

    emission to thecation and the local symmetry is discussed.

  • 2. 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.

  • 3. 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.

  • 4. 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.

  • 5.
    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.

  • 6.
    Camisasca, Gaia
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Galamba, Nuno
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Translational and rotational dynamics of high and low density TIP4P/2005 water2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 22, article id 224507Article in journal (Refereed)
    Abstract [en]

    We use molecular dynamics simulations using TIP4P/2005 to investigate the self- and distinct-van Hove functions for different local environments of water, classified using the local structure index as an order parameter. The orientational dynamics were studied through the calculation of the time-correlation functions of different-order Legendre polynomials in the OH-bond unit vector. We found that the translational and orientational dynamics are slower for molecules in a low-density local environment and correspondingly the mobility is enhanced upon increasing the local density, consistent with some previous works, but opposite to a recent study on the van Hove function. From the analysis of the distinct dynamics, we find that the second and fourth peaks of the radial distribution function, previously identified as low density-like arrangements, show long persistence in time. The analysis of the time-dependent interparticle distance between the central molecule and the first coordination shell shows that particle identity persists longer than distinct van Hove correlations. The motion of two first-nearest-neighbor molecules thus remains coupled even when this correlation function has been completely decayed. With respect to the orientational dynamics, we show that correlation functions of molecules in a low-density environment decay exponentially, while molecules in a local high-density environment exhibit bi-exponential decay, indicating that dynamic heterogeneity of water is associated with the heterogeneity among high-density and between high-density and low-density species. This bi-exponential behavior is associated with the existence of interstitial waters and the collapse of the second coordination sphere in high-density arrangements, but not with H-bond strength.

  • 7.
    Camisasca, Gaia
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pathak, Harshad
    Stockholm University, Faculty of Science, Department of Physics.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Radial distribution functions of water: Models vs experiments2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 4, article id 044502Article in journal (Refereed)
    Abstract [en]

    We study the temperature behavior of the first four peaks of the oxygen-oxygen radial distribution function of water, simulated by the TIP4P/2005, MB-pol, TIP5P, and SPC/E models and compare to experimental X-ray diffraction data, including a new measurement which extends down to 235 K [H. Pathak et al., J. Chem. Phys. 150, 224506 (2019)]. We find the overall best agreement using the MB-pol and TIP4P/2005 models. We observe, upon cooling, a minimum in the position of the second shell simulated with TIP4P/2005 and SPC/E potentials, located close to the temperature of maximum density. We also calculated the two-body entropy and the contributions coming from the first, second, and outer shells to this quantity. We show that, even if the main contribution comes from the first shell, the contribution of the second shell can become important at low temperature. While real water appears to be less ordered at short distance than obtained by any of the potentials, the different water potentials show more or less order compared to the experiments depending on the considered length-scale.

  • 8.
    Camisasca, Gaia
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Zhovtobriukh, Iurii
    Stockholm University, Faculty of Science, Department of Physics.
    Pitsevich, George
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    A proposal for the structure of high- and low-density fluctuations in liquid water2019In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 151, no 3, article id 034508Article in journal (Refereed)
    Abstract [en]

    Based on recent experimental data that can be interpreted as indicating the presence of specific structures in liquid water, we build and optimize two structural models which we compare with the available experimental data. To represent the proposed high-density liquid structures, we use a model consisting of chains of water molecules, and for low-density liquid, we investigate fused dodecahedra as templates for tetrahedral fluctuations. The computed infrared spectra of the models are in very good agreement with the extracted experimental spectra for the two components, while the extracted structures from molecular dynamics (MD) simulations give spectra that are intermediate between the experimentally derived spectra. Computed x-ray absorption and emission spectra as well as the O-O radial distribution functions of the proposed structures are not contradicted by experiment. The stability of the proposed dodecahedral template structures is investigated in MD simulations by seeding the starting structure, and remnants found to persist on an similar to 30 ps time scale. We discuss the possible significance of such seeds in simulations and whether they can be viable candidates as templates for structural fluctuations below the compressibility minimum of liquid water.

  • 9.
    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.

  • 10.
    Cavalleri, M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Nordlund, D.
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, M.
    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.
    Half or full core hole in density functional theory X-ray absorption spectrum calculations of water?2005In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 7, no 15, p. 2854-2858Article in journal (Refereed)
    Abstract [en]

    We analyze the performance of two different core-hole potentials in the theoretical modeling of XAS of ice, liquid and gas phase water; the use of a full core-hole (FCH) in the calculations, as suggested by Hetenyi et al. [B. Hetenyi, F. De Angelis, P. Giamozzi and R. Car, J. Chem. Phys., 2004, 120(18), 8632], gives poor agreement with experiment in terms of intensity distribution as well as transition energies, while the half core hole (HCH) potential, in the case of water, provides a better compromise between initial and final state effects, leading to good agreement with the experimental data.

  • 11.
    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.

  • 12. Chen, Chen
    et al.
    Huang, Congcong
    Waluyo, Iradwikanari
    Nordlund, Dennis
    Weng, Tsu-Chien
    Sokaras, Dimosthenis
    Weiss, Thomas
    Bergmann, Uwe
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Solvation structures of protons and hydroxide ions in water2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 15, p. 154506-Article in journal (Refereed)
    Abstract [en]

    X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H+ and OH- are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H+ and OH- have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water.

  • 13. 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.

  • 14.
    Damian, Emiliana
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Jalilehvand, Farideh
    Leung, Bonnie
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Sandström, Magnus
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Theoretical and experimental sulfur K-edge X-ray absorption spectroscopic (XANES) study of cysteine, cystine, methionine and methionine sulfoxide2009In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 18, p. 3542-3558Article in journal (Refereed)
    Abstract [en]

    The experimental sulfur K-edge x-ray absorption near edge structure (XANES) spectra of the amino acids cysteine, homocysteine, penicillamine, methionine, the oxidation products methionine sulfoxide and the disulfide cystine, have been analyzed by transition potential DFT calculations. The absolute energies and intensities of the main pre-edge sulfur 1s electron transitions have been computed to determine the character of the receiving low energy unoccupied molecular orbitals (MO), and to investigate the influence of external interactions, especially by introducing water molecules hydrogen bonded to the ionic species present in different pH ranges. When the thiol group deprotonates for the three amino acids cysteine, homocysteine and penicillamine the energy of the main transition, to an MO with antibonding σ*(S-H) character, reduces by ~1.1 eV and the receiving MO obtains σ*(S-C) character. The energy shifts due to hydrogen bonding were in most cases found to be relatively small, although the transition intensities could vary significantly due to the changes induced in the molecular charge distribution, thereby affecting the shapes of the spectral features. For the cysteine and penicillamine zwitterions deconvolution of the experimental spectra allowed the microscopic acid dissociation constants to be extracted separately for the thiol and the protonated amine groups, pKaT = 8.5 ± 0.1 and 8.2 ± 0.1, and pKaN = 8.9 ± 0.1 and 8.8 ± 0.1, respectively, in both cases with the thiol group the more acidic. Coordination of cysteine to nickel(II) or mercury(II) introduced a new low energy transition involving metal ion orbitals in the receiving LUMO. The small experimentally observed energy differences between the similar main absorption features of the cysteine and methionine zwitterions, 0.2-0.3 eV in comparable surrounding, as well as a minor difference in their intensity ratio, are reflected in the calculated transitions. The S K-edge XANES spectrum of the disulfide cystine displays a characteristic double peak with the lower energy transition (2469.9 eV) into the antibonding σ*(S-S) MO. The second peak, at 2471.5 eV in aqueous solution, contains several transitions into MO:s with σ*(S-C) character involving also charge transfer to the water molecules hydrating the protonated amine groups (NH3+) of cystine. The experimental energy difference between the two peaks increases by 0.2 eV for solid cystine, while no such increase occurs for the –S-S- bond between the cysteine residues in the oxidized disulfide of glutathione, where the amine groups are engaged in peptide bonds. This study shows that externally induced changes in the intramolecular bonding, e.g. by coordination, conformation geometry or hydrogen bonding, can significantly influence the S K-edge spectra, and emphasizes the importance of a similar chemical surrounding of the model compounds for standard spectra of sulfur functional groups, used to deconvolute composite experimental spectra.

  • 15.
    de Wijn, Astrid S.
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Norwegian University of Science and Technology, Norway.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    How square ice helps lubrication2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 16, article id 165433Article in journal (Refereed)
    Abstract [en]

    In the context of friction we use atomistic molecular-dynamics simulations to investigate water confined between graphene sheets over a wide range of pressures. We find that thermal equilibration of the confined water is hindered at high pressures. We demonstrate that, under the right conditions, square ice can form in an asperity, and that it is similar to cubic ice VII and ice X. We simulate sliding of atomically flat graphite on the square ice and find extremely low friction due to structural superlubricity. The conditions needed for square ice to form correspond to low sliding speeds, and we suggest that the ice observed in experiments of friction on wet graphite is of this type.

  • 16. Delesma, Francisco A.
    et al.
    Van den Bossche, Maxime
    Grönbeck, Henrik
    Calaminici, Patrizia
    Köster, Andreas M.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    A Chemical View on X-ray Photoelectron Spectroscopy: the ESCA Molecule and Surface-to-Bulk XPS Shifts2018In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 19, no 2, p. 169-174Article in journal (Refereed)
    Abstract [en]

    In this paper we remind the reader of a simple, intuitive picture of chemical shifts in X-ray photoelectron spectroscopy (XPS) as the difference in chemical bonding between the probed atom and its neighbor to the right in the periodic table, the so called Z + 1 approximation. We use the classical ESCA molecule, ethyl trifluoroacetate, and 4d-transition metals to explicitly demonstrate agreement between core-level shifts computed as differences between final core-hole states and the approach where each core-ionized atom is replaced by a Z + 1 atom. In this final state, or total energy picture, the XPS shift arises due to the more or less unfavorable chemical bonding of the effective nitrogen in the carbon geometry for the ESCA molecule. Surface core level shifts in metals are determined by whether the Z + 1 atom as an alloy segregates to the surface or is more soluble in the bulk. As further illustration of this more chemical picture, we compare the geometry of C 1s and O 1s core-ionized CO with that of, respectively, NO+ and CF+. The scope is not to propose a new method to compute XPS shifts but rather to stress the validity of this simple interpretation.

  • 17. 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.

  • 18. Dos Santos, Egon C.
    et al.
    Lourenco, Maicon P.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Duarte, Hélio A.
    Stability, Structure, and Electronic Properties of the Pyrite/Arsenopyrite Solid-Solid Interface-A DFT Study2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 14, p. 8042-8051Article in journal (Refereed)
    Abstract [en]

    Pyrite is the most common sulfide in the Earth. In the presence of arsenopyrite its oxidation is delayed, and instead, the arsenopyrite increases its oxidation rate, releasing As(III) and As(V) species in the medium. DFT/plane waves calculations were performed on pyrite/arsenopyrite interface models to understand the stability, structure, and electronic properties of the interface. This is the first step to understand the influence of the inlaid arsenopyrite in the pyrite oxidation mechanism. The interface is slightly stressed with minor changes in the bond lengths and lattice parameters with respect to the pure phases. The work of adhesion and the formation energy indicate that the miscibility of the two phases is not favorable, explaining the presence of large domains of either pyrite or arsenopyrite forming bulk granular regions. The valence band of the pyrite/arsenopyrite interface has large contributions from the pyrite phase, while the conduction band has large contributions from the arsenopyrite. This is consistent with the pyrite as cathode and arsenopyrite as anode in a galvanic contact. Furthermore, the interface formation shifts the valence states upward and decreases the band gap, facilitating interfacial electron transfer.

  • 19.
    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.

  • 20. Ferreira de Lima, Guilherme
    et al.
    Anderson Duarte, Hélio
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray Absorption Near-Edge Spectroscopy Calculations on Pristine and Modified Chalcopyrite Surfaces2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 35, p. 20200-20209Article in journal (Refereed)
    Abstract [en]

    Understanding chemical modifications on the chalcopyrite surface is an important issue to improve hydrometallurgical processes to recover copper from the mineral. X-ray absorption near-edge spectroscopy (XANES) can be used for this task, but the interpretation of the spectrum and the correlation with chemical changes in the first atomic layers are not straightforward. The present study demonstrates the potential of combining spectrum measurements with theoretical X-ray spectrum simulations to elucidate the chemistry behind weathering of important classes of minerals. We simulated the S and Fe K-edge XANES spectrum for pristine and modified chalcopyrite surfaces using periodic DFT calculations and the transition-potential model. The calculated S K-edge XANES spectra are in good agreement with experimental data and the peaks were attributed using the density of states. The simulated Fe K-edge XANES spectra do not reproduce all features observed experimentally. The effect of surface changes due to reconstruction, hydration, and oxidation on the spectrum was analyzed. Our results show that the S K-edge XANES spectrum is more sensitive to surface modifications than the Fe K-edge XANES spectrum, and this sensitivity could be used to follow the evolution of the surface.

  • 21. 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.

  • 22. Fransson, Thomas
    et al.
    Zhovtobriukh, Iurii
    Stockholm University, Faculty of Science, Department of Physics.
    Coriani, Sonia
    Wikfeldt, Kjartan T.
    Stockholm University, Faculty of Science, Department of Physics. University of Iceland, Iceland .
    Norman, Patrick
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Requirements of first-principles calculations of X-ray absorption spectra of liquid water2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 1, p. 566-583Article in journal (Refereed)
    Abstract [en]

    A computational benchmark study on X-ray absorption spectra of water has been performed by means of transition-potential density functional theory (TP-DFT), damped time-dependent density functional theory (TDDFT), and damped coupled cluster (CC) linear response theory. For liquid water, using TDDFT with a tailored CAM-B3LYP functional and a polarizable embedding, we find that an embedding with over 2000 water molecules is required to fully converge spectral features for individual molecules, but a substantially smaller embedding can be used within averaging schemes. TP-DFT and TDDFT calculations on 100 MD structures demonstrate that TDDFT produces a spectrum with spectral features in good agreement with experiment, while it is more difficult to fully resolve the spectral features in the TP-DFT spectrum. Similar trends were also observed for calculations of bulk ice. In order to further establish the performance of these methods, small water clusters have been considered also at the CC2 and CCSD levels of theory. Issues regarding the basis set requirements for spectrum simulations of liquid water and the determination of gas-phase ionization potentials are also discussed.

  • 23. 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.

  • 24. Gallo, Paola
    et al.
    Arnann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics.
    Angell, Charles Austen
    Anisimov, Mikhail Alexeevich
    Caupin, Frederic
    Chakravarty, Charusita
    Lascaris, Erik
    Loerting, Thomas
    Panagiotopoulos, Athanassios Zois
    Russo, John
    Sellberg, Jonas Alexander
    Stanley, Harry Eugene
    Tanaka, Hajime
    Vega, Carlos
    Xu, Limei
    Pettersson, Lars Gunnar Moody
    Stockholm University, Faculty of Science, Department of Physics.
    Water: A Tale of Two Liquids2016In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 116, no 13, p. 7463-7500Article, review/survey (Refereed)
    Abstract [en]

    Water is the most abundant liquid on earth and also the substance with the largest number of anomalies in its properties. It is a prerequisite for life and as such a most important subject of current research in chemical physics and physical chemistry. In spite of its simplicity as a liquid, it has an enormously rich phase diagram where different types of ices, amorphous phases, and anomalies disclose a path that points to unique thermodynamics of its supercooled liquid state that still hides many unraveled secrets. In this review we describe the behavior of water in the regime from ambient conditions to the deeply supercooled region. The review describes simulations and experiments on this anomalous liquid. Several scenarios have been proposed to explain the anomalous properties that become strongly enhanced in the supercooled region. Among those, the second critical-point scenario has been investigated extensively, and at present most experimental evidence point to this scenario. Starting from very low temperatures, a coexistence line between a high-density amorphous phase and a low-density amorphous phase would continue in a coexistence line between a high-density and a low-density liquid phase terminating in a liquid liquid critical point, LLCP. On approaching this LLCP from the one-phase region, a crossover in thermodynamics and dynamics can be found. This is discussed based on a picture of a temperature-dependent balance between a high-density liquid and a low-density liquid favored by, respectively, entropy and enthalpy, leading to a consistent picture of the thermodynamics of bulk water. Ice nucleation is also discussed, since this is what severely impedes experimental investigation of the vicinity of the proposed LLCP. Experimental investigation of stretched water, i.e., water at negative pressure, gives access to a different regime of the complex water diagram. Different ways to inhibit crystallization through confinement and aqueous solutions are discussed through results from experiments and simulations using the most sophisticated and advanced techniques. These findings represent tiles of a global picture that still needs to be completed. Some of the possible experimental lines of research that are essential to complete this picture are explored.

  • 25.
    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.

  • 26.
    Gladh, Jörgen
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Öberg, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy2015In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 633, p. 77-81Article in journal (Refereed)
    Abstract [en]

    We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2x1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments.

  • 27.
    Halldin Stenlid, Joakim
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Campos dos Santos, Egon
    Stockholm University, Faculty of Science, Department of Physics. Universidade Federal de Minas Gerais, Brazil.
    Johansson, Adam Johannes
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    On the Nature of the Cathodic Reaction during Corrosion of Copper in Anoxic Sulfide Solutions2019In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 166, no 6, p. C196-C208Article in journal (Refereed)
    Abstract [en]

    Sulfide-induced corrosion is expected to be the dominating long-term corrosion process for copper containers in technical concepts for deep geological disposal of spent nuclear fuel (SNF), adapted in several waste management programs around the world. The present study investigates the atomic-scale mechanism of the cathode side of the corrosion reaction using Density Functional Theory (DFT) calculations. Despite the central role of the reaction, neither the site of reaction nor the active species has been previously established. Here we compare the cathodic reaction leading to H-2-evolution on pure copper and on chalcocite (Cu2S) surfaces. The considered H-donors are OH-/H2O and HS-/H2S which are all available at the neutral to alkaline conditions anticipated at the SNF disposal sites. Assuming Volmer-Tafel-Heyrovsky kinetics, we find that the cathodic reactions are many orders of magnitude faster on copper compared to copper sulfide. Although we find that HS-/H2S have lower reaction barriers than H2O, our kinetic analysis suggest that H2O is expected to be the main H-source for the cathodic reaction under SNF repository conditions as results of the low sulfide concentrations (less than or similar to 10 mu M) expected in SNF repositories in Sweden, Finland and Canada.

  • 28. 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).

  • 29. 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.

  • 30.
    Hedström, Svante
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Campos dos Santos, Egon
    Stockholm University, Faculty of Science, Department of Physics. Universidade Federal de Minas Gerais, Brazil.
    Liu, Chang
    Stockholm University, Faculty of Science, Department of Physics.
    Chan, Karen
    Abild-Pedersen, Frank
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Spin Uncoupling in Chemisorbed OCCO and CO2: Two High-Energy Intermediates in Catalytic CO2 Reduction2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 23, p. 12251-12258Article in journal (Refereed)
    Abstract [en]

    The production of useful compounds via the electrochemical carbon dioxide reduction reaction (CO2RR) is a matter of intense research. Although the thermodynamics and kinetic barriers of CO2RR are reported in previous computational studies, the electronic structure details are often overlooked. We study two important CO2RR intermediates: ethylenedione (OCCO) and CO, covalently bound to cluster and slab models of the Cu(100) surface. Both molecules exhibit a near-unity negative charge as chemisorbed, but otherwise they behave quite differently, as explained by a spin uncoupling perspective. OCCO adopts a high-spin, quartetlike geometry, allowing two covalent bonds to the surface with an average gross interaction energy of -1.82 eV/bond. The energy cost for electronically exciting OCCO- to the quartet state is 1.5 eV which is readily repaid via the formation of its two surface bonds. CO2, conversely, retains a low-spin, doubletlike structure upon chemisorption, and its single unpaired electron forms a single covalent surface bond of -2.07 eV. The 5.0 eV excitation energy to the CO2- quartet state is prohibitively costly and cannot be compensated for by an additional surface bond.

  • 31. 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.

  • 32. 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

  • 33. 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.

  • 34. 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)
  • 35. 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.

  • 36. 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.

  • 37.
    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.

  • 38. Juarez-Mosqueda, Rosalba
    et al.
    Mavrandonakis, Andreas
    Kuc, Agnieszka B.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Heine, Thomas
    Theoretical analysis of hydrogen spillover mechanism on carbon nanotubes2015In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 3, article id 2Article in journal (Refereed)
    Abstract [en]

    The spillover mechanism of molecular hydrogen on carbon nanotubes in the presence of catalytically active platinum clusters was critically and systematically investigated by using density-functional theory. Our simulation model includes a Pt-4 cluster for the catalyst nanoparticle and curved and planar circumcoronene for two exemplary single-walled carbon nanotubes (CNT), the (10,10) CNT and one of large diameter, respectively. Our results show that the H-2 molecule dissociates spontaneously on the Pt-4 cluster. However, the dissociated H atoms have to overcome a barrier of more than 2 eV to migrate from the catalyst to the CNT, even if the Pt-4 cluster is at full saturation with six adsorbed and dissociated hydrogen molecules. Previous investigations have shown that the mobility of hydrogen atoms on the CNT surface is hindered by a barrier. We find that instead the Pt-4 catalyst may move along the outer surface of the CNT with activation energy of only 0.16 eV, and that this effect offers the possibility of full hydrogenation of the CNT. Thus, although we have not found a low-energy pathway to spillover onto the CNT, we suggest, based on our calculations and calculated data reported in the literature, that in the hydrogen-spillover process the observed saturation of the CNT at hydrogen background pressure occurs through mobile Pt nanoclusters, which move on the substrate more easily than the substrate-chemisorbed hydrogens, and deposit or reattach hydrogens in the process. Initial hydrogenation of the carbon substrate, however, is thermodynamically unfavoured, suggesting that defects should play a significant role.

  • 39. Karlsson, Rasmus K. B.
    et al.
    Cornell, Ann
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Structural Changes in RuO2 during Electrochemical Hydrogen Evolution2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 13, p. 7094-7102Article in journal (Refereed)
    Abstract [en]

    A comprehensive theoretical study of the X-ray photoelectron shifts for RuO2 during hydrogen evolution has been performed. The shifts have been calculated using first principles density functional theory and are compared with previous theoretical and experimental results to reconsider the proposed structural changes occurring during hydrogen evolution on RuO2. We find that during hydrogen evolution hydrogen enters the rutile RuO2 lattice and converts oxygen groups into hydroxyl groups and that this process explains the experimentally observed increase in unit cell dimensions as well as observed chemical shifts. Furthermore, carbon contamination is the most likely explanation for a set of peaks previously identified as caused by a new RuO(OH)(2) phase. We find that formation of metallic Ru is just one possible explanation for another peak in the X-ray photoelectron spectrum and that explanations including conversion of RuO2 into Ru(OH)(3), or removal of oxygen from Ru active surface sites, also can explain the observed shifts.

  • 40. Karlsson, Rasmus K. B.
    et al.
    Cornell, Ann
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    The electrocatalytic properties of doped TiO22015In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 180, p. 514-527Article in journal (Refereed)
    Abstract [en]

    To rationally control the catalytic properties of heterogeneous catalysts is the goal in heterogeneous (electro)catalysis research. Recent developments of theoretical methods based on density functional theory have enabled computational screening of catalysts, to achieve fundamental understanding of which catalyst is optimal for a certain reaction. In the present work, such screening is employed to elucidate the electrocatalytic properties of doped rutile TiO2. Electrodes based on this material are widely used in industrial production of, e.g., chlorine and sodium chlorate. The screening covers 38 different dopants, including all fourth, fifth and sixth row transition metals. Several dopants are predicted to activate TiO2, resulting in a material optimal either for the oxygen evolution reaction, or for selective chlorine evolution. The results can serve as a map for the rational design of electrocatalysts based on TiO2.

  • 41. Karlsson, Rasmus K. B.
    et al.
    Hansen, Heine A.
    Bligaard, Thomas
    Cornell, Ann
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Ti atoms in Ru0.3Ti0.7O2 mixed oxides form active and selective sites for electrochemical chlorine evolution2014In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 146, p. 733-740Article in journal (Refereed)
    Abstract [en]

    The electrocatalytic properties of the (1 1 0) surface of Ru-doped TiO2, Ti-doped RuO2 and the industrially important Dimensionally Stable Anode (DSA) composition Ru0.3Ti0.7O2 have been examined using density functional theory. It is found that the oxygen adsorption energy on a Ti site is strongly affected by the presence of small amounts of Ru dopant, whereas oxygen adsorption is relatively unaffected by Ti dopants in RuO2. The calculations also indicate that coordinatively unsaturated Ti sites on Ru-doped TiO2 and on Ru0.3Ti0.7O2 could form active and selective sites for Cl-2 evolution. These results suggest a reason for why DSA shows a higher chlorine selectivity than RuO2 and propose an experimental test of the hypothesis.

  • 42. 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.

  • 43.
    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.

  • 44. Kumar, P.
    et al.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Stanley, H. E.
    Possible origin of low-frequency excitations in supercooled bulk and protein-hydration waterManuscript (preprint) (Other academic)
  • 45. Kumar, Pradeep
    et al.
    Wikfeldt, Thor Kjartan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Stanley, H. Eugene
    The Boson peak in supercooled water2013In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, article id 1980Article in journal (Refereed)
    Abstract [en]

    We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line T-W. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

  • 46. 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.

  • 47.
    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.

  • 48.
    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.

  • 49.
    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)
  • 50.
    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, article id 084502Article 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.

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