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  • 1. 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.

  • 2. Enkovaara, J.
    et al.
    Rostgaard, C.
    Mortensen, J. J.
    Chen, J.
    Dulak, M.
    Ferrighi, L.
    Gavnholt, J.
    Glinsvad, C.
    Haikola, V.
    Hansen, H. A.
    Kristoffersen, H. H.
    Kuisma, M.
    Larsen, A. H.
    Lehtovaara, L.
    Ljungberg, Mathias
    Stockholm University, Faculty of Science, Department of Physics.
    Lopez-Acevedo, O.
    Moses, P. G.
    Ojanen, J.
    Olsen, T.
    Petzold, V.
    Romero, N. A.
    Stausholm-Moller, J.
    Strange, M.
    Tritsaris, G. A.
    Vanin, M.
    Walter, M.
    Hammer, B.
    Hakkinen, H.
    Madsen, G. K. H.
    Nieminen, R. M.
    Norskov, J. K.
    Puska, M.
    Rantala, T. T.
    Schiotz, J.
    Thygesen, K. S.
    Jacobsen, K. W.
    Electronic structure calculations with GPAW: a real-space implementation of the projector augmented-wave method2010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 25, p. 253202-Article, review/survey (Refereed)
    Abstract [en]

    Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between the two representations provides great flexibility. While DFT allows one to study ground state properties, time-dependent density-functional theory (TDDFT) provides access to the excited states. We have implemented the two common formulations of TDDFT, namely the linear-response and the time propagation schemes. Electron transport calculations under finite-bias conditions can be performed with GPAW using non-equilibrium Green functions and the localized basis set. In addition to the basic features of the real-space PAW method, we also describe the implementation of selected exchange-correlation functionals, parallelization schemes, Delta SCF-method, x-ray absorption spectra, and maximally localized Wannier orbitals.

  • 3. 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.

  • 4.
    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.

  • 5.
    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

  • 6.
    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.

  • 7.
    Ljungberg, Mathias
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Lyubartsev, Alexander
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Nilsson, Anders
    Stanford Synchrotron Radiation Lightsource.
    Pettersson, Lars
    Stockholm University, Faculty of Science, Department of Physics.
    Assessing the electric-field approximation to IR and Raman spectra of dilute HOD in D2O2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, p. 034501-Article in journal (Refereed)
    Abstract [en]

    We analyze the validity of the commonly used electric-field (E-field) approximation to vibrational OH stretch Raman spectra of dilute HOD in D2O by computing the OH stretch frequency of all molecules in several different structure models, each containing around 2000 molecules. The calculations are done at the B3LYP level using clusters containing 32 molecules centered around the molecule for which the frequencies are calculated; the large cluster size is required due to significant nonlocal contributions influencing the computed frequencies. The vibrational frequencies are determined using a six-point potential optimized discrete variable representation. Raman and infrared intensities are furthermore computed to generate the spectra. We find that a quadratic fit of E-field versus frequency gives a reasonable representation of the calculated distribution of frequencies. However, the mapping depends significantly on the structural model and is thus not universal. Anharmonic couplings are calculated for several optimized clusters showing a general trend to compress the computed frequency distributions, which is in agreement with dynamical simulations (motional narrowing).

  • 8.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and surface adsorbates2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis presents results of theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and to CO adsorbed on a Ni(100) surface. The Reverse Monte Carlo method is used to search for  water structures that reproduce diffraction, IR/Raman and x-ray absorption by fitting them to experimental data and imposed constraints. Some of the structures are created to have a large fraction of broken hydrogen bonds because recent x-ray absorption and emission studies have been seen to support the existence of such structures. In the fitting procedure a fast way of computing the IR/Raman spectrum for an isolated OH stretch is used, where the frequency is represented by the electric field projected in the direction of the stretch coordinate. This method is critically evaluated by comparing it to quantum chemical cluster calculations. Furthermore, the x-ray emission spectrum of water is investigated, the modeling of which is complicated by the necessity of including vibrational effects in the spectrum calculations due to a dissociative intermediate state. Based on the Kramers-Heisenberg formula a new semi-classical method is developed to include vibrational effects in x-ray emission calculations. The method is seen to work very well for a one-dimensional test system. Moreover, x-ray absorption and emission are implemented in a periodic Density Functional Theory code which is applied to ice and to the surface adsorbate system CO on Ni(100).

  • 9.
    Ljungberg, Mathias P.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Mortensen, Jens-Jørgen
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    An implementation of core level spectroscopies in a real space Projector Augmented Wave codeManuscript (preprint) (Other academic)
    Abstract [en]

    We describe the implementation of K-shell core level spectroscopies (x-ray absorption (XAS), x-ray emission (XES), x-ray photoemission (XPS)) in the real-space-grid-based Projector Augmented Wave (PAW) GPAW code. The implementation for XAS is based on the Haydock recursion method avoiding computation of unoccupied states. The absolute energy scale is computed with the Delta Kohn-Sham method which is possible using specific PAW setups for the core-hole states. We show computed spectra for selected test cases (gas phase H2O and bulk diamond) and discuss the dependence on grid spacing, box size and core hole occupation. We apply the method to XES and XAS of CO adsorbed on Ni(100) and compare to experimental data where possible.

  • 10.
    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.

  • 11.
    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.

  • 12.
    Wikfeldt, Kjartan Thor
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Leetmaa, Mikael
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias
    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.
    On the Range of Water Structure Models Compatible with X-ray and Neutron Diffraction Data2009In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 113, no 18, p. 6246-6255Article in journal (Refereed)
    Abstract [en]

    We use the reverse Monte Carlo (RMC) method to critically evaluate the structural information content of diffraction data on bulk water by fitting simultaneously or separately to X-ray and neutron data; the O-H and H-H, but not the O-O, pair-correlation functions (PCFs) are well-described by the neutron data alone. Enforcing at the same time different H-bonding constraints, we generate four topologically different structure models of liquid water, including a simple mixture model, that all equally well reproduce the diffraction data. Although earlier work [Leetmaa, M.; et al. J. Chem. Phys. 2008, 129, 084502] has focused on tetrahedrality in the H-bond network in liquid water, we show here that, even for the O-O-O three-body correlation, tetrahedrality is not strictly defined by the data. We analyze how well two popular MD models (TIP4P-pol2 and SPC/E) reproduce the neutron data in q-space and find differences in important aspects from the experiment. From the RMC fits, we obtain pair-correlation functions (PCFs) that are in optimal agreement with the diffraction data but still show a surprisingly strong variability both in position and height of the first intermolecular (H-bonding) O-H peak. We conclude that, although diffraction data impose important constraints on the range of possible water structures, additional data are needed to narrow the range of possible structure models.

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