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  • 1.
    Agosta, Lorenzo
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Brandt, Erik G.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Lyubartsev, Alexander P.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Diffusion and reaction pathways of water near fully hydrated TiO2 surfaces from ab initio molecular dynamics2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 2, article id 024704Article in journal (Refereed)
    Abstract [en]

    Ab initio molecular dynamics simulations are reported forwater-embedded TiO2 surfaces to determine the diffusive and reactive behavior at full hydration. A three-domain model is developed for six surfaces [rutile (110), (100), and (001), and anatase (101), (100), and (001)] which describes waters as hard (irreversibly bound to the surface), soft (with reduced mobility but orientation freedom near the surface), or bulk. The model explains previous experimental data and provides a detailed picture of water diffusion near TiO2 surfaces. Water reactivity is analyzed with a graph-theoretic approach that reveals a number of reaction pathways on TiO2 which occur at full hydration, in addition to direct water splitting. Hydronium (H3O+) is identified to be a key intermediate state, which facilitates water dissociation by proton hopping between intact and dissociated waters near the surfaces. These discoveries significantly improve the understanding of nanoscale water dynamics and reactivity at TiO2 interfaces under ambient conditions.

  • 2. Andersson, E.
    et al.
    Niskanen, J.
    Hedin, L.
    Eland, J. H. D.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Karlsson, L.
    Rubensson, J. -E
    Carravetta, V.
    Agren, H.
    Feifel, R.
    Core-valence double photoionization of the CS2 molecule2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 9, p. 94305-Article in journal (Refereed)
    Abstract [en]

    Double photoionization spectra of the CS2 molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies h nu=220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a (2p(3/2))(-1)(2 pi(g))(-1) double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a (C 1s)(-1)(2 pi(g))(-1) double vacancy. (C-) 2010 American Institute of Physics. [doi: 10.1063/1.3469812]

  • 3. Behringer, Hans
    et al.
    Eichhorn, Ralf
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Brownian dynamics simulations with hard-body interactions: Spherical particles2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 16, p. 164108-Article in journal (Refereed)
    Abstract [en]

    A novel approach to account for hard-body interactions in (overdamped) Brownian dynamics simulations is proposed for systems with non-vanishing force fields. The scheme exploits the analytically known transition probability for a Brownian particle on a one-dimensional half-line. The motion of a Brownian particle is decomposed into a component that is affected by hard-body interactions and into components that are unaffected. The hard-body interactions are incorporated by replacing the affected component of motion by the evolution on a half-line. It is discussed under which circumstances this approach is justified. In particular, the algorithm is developed and formulated for systems with space-fixed obstacles and for systems comprising spherical particles. The validity and justification of the algorithm is investigated numerically by looking at exemplary model systems of soft matter, namely at colloids in flow fields and at protein interactions. Furthermore, a thorough discussion of properties of other heuristic algorithms is carried out.

  • 4.
    Ben-Naim, Arieh
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    The Kirkwood-Buff integrals for one-component liquids2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 23, p. 234501-Article in journal (Refereed)
    Abstract [en]

    The Kirkwood-Buff integrals (KBIs) for one-component systems are calculated from either the pair correlation functions or from experimental macroscopic quantities. As in the case of mixtures, the KBIs provide important information on the local densities around a molecule. In the low density limit (rho -> 0) one can extract from the KBI some information on the strength of the intermolecular forces. No such information may be extracted from the KBIs at higher densities. We used experimental data on densities and isothermal compressibilities to calculate the KBIs for various liquids ranging from inert molecules, to hydrocarbons, alcohols, and liquid water.

  • 5. Besharat, Zahra
    et al.
    Halldin Stenlid, Joakim
    Soldemo, Markus
    Marks, Kess
    Stockholm University, Faculty of Science, Department of Physics.
    Önsten, Anneli
    Johnson, Magnus
    Öström, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Weissenrieder, Jonas
    Brinck, Tore
    Göthelid, Mats
    Dehydrogenation of methanol on Cu2O(100) and (111)2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 24, article id 244702Article in journal (Refereed)
    Abstract [en]

    Adsorption and desorption of methanol on the (111) and (100) surfaces of Cu2O have been studied using high-resolution photoelectron spectroscopy in the temperature range 120-620 K, in combination with density functional theory calculations and sum frequency generation spectroscopy. The bare (100) surface exhibits a (3,0; 1,1) reconstruction but restructures during the adsorption process into a Cu-dimer geometry stabilized by methoxy and hydrogen binding in Cu-bridge sites. During the restructuring process, oxygen atoms from the bulk that can host hydrogen appear on the surface. Heating transforms methoxy to formaldehyde, but further dehydrogenation is limited by the stability of the surface and the limited access to surface oxygen. The (root 3 x root 3)R30 degrees-reconstructed (111) surface is based on ordered surface oxygen and copper ions and vacancies, which offers a palette of adsorption and reaction sites. Already at 140 K, a mixed layer of methoxy, formaldehyde, and CHxOy is formed. Heating to room temperature leaves OCH and CHx. Thus both CH-bond breaking and CO-scission are active on this surface at low temperature. The higher ability to dehydrogenate methanol on (111) compared to (100) is explained by the multitude of adsorption sites and, in particular, the availability of surface oxygen.

  • 6. Carrascosa, E.
    et al.
    Bawart, M.
    Stei, M.
    Lindén, Fredrik
    Stockholm University, Faculty of Science, Department of Physics.
    Carelli, F.
    Meyer, J.
    Geppert, Wolf D.
    Stockholm University, Faculty of Science, Department of Physics.
    Gianturco, F. A.
    Wester, R.
    Nucleophilic substitution with two reactive centers: The CN- + CH3I case2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, no 18, article id 184309Article in journal (Refereed)
    Abstract [en]

    The nucleophilic substitution reaction CN- + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle-and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.

  • 7.
    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.

  • 8. Chaudhuri, A
    et al.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Jones, R G
    Lee, T-L
    Detlefs, B
    Woodruff, D P
    The structure of the Au(111)/methylthiolate interface: new insights from near-edge x-ray absorption spectroscopy and x-ray standing waves.2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, no 12, p. 124708-Article in journal (Refereed)
    Abstract [en]

    The local structure of the Au(111)(√3x√3)R30 degrees-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular σ*S-C resonance but also into a σ* S-Au orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second σ* S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the σ* S-C resonance. The new NIXSW data show the S-C bond to be tilted by 61 degrees relative to the surface normal, with a preferred azimuthal orientation in <211>, corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data.

  • 9. 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.

  • 10.
    Chen, Tao
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gatchell, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Stockett, Mark H.
    Stockholm University, Faculty of Science, Department of Physics.
    Alexander, John D.
    Stockholm University, Faculty of Science, Department of Physics.
    Zhang, Y.
    Rousseau, P.
    Domaracka, A.
    Maclot, S.
    Delaunay, R.
    Adoui, L.
    Huber, B. A.
    Schlatholter, T.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Absolute fragmentation cross sections in atom-molecule collisions: Scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 22, article id 224306Article in journal (Refereed)
    Abstract [en]

    We present scaling laws for absolute cross sections for non-statistical fragmentation in collisions between Polycyclic Aromatic Hydrocarbons (PAH/PAH(+)) and hydrogen or helium atoms with kinetic energies ranging from 50 eV to 10 keV. Further, we calculate the total fragmentation cross sections (including statistical fragmentation) for 110 eV PAH/PAH(+) + He collisions, and show that they compare well with experimental results. We demonstrate that non-statistical fragmentation becomes dominant for large PAHs and that it yields highly reactive fragments forming strong covalent bonds with atoms (H and N) and molecules (C6H5). Thus nonstatistical fragmentation may be an effective initial step in the formation of, e. g., Polycyclic Aromatic Nitrogen Heterocycles (PANHs). This relates to recent discussions on the evolution of PAHNs in space and the reactivities of defect graphene structures.

  • 11.
    Chen, Tao
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gatchell, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Stockett, Mark H.
    Stockholm University, Faculty of Science, Department of Physics.
    Rudy, Delaunay
    Domaracka, Alicja
    Micelotta, Elisabetta R.
    Tielens, Alexander G. G. M.
    Rousseau, Patrick
    Adoui, Lamri
    Huber, Bernd A.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Formation of H2 from internally heated polycyclic aromatic hydrocarbons: Excitation energy dependence2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 14, article id 144305Article in journal (Refereed)
    Abstract [en]

    We have investigated the effectiveness of molecular hydrogen (H-2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H-2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H-2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H-2-formation rates.

  • 12.
    Danilo, Cecile
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Vallet, Valerie
    Flament, Jean-Pierre
    Wahlgren, Ulf
    Stockholm University, Faculty of Science, Department of Physics.
    Spin-orbit configuration interaction study of the electronic structure of the 5f(2) manifold of U4+ and the 5f manifold of U5+2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 15, p. 154310-Article in journal (Refereed)
    Abstract [en]

    The energy levels of the 5f configuration of U5+ and 5f(2) configuration of U4+ have been calculated in a dressed effective Hamiltonian relativistic spin-orbit configuration interaction framework. Electron correlation is treated in the scalar relativistic scheme with either the multistate multireference second-order multiconfigurational perturbation theory (MS-CASPT2) or with the multireference single and double configuration interaction (MRCI) and its size-extensive Davidson corrected variant. The CASPT2 method yields relative energies which are lower than those obtained with the MRCI method, the differences being the largest for the highest state S-1(0) of the 5f(2) manifold. Both valence correlation effects and spin-orbit polarization of the outer-core orbitals are shown to be important. The satisfactory agreement of the results with experiments and four-component correlated calculations illustrates the relevance of dressed spin-orbit configuration interaction methods for spectroscopy studies of heavy elements. (c) 2008 American Institute of Physics.

  • 13. Eland, J. H. D.
    et al.
    Andric, L.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Hedin, L.
    Plogmaker, S.
    Palaudoux, J.
    Penent, F.
    Lablanquie, P.
    Feifel, R.
    Triple ionization of CO(2) by valence and inner shell photoionization2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, no 13, p. 134309-Article in journal (Refereed)
    Abstract [en]

    Spectra of triply ionized CO(2) have been obtained from photoionization of the molecule using soft x-ray synchrotron light and an efficient multi-electron coincidence technique. Although all states of the CO(2)(+++) trication are unstable, the ionization energy for formation of molecular ions at a geometry similar to that of the neutral molecule is determined as 74 +/- 0.5 eV.

  • 14. Eland, J. H. D.
    et al.
    Hochlaf, M.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Andersson, E.
    Hedin, L.
    Feifel, R.
    Triple ionization spectra by coincidence measurements of double Auger decay: The case of OCS2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, no 1, p. 14311-Article in journal (Refereed)
    Abstract [en]

    By combining multiple electron coincidence detection with ionization by synchrotron radiation, we have obtained resolved spectra of the OCS3+ ion created through the double Auger effect. The form of the spectra depends critically on the identity of the atom bearing the initial hole. High and intermediate level electron structure calculations lead to an assignment of the resolved spectrum from ionization via the S 2p hole. From the analysis it appears that the double Auger effect from closed shell molecules favors formation of doublet states over quartet states. Molecular field effects in the double Auger effect are similar to those in the single Auger effect in linear molecules.

  • 15. Eland, J. H. D.
    et al.
    Rigby, C. F.
    Andersson, E.
    Palaudoux, J.
    Andric, L.
    Penent, F.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Hedin, L.
    Karlsson, L.
    Rubensson, J. -E
    Hikosaka, Y.
    Ito, K.
    Lablanquie, P.
    Feifel, R.
    Spectra of the triply charged ion CS23+ and selectivity in molecular Auger effects2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, no 10, p. 104311-Article in journal (Refereed)
    Abstract [en]

    Spectra of triply charged carbon disulphide have been obtained by measuring, in coincidence, all three electrons ejected in its formation by photoionization. Measurements of the CS23+ ion in coincidence with the three electrons identify the energy range where stable trications are formed. A sharp peak in this energy range is identified as the (2)Pi ground state at 53.1 +/- 0.1 eV, which is the lowest electronic state according to ab initio molecular orbital calculations. Triple ionization by the double Auger effect is provisionally divided, on the basis of the pattern of energy sharing between the two Auger electrons into contributions from direct and cascade Auger processes. The spectra from the direct double Auger effect via S 2p, S 2s, and C 1s hole states contain several resolved features and show selectivity based on the initial charge localization and on the identity of the initial state. Triple ionization spectra from single Auger decay of S 2p-based core-valence states CS22+ show retention of the valence holes in this Auger process. Related ion-electron coincidence measurements give the triple ionization yields and the breakdown patterns in triple photoionization at selected photon energies from 90 eV to above the inner shell edges.

  • 16.
    Elenius, Måns
    et al.
    Stockholm University, Faculty of Science, Numerical Analysis and Computer Science (NADA) (together with KTH).
    Dzugutov, Mikhail
    Dept. of Materials Science and Engineering, Royal Institute of Technology.
    Evidence for a liquid-solid critical point in a simple monatomic system2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 104502Article in journal (Refereed)
    Abstract [en]

    It is commonly believed that the transition line  separating a liquid and a solid cannot be interrupted by a  critical point. This opinion is based on the traditional  symmetry argument that an isotropic liquid cannot be  continuously transformed into a crystal with a discrete  rotational and translational symmetry. We present here a  molecular-dynamics simulation of a simple monatomic system  suggesting the existence of a liquid-solid spinodal terminating  at a critical point. We show that, in the critical region, the  isotropic liquid continuously transforms into a phase with a  mesoscopic order similar to that of the smectic liquid  crystals. We argue that the existence of both the spinodal and  the critical point can be explained by the close structural  proximity between the mesophase and the crystal. This indicates  a possibility of finding a similar thermodynamic behaviour in  gelating colloids, liquid crystals and polymers.

  • 17.
    Elshakre, M.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Storchi, L.
    Kloda, Tomasz
    Stockholm University, Faculty of Science, Department of Physics.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Heijkenskjold, F.
    Gengelbach, A.
    Karlsson, L.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Tarantelli, F.
    Feifel, R.
    A photoelectron and double photoionization study of the valence electronic structure of 1,4-bromofluorobenzene2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 18, p. 184302-Article in journal (Refereed)
    Abstract [en]

    Conventional photoelectron and time-of-flight photoelectron-photoelectron coincidence (TOF-PEPECO) spectra have been measured for the outer valence region of the 1,4-bromofluorobenzene molecule. The photoelectron spectra were recorded using Hela radiation from a resonance Source, and the TOF-PEPECO spectra were recorded using HeII alpha radiation from a pulsed resonance source. The former provide energies of the cationic states and the latter of the dicationic states. The spectra are adequately interpreted with the aid of accurate Green's function calculations, showing very significant correlation effects. The lowest double ionization energy is found at 23.45 eV associated with the (4b(1))X-2 (1)A(1) dicationic state.

  • 18.
    Forsberg, B. O.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Alexander, John D.
    Stockholm University, Faculty of Science, Department of Physics.
    Chen, Tao
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, A. T.
    Stockholm University, Faculty of Science, Department of Physics.
    Gatchell, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Ions interacting with planar aromatic molecules: Modeling electron transfer reactions2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 5, p. 054306-Article in journal (Refereed)
    Abstract [en]

    We present theoretical absolute charge exchange cross sections for multiply charged cations interacting with the Polycyclic Aromatic Hydrocarbon (PAH) molecules pyrene C14H10, coronene C24H12, or circumcoronene C54H18. These planar, nearly circular, PAHs are modelled as conducting, infinitely thin, and perfectly circular discs, which are randomly oriented with respect to straight line ion trajectories. We present the analytical solution for the potential energy surface experienced by an electron in the field of such a charged disc and a point-charge at an arbitrary position. The location and height of the corresponding potential energy barrier from this simple model are in close agreement with those from much more computationally demanding Density Functional Theory (DFT) calculations in a number of test cases. The model results compare favourably with available experimental data on single-and multiple electron transfer reactions and we demonstrate that it is important to include the orientation dependent polarizabilities of the molecules (model discs) in particular for the larger PAHs. PAH ionization energy sequences from DFT are tabulated and used as model inputs. Absolute cross sections for the ionization of PAH molecules, and PAH ionization energies such as the ones presented here may be useful when considering the roles of PAHs and their ions in, e. g., interstellar chemistry, stellar atmospheres, and in related photoabsorption and photoemission spectroscopies.

  • 19. Fromager, Emmanuel
    et al.
    Réal, Florent
    Stockholm University, Faculty of Science, Department of Physics. Université Lille 1 (Sciences et Technologies), France.
    Wåhlin, Pernilla
    Stockholm University, Faculty of Science, Department of Physics.
    Wahlgren, Ulf
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Jensen, Hans Jørgen Aa.
    On the universality of the long-/short-range separation in multiconfigurational density-functional theory. II. Investigating f0 actinide species2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, article id 054107Article in journal (Refereed)
    Abstract [en]

    In a previous paper [ Fromager et al., J. Chem. Phys. 126, 074111 (2007) ], some of the authors proposed a recipe for choosing the optimal value of the μ parameter that controls the long-range/short-range separation of the two-electron interaction in hybrid multiconfigurational self-consistent field short-range density-functional theory (MC-srDFT) methods. For general modeling with MC-srDFT methods, it is clearly desirable that the same universal value of μ can be used for any molecule. Their calculations on neutral light element compounds all yielded μopt = 0.4 a.u. In this work the authors investigate the universality of this value by considering “extreme” study cases, namely, neutral and charged isoelectronic f0 actinide compounds (ThO2, PaO2+, UO22+, UN2, CUO, and NpO23+). We find for these compounds that μopt = 0.3 a.u. but show that 0.4 a.u. is still acceptable. This is a promising result in the investigation of a universal range separation. The accuracy of the currently best MC-srDFT (μ = 0.3 a.u.) approach has also been tested for equilibrium geometries. Though it performs as well as wave function theory and DFT for static-correlation-free systems, it fails in describing the neptunyl (VII) ion NpO23+ where static correlation is significant; bending is preferred at the MC-srDFT (μ = 0.3 a.u.) level, whereas the molecule is known to be linear. This clearly shows the need for better short-range functionals, especially for the description of the short-range exchange. It also suggests that the bending tendencies observed in DFT for NpO23+ cannot be fully explained by the bad description of static correlation effects by standard functionals. A better description of the exchange seems to be essential too.

  • 20.
    Fromager, Emmanuel
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Teichteil, Christian
    Maron, Laurent
    Atomic spin-orbit pseudopotential definition and its relation to the different relativistic approximations2005In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 123Article in journal (Refereed)
    Abstract [en]

    A critical analysis of usual shape-consistent spin-orbit pseudopotential extraction procedures is presented, considering the basic requirements of the atomic pseudopotentials. It is based on a perturbative analysis of both reference all-electron Dirac–Coulomb and pseudopotential calculations by means of the formalism developed by Lindgren and Morrisson. In the light of this analysis, we propose a new hybrid extraction of spin-orbit pseudopotentials, taking advantage of both shape-consistent and energy-consistent procedures. These new pseudopotentials are extracted and checked for the ground state of the halogens.

  • 21. Gadegaard, Ane Riis
    et al.
    Thogersen, Jan
    Jensen, Svend Knak
    Nielsen, Jakob Brun
    Jena, Naresh K.
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Jensen, Frank
    Keiding, Søren Rud
    Spectroscopy and picosecond dynamics of aqueous NO22014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 6, p. 064310-Article in journal (Refereed)
    Abstract [en]

    We investigate the formation of aqueous nitrogen dioxide, NO2 formed through femtosecond photolysis of nitrate, NO3- ( aq) and nitromethane CH3NO2(aq). Common to the experiments is the observation of a strong induced absorption at 1610 +/- 10 cm(-1), assigned to the asymmetric stretch vibration in the ground state of NO2. This assignment is substantiated through isotope experiments substituting N-14 by N-15, experiments at different pH values, and by theoretical calculations and simulations of NO2-D2O clusters.

  • 22.
    Geng, Ting
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics.
    Neville, Simon P.
    Hansson, Tony
    Stockholm University, Faculty of Science, Department of Physics.
    Thomas, Richard D.
    Stockholm University, Faculty of Science, Department of Physics.
    Dynamics in higher lying excited states: Valence to Rydberg transitions in the relaxation paths of pyrrole and methylated derivatives2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, no 14, article id 144307Article in journal (Refereed)
    Abstract [en]

    The involvement of intermediate Rydberg states in the relaxation dynamics of small organic molecules which, after excitation to the valence manifold, also return to the valence manifold is rarely observed. We report here that such a transiently populated Rydberg state may offer the possibility to modify the outcome of a photochemical reaction. In a time resolved photoelectron study on pyrrole and its methylated derivatives, N-methyl pyrrole and 2,5-dimethyl pyrrole, 6.2 eV photons (200 nm) are used to excite these molecules into a bright pi pi* state. In each case, a pi 3p-Rydberg state, either the B-1(pi 3p(y)) or the A(2)(pi 3p(z)) state, is populated within 20-50 fs after excitation. The wavepacket then proceeds to the lower lying A(2)(pi sigma*) state within a further 20 fs, at which point two competing reaction channels can be accessed: prompt N-H (N-CH3) bond cleavage or return to the ground state via a conical intersection accessed after ring puckering, the latter of which is predicted to require an additional 100-160 fs depending on the molecule.

  • 23. Ghassemi, Elham Nour
    et al.
    Larson, Jonas
    Stockholm University, Faculty of Science, Department of Physics. University of Cologne, Germany.
    Larson, Åsa
    Stockholm University, Faculty of Science, Department of Physics.
    A diabatic representation of the two lowest electronic states of Li-32014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 15, p. 154304-Article in journal (Refereed)
    Abstract [en]

    Using the Multi-Reference Configuration Interaction method, the adiabatic potential energy surfaces of Li-3 are computed. The two lowest electronic states are bound and exhibit a conical intersection. By fitting the calculated potential energy surfaces to the cubic E circle times epsilon Jahn-Teller model we extract the effective Jahn-Teller parameters corresponding to Li-3. These are used to set up the transformationmatrix which transforms from the adiabatic to a diabatic representation. This diabatization method gives a Hamiltonian for Li-3 which is free from singular non-adiabatic couplings and should be accurate for large internuclear distances, and it thereby allows for bound dynamics in the vicinity of the conical intersection to be explored.

  • 24.
    Giacomozzi, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kjaer, C.
    Langeland Knudsen, J.
    Andersen, L. H.
    Brøndsted Nielsen, S.
    Stockett, Mark H.
    Stockholm University, Faculty of Science, Department of Physics.
    Absorption and luminescence spectroscopy of mass-selected flavin adenine dinucleotide mono-anions2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 21, article id 214309Article in journal (Refereed)
    Abstract [en]

    We report the absorption profile of isolated Flavin Adenine Dinucleotide (FAD) mono-anions recorded using photo-induced dissociation action spectroscopy. In this charge state, one of the phosphoric acid groups is deprotonated and the chromophore itself is in its neutral oxidized state. These measurements cover the first four optical transitions of FAD with excitation energies from 2.3 to 6.0 eV (210-550 nm). The S-0 -> S-2 transition is strongly blue shifted relative to aqueous solution, supporting the view that this transition has a significant charge-transfer character. The remaining bands are close to their solution-phase positions. This confirms that the large discrepancy between quantum chemical calculations of vertical transition energies and solution-phase band maxima cannot be explained by solvent effects. We also report the luminescence spectrum of FAD mono-anions in vacuo. The gas-phase Stokes shift for S-1 is 3000 cm(-1), which is considerably larger than any previously reported for other molecular ions and consistent with a significant displacement of the ground and excited state potential energy surfaces. Consideration of the vibronic structure is thus essential for simulating the absorption and luminescence spectra of flavins.

  • 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. Glover, William J.
    et al.
    Mori, Toshifumi
    Schuurman, Michael S.
    Boguslavskiy, Andrey E.
    Schalk, Oliver
    Stockholm University, Faculty of Science, Department of Physics. National Research Council Canada, Canada.
    Stolow, Albert
    Martinez, Todd J.
    Excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene. II. Ab initio multiple spawning simulations2018In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 148, no 16, article id 164303Article in journal (Refereed)
    Abstract [en]

    The excited state non-adiabatic dynamics of the smallest polyene, trans 1,3-butadiene (BD), has long been the subject of controversy due to its strong coupling, ultrafast time scales and the difficulties that theory faces in describing the relevant electronic states in a balanced fashion. Here we apply Ab Initio Multiple Spawning (AIMS) using state-averaged complete active space multistate second order perturbation theory [SA-3-CAS(4/4)-MSPT2] which describes both static and dynamic electron correlation effects, providing a balanced description of both the initially prepared bright 1(1)B(u) (pi pi*) state and non-adiabatically coupled dark 2(1)A(g) state of BD. Importantly, AIMS allows for on-the-fly calculations of experimental observables. We validate our approach by directly simulating the time resolved photoelectron-photoion coincidence spectroscopy results presented in Paper I [A. E. Boguslavskiy et al., J. Chem. Phys. 148, 164302 (2018)], demonstrating excellent agreement with experiment. Our simulations reveal that the initial excitation to the 11Bu state rapidly evolves via wavepacket dynamics that follow both bright-and dark-state pathways as well as mixtures of these. In order to test the sensitivity of the AIMS results to the relative ordering of states, we considered two hypothetical scenarios biased toward either the bright B-1(u) or the dark 2(1)A(g) state. In contrast with AIMS/SA-3-CAS(4/4)-MSPT2 simulations, neither of these scenarios yields favorable agreement with experiment. Thus, we conclude that the excited state non-adiabatic dynamics in BD involves both of these ultrafast pathways.

  • 27.
    Haag, Nicole
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Holm, Anne I. S.
    Stockholm University, Faculty of Science, Department of Physics.
    Johansson, Henrik A. B.
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Brøndsted Nielsen, Steen
    Hvelplund, Preben
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Electron capture induced dissociation of doubly protonated pentapeptides: Dependence on molecular structure and charge separation2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 3, p. 035102-Article in journal (Refereed)
    Abstract [en]

    We have studied electron capture induced dissociation of a set of doubly protonated pentapeptides, all composed of one lysine (K) and either four glycine (G) or four alanine (A) residues, as a function of the sequence of these building blocks. Thereby the separation of the two charges, sequestered on the N-terminal amino group and the lysine side chain, is varied. The characteristic cleavage of N–Cα bonds is observed for all peptides over the whole backbone length, with the charge carrying fragments always containing K. The resulting fragmentation patterns are very similar if G is replaced by A. In the case of [XKXXX+2H]2+ (X=A or G), a distinct feature is observed in the distribution of backbone cleavage fragments and the probability for ammonia loss is drastically reduced. This may be due to an isomer with an amide oxygen as protonation site giving rise to the observed increase in breakage at a specific site in the molecule. For the other peptides, a correlation with the distance between amide oxygen and the charge at the lysine side chain has been found. This may be an indication that it is only the contribution from this site to the charge stabilization of the amide π* orbitals which determines relative fragment intensities. For comparison, complexes with two crown ether molecules have been studied as well. The crown ether provides a shielding of the charge and prevents the peptide from folding and internal hydrogen bonding, which leads to a more uniform fragmentation behavior.

  • 28. Hedin, L.
    et al.
    Tashiro, M.
    Linusson, Per
    Stockholm University, Faculty of Science, Department of Physics.
    Eland, J. H. D.
    Ehara, M.
    Ueda, K.
    Zhaunerchyk, V.
    Karlsson, L.
    Pernestal, K.
    Feifel, R.
    N1s and O1s double ionization of the NO and N2O molecules2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 4, p. 044309-Article in journal (Refereed)
    Abstract [en]

    Single-site N1s and O1s double core ionisation of the NO and N2O molecules has been studied using a magnetic bottle many-electron coincidence time-of-flight spectrometer at photon energies of 1100 eV and 1300 eV. The double core hole energies obtained for NO are 904.8 eV (N1s(-2)) and 1179.4 eV (O1s(-2)). The corresponding energies obtained for N2O are 896.9 eV (terminal N1s(-2)), 906.5 eV (central N1s(-2)), and 1174.1 eV (O1s(-2)). The ratio between the double and single ionisation energies are in all cases close or equal to 2.20. Large chemical shifts are observed in some cases which suggest that reorganisation of the electrons upon the double ionization is significant. Delta-self-consistent field and complete active space self-consistent field (CASSCF) calculations were performed for both molecules and they are in good agreement with these results. Auger spectra of N2O, associated with the decay of the terminal and central N1s(-2) as well as with the O1s(-2) dicationic states, were extracted showing the two electrons emitted as a result of filling the double core holes. The spectra, which are interpreted using CASSCF and complete active space configuration interaction calculations, show atomic-like character. The cross section ratio between double and single core hole creation was estimated as 1.6 x 10(-3) for nitrogen at 1100 eV and as 1.3 x 10(-3) for oxygen at 1300 eV.

  • 29.
    Holm, Anne I. S.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Johansson, Henrik A. B.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Dissociation and multiple ionization energies for five polycyclic aromatic hydrocarbon molecules2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 4, p. 044301-Article in journal (Refereed)
    Abstract [en]

    We have performed density functional theory calculations for a range of neutral, singly, and multiply charged polycyclic aromatic hydrocarbons (PAHs), and their fragmentation products for H-, H+-, C2H2-, and C2H2+-emissions. The adiabatic and vertical ionization energies follow linear dependencies as functions of charge state for all five intact PAHs (naphthalene, biphenylene, anthracene, pyrene, and coronene). First estimates of the total ionization and fragmentation cross sections in ion-PAH collisions display markedly different size dependencies for pericondensed and catacondensed PAH species, reflecting differences in their first ionization energies. The dissociation energies show that the PAHq+-molecules are thermodynamically stable for q <= 2 (naphthalene, biphenylene, and anthracene), q <= 3 (pyrene), and q <= 4 (coronene). PAHs in charge states above these limits may also survive experimental time scales due to the presence of reaction barriers as deduced from explorations of the potential energy surface regions for H+-emissions from all five PAHs and for C2H2+-emission from naphthalene - the smallest PAH.

  • 30. 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

  • 31. 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.

  • 32.
    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.

  • 33.
    Ivanov, Sergei
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Lyubartsev, Alexander
    Bead-Fourier path integral molecular dynamics for identical particles2005In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 123, p. 034105-Article in journal (Other academic)
    Abstract [en]

    The Bead-Fourier path integral molecular dynamics technique introduced earlier [ S. D. Ivanov, A. P. Lyubartsev, and A. Laaksonen, Phys. Rev. E 67 066710 (2003) ] is applied for simulation of electrons in the simplest molecules: molecular hydrogen, helium atom, and their ions. Special attention is paid to the correct description of electrons in the core region of a nucleus. In an attempt to smooth the Coulomb potential at small distances, a recipe is suggested. The simulation results are in excellent agreement with the analytical solution for the “harmonic helium atom”, as well as with the vibrational potential of the H2 molecule and He ionization energies. It is demonstrated, that the Bead-Fourier path integral molecular dynamics technique is able to provide the accuracy required for the description of electron structure and chemical bonds in cases when electron exchange effects need not be taken into account.

  • 34.
    Johansson, Adam Johannes
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Blomberg, Margareta R. A.
    Stockholm University, Faculty of Science, Department of Physics.
    Siegbahn, Per. E. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Quantifying the Effects of the Self-interaction Error in Density Functional Theory: When do the Delocalized States Appear? II. Iron-oxo Complexes and Closed-shell Substrate Molecules2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, p. 154301-Article in journal (Refereed)
    Abstract [en]

    Effects of the self-interaction error (SIE) in approximate density functional theory have several times been reported and quantified for the dissociation of charged radicals, charge transfer complexes, polarizabilities, and for transition states of reactions involving main-group molecules. In the present contribution, effects of the SIE in systems composed of a catalytic transition metal complex and a closed-shell substrate molecule are investigated. For this type of system, effects of the SIE have not been reported earlier. It is found that although the best density functionals (e.g., B3LYP) are capable of accurate predictions of structure, thermodynamics, and reactivity of such systems, there are situations and systems for which the magnitude of the SIE can be large, and for which the effects can be severe for the modeling of chemical reactivity. The largest energetic effect reported here is the artificial stabilization of a catalyst-substrate complex by as much as 18 kcal/mol. Also, the disappearance of significant energy barriers for hydrogen atom transfer in certain systems are reported. In line with earlier work, it is found that the magnitude of the SIE is related to the energetics of electron transfer between the metal catalyst and the substrate molecule. It is suggested that these problems might be circumvented by the inclusion of counterions or point charges that would alter the energetics of electron transfer. It is also pointed out that the effects of SIE in the modeling of transition metal reactivity need to be investigated further.

  • 35.
    Johansson, Anna C V
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Lindahl, Erik
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    The role of lipid composition for insertion and stabilization of amino acids in membranes2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, no 18, p. 185101-Article in journal (Refereed)
    Abstract [en]

    While most membrane protein helices are clearly hydrophobic, recent experiments have indicated that it is possible to insert marginally hydrophobic helices into bilayers and have suggested apparent in vivo free energies of insertion for charged residues that are low, e.g., a few kcals for arginine. In contrast, a number of biophysical simulation studies have predicted that the bilayer interior is close to a pure hydrophobic environment with large penalties for hydrophilic amino acids--and yet the experimental scales do significantly better at predicting actual membrane proteins from sequence. Here, we have systematically studied the dependence of the free energy profiles on lipid properties, including tail length, saturation, headgroup hydrogen bond strength, and charge, both to see to whether the in vivo insertion can be explained in whole or part from lipid composition of the endoplasmic reticulum (ER) membranes, and if the solvation properties can help interpret how protein function depends on the lipids. We find that lipid charge is important to stabilize charged amino acids inside the bilayer (with implications, e.g., for ion channels), that thicker bilayers have higher solvation costs for hydrophilic side chains, and that headgroup hydrogen bond strength determines how adaptive the lipids are as a hydrophobic/hydrophilic solvent. None of the different free energy profiles are even close to the low apparent in vivo insertion cost, which suggests that regardless of the specific ER membrane composition the current experimental results cannot be explained by normal lipid-type variation.

  • 36. Johansson, Erik M J
    et al.
    Hedlund, Maria
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Siegbahn, Hans
    Rensmo, Håkan
    Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: a photoelectron spectroscopy study.2007In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 126, no 24, p. 244303-Article in journal (Refereed)
    Abstract [en]

    The frontier electronic structures of Ru(tcterpy)(NCS)3 [black dye (BD)] and Ru(dcbpy)2(NCS)2 (N719) have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). N1s XAS has been used to probe the nitrogen contribution in the unoccupied density of states, and PES, together with RPES over the N1s edge, has been used to delineate the character of the occupied density of states. The experimental findings of the frontier electron structure are compared to calculations of the partial density of states for the nitrogens in the different ligands (NCS and terpyridine/bipyridine) and for Ru4d. The result indicates large similarities between the two complexes. Specifically, the valence level spectra show two well separated structures at low binding energy. The experimental results indicate that the outermost structure in the valence region largely has a Ru4d character but with a substantial character also from the NCS ligand. Interestingly, the second lowest structure also has a significant Ru4d character mixed into the structure otherwise dominated by NCS. Comparing the two complexes the BD valence structures lowest in binding energy contains a large contribution from the NCS ligands but almost no contribution from the terpyridine ligands, while for N719 also some contribution from the bipyridine ligands is mixed into the energy levels

  • 37.
    Johansson, Henrik A. B.
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Zettergren, Henning
    Stockholm University, Faculty of Science, Department of Physics.
    Holm, Anne I. S.
    Stockholm University, Faculty of Science, Department of Physics.
    Haag, Nicole
    Stockholm University, Faculty of Science, Department of Physics.
    Brøndsted Nielsen, S.
    Wyer, J. A.
    Kirketerp, M.-B. S.
    Støchkel, K.
    Hvelplund, P.
    Schmidt, Henning T.
    Stockholm University, Faculty of Science, Department of Physics.
    Cederquist, Henrik
    Stockholm University, Faculty of Science, Department of Physics.
    Unimolecular dissociation of anthracene and acridine cations: The importance of isomerization barriers for the C2H2 loss and HCN loss channels2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, p. 084304-Article in journal (Refereed)
    Abstract [en]

    The loss of C2H2 is a low activation energy dissociation channel for anthracene (C14H10) and acridine (C13H9N) cations. For the latter ion another prominent fragmentation pathway is the loss of HCN. We have studied these two dissociation channels by collision induced dissociation experiments of 50 keV anthracene cations and protonated acridine, both produced by electrospray ionization, in collisions with a neutral xenon target. In addition, we have carried out density functional theory calculations on possible reaction pathways for the loss of C2H2 and HCN. The mass spectra display features of multi-step processes, and for protonated acridine the dominant first step process is the loss of a hydrogen from the N site, which then leads to C2H2/HCN loss from the acridine cation. With our calculations we have identified three pathways for the loss of C2H2 from the anthracene cation, with three different cationic products: 2-ethynylnaphthalene, biphenylene, and acenaphthylene. The third product is the one with the overall lowest dissociation energy barrier. For the acridine cation our calculated pathway for the loss of C2H2 leads to the 3-ethynylquinoline cation, and the loss of HCN leads to the biphenylene cation. Isomerization plays an important role in the formation of the non-ethynyl containing products. All calculated fragmentation pathways should be accessible in the present experiment due to substantial energy deposition in the collisions.

  • 38.
    Khan, Shehryar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kubica-Misztal, Aleksandra
    Kruk, Danuta
    Kowalewski, Jozef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Systematic theoretical investigation of the zero-field splitting in Gd(III) complexes: Wave function and density functional approaches2015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 3, article id 034304Article in journal (Refereed)
    Abstract [en]

    The zero-field splitting (ZFS) of the electronic ground state in paramagnetic ions is a sensitive probe of the variations in the electronic and molecular structure with an impact on fields ranging from fundamental physical chemistry to medical applications. A detailed analysis of the ZFS in a series of symmetric Gd(III) complexes is presented in order to establish the applicability and accuracy of computational methods using multiconfigurational complete-active-space self-consistent field wave functions and of density functional theory calculations. The various computational schemes are then applied to larger complexes Gd(III)DOTA(H2O)(-), Gd(III)DTPA(H2O)(2-), and Gd(III)(H2O)(8)(3+) in order to analyze how the theoretical results compare to experimentally derived parameters. In contrast to approximations based on density functional theory, the multiconfigurational methods produce results for the ZFS of Gd(III) complexes on the correct order of magnitude.

  • 39.
    Khan, Shehryar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pollet, Rodolphe
    Vuilleumier, Rodolphe
    Kowalewski, Jozef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    An ab initio CASSCF study of zero field splitting fluctuations in the octet ground state of aqueous [Gd(iii)(HPDO3A)(H2O)]2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 24, article id 244306Article in journal (Refereed)
    Abstract [en]

    In this work, we present ab initio calculations of the zero-field splitting (ZFS) of a gadolinium complex [Gd(m)(HPDO3A)(H2O)] sampled from an ab initio molecular dynamics (AIMD) simulation. We perform both post-Hartree-Fock (complete active space self-consistent field-CASSCF) and density functional theory (DFT) calculations of the ZFS and compare and contrast the methods with experimental data. Two different density functional approximations (TPSS and LC-BLYP) were investigated. The magnitude of the ZFS from the CASSCF calculations is in good agreement with experiment, whereas the DFT results in varying degrees overestimate the magnitude of the ZFS for both functionals and exhibit a strong functional dependence. It was found in the sampling over the AIMD trajectory that the fluctuations in the transient ZFS tensor derived from DFT are not correlated with those of CASSCF nor does the magnitude of the ZFS from CASSCF and DFT correlate. From the fluctuations in the ZFS tensor, we extract a correlation time of the transient ZFS which is on the sub-picosecond time scale, showing a faster decay than experimental estimates.

  • 40. Kruk, D.
    et al.
    Hoffmann, S. K.
    Goslar, J.
    Lijewski, S.
    Kubica-Misztal, A.
    Korpala, A.
    Oglodek, I.
    Kowalewski, Josef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Roessler, E. A.
    Moscicki, J.
    ESR lineshape and H-1 spin-lattice relaxation dispersion in propylene glycol solutions of nitroxide radicals - Joint analysis2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 139, no 24, p. 244502-Article in journal (Refereed)
    Abstract [en]

    Electron Spin Resonance (ESR) spectroscopy and Nuclear Magnetic Relaxation Dispersion (NMRD) experiments are reported for propylene glycol solutions of the nitroxide radical: 4-oxo-TEMPO-d(16) containing N-15 and N-14 isotopes. The NMRD experiments refer to H-1 spin-lattice relaxation measurements in a broad frequency range (10 kHz-20 MHz). A joint analysis of the ESR and NMRD data is performed. The ESR lineshapes give access to the nitrogen hyperfine tensor components and the rotational correlation time of the paramagnetic molecule. The NMRD data are interpreted in terms of the theory of paramagnetic relaxation enhancement in solutions of nitroxide radicals, recently presented by Kruk et al. [J. Chem. Phys. 138, 124506 (2013)]. The theory includes the effect of the electron spin relaxation on the H-1 relaxation of the solvent. The H-1 relaxation is caused by dipole-dipole interactions between the electron spin of the radical and the proton spins of the solvent molecules. These interactions are modulated by three dynamic processes: relative translational dynamics of the involved molecules, molecular rotation, and electron spin relaxation. The sensitivity to rotation originates from the non-central positions of the interacting spin in the molecules. The electronic relaxation is assumed to stem from the electron spin-nitrogen spin hyperfine coupling, modulated by rotation of the radical molecule. For the interpretation of the NMRD data, we use the nitrogen hyperfine coupling tensor obtained from ESR and fit the other relevant parameters. The consistency of the unified analysis of ESR and NMRD, evaluated by the agreement between the rotational correlation times obtained from ESR and NMRD, respectively, and the agreement of the translation diffusion coefficients with literature values obtained for pure propylene glycol, is demonstrated to be satisfactory.

  • 41. Kruk, D.
    et al.
    Korpala, A.
    Kowalewski, Josef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Roessler, E. A.
    Moscicki, J.
    H-1 relaxation dispersion in solutions of nitroxide radicals: Effects of hyperfine interactions with N-14 and N-15 nuclei2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 4, p. 044512-Article in journal (Refereed)
    Abstract [en]

    H-1 relaxation dispersion of decalin and glycerol solutions of nitroxide radicals, 4-oxo-TEMPO-d(16)-N-15 and 4-oxo-TEMPO-d(16)-N-14 was measured in the frequency range of 10 kHz-20 MHz (for H-1) using STELAR Field Cycling spectrometer. The purpose of the studies is to reveal how the spin dynamics of the free electron of the nitroxide radical affects the proton spin relaxation of the solvent molecules, depending on dynamical properties of the solvent. Combining the results for both solvents, the range of translational diffusion coefficients, 10(-9)-10(-11) m(2)/s, was covered (these values refer to the relative diffusion of the solvent and solute molecules). The data were analyzed in terms of relaxation formulas including the isotropic part of the electron spin - nitrogen spin hyperfine coupling (for the case of N-14 and N-15) and therefore valid for an arbitrary magnetic field. The influence of the hyperfine coupling on H-1 relaxation of solvent molecules depending on frequency and time-scale of the translational dynamics was discussed in detail. Special attention was given to the effect of isotope substitution (N-14/N-15). In parallel, the influence of rotational dynamics on the inter-molecular (radical - solvent) electron spin - proton spin dipole-dipole coupling (which is the relaxation mechanism of solvent protons) was investigated. The rotational dynamics is of importance as the interacting spins are not placed in the molecular centers. It was demonstrated that the role of the isotropic hyperfine coupling increases for slower dynamics, but it is of importance already in the fast motion range (10(-9)m(2)/s). The isotope effects is small, however clearly visible; the H-1 relaxation rate for the case of N-15 is larger (in the range of lower frequencies) than for N-14. It was shown that when the diffusion coefficient decreases below 5 x 10(-11) m(2)/s electron spin relaxation becomes of importance and its role becomes progressively more significant when the dynamics slows done. As far as the influence of the rotational dynamics is concerned, it was show that this process is of importance not only in the range of higher frequencies (like for diamagnetic solutions) but also at low and intermediate frequencies.

  • 42. Kruk, D.
    et al.
    Korpala, A.
    Kubica, A.
    Kowalewski, Jozef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Roessler, E. A.
    Moscicki, J.
    H-1 relaxation dispersion in solutions of nitroxide radicals: Influence of electron spin relaxation2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 12, p. 124506-Article in journal (Refereed)
    Abstract [en]

    The work presents a theory of nuclear (H-1) spin-lattice relaxation dispersion for solutions of N-15 and N-14 radicals, including electron spin relaxation effects. The theory is a generalization of the approach presented by Kruk et al. [J. Chem. Phys. 137, 044512 (2012)]. The electron spin relaxation is attributed to the anisotropic part of the electron spin-nitrogen spin hyperfine interaction modulated by rotational dynamics of the paramagnetic molecule, and described by means of Redfield relaxation theory. The H-1 relaxation is caused by electron spin-proton spin dipole-dipole interactions which are modulated by relative translational motion of the solvent and solute molecules. The spectral density characterizing the translational dynamics is described by the force-free-hard-sphere model. The electronic relaxation influences the H-1 relaxation by contributing to the fluctuations of the inter-molecular dipolar interactions. The developed theory is tested against H-1 spin-lattice relaxation dispersion data for glycerol solutions of 4-oxo-TEMPO-d(16)-N-15 and 4-oxo-TEMPO-d(16)-N-14 covering the frequency range of 10 kHz-20 MHz. The studies are carried out as a function of temperature starting at 328 K and going down to 290 K. The theory gives a consistent overall interpretation of the experimental data for both N-14 and N-15 systems and explains the features of H-1 relaxation dispersion resulting from the electron spin relaxation.

  • 43. Kruk, D.
    et al.
    Kowalewski, Josef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Tipikin, S.
    Freed, J. H.
    Moscicki, M.
    Mielczarek, A.
    Port, M.
    Joint analysis of ESR lineshapes and (1)H NMRD profiles of DOTA-Gd derivatives by means of the slow motion theory2011In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 2, p. 024508-Article in journal (Refereed)
    Abstract [en]

    The Swedish slow motion theory [Nilsson and Kowalewski, J. Magn. Reson. 146, 345 (2000)] applied so far to Nuclear Magnetic Relaxation Dispersion (NMRD) profiles for solutions of transition metal ion complexes has been extended to ESR spectral analysis, including in addition g-tensor anisotropy effects. The extended theory has been applied to interpret in a consistent way (within one set of parameters) NMRD profiles and ESR spectra at 95 and 237 GHz for two Gd(III) complexes denoted as P760 and P792 (hydrophilic derivatives of DOTA-Gd, with molecular masses of 5.6 and 6.5 kDa, respectively). The goal is to verify the applicability of the commonly used pseudorotational model of the transient zero field splitting (ZFS). According to this model the transient ZFS is described by a tensor of a constant amplitude, defined in its own principal axes system, which changes its orientation with respect to the laboratory frame according to the isotropic diffusion equation with a characteristic time constant (correlation time) reflecting the time scale of the distortional motion. This unified interpretation of the ESR and NMRD leads to reasonable agreement with the experimental data, indicating that the pseudorotational model indeed captures the essential features of the electron spin dynamics.

  • 44. Kruk, D
    et al.
    Kowalewski, Jozef
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    General treatment of paramagnetic relaxation enhancement associated with translational diffusion2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 130, p. -174104Article in journal (Refereed)
    Abstract [en]

    A theory of nuclear spin relaxation in isotropic liquids for nuclear spins interacting with electron spins, residing in other molecules (the outer-sphere relaxation), is presented. The approach, valid outside of the Redfield limit for electron spin relaxation, is an extension of the Swedish slow motion theory [ Benetis et al., Mol. Phys. 48, 329 (1983) ; Nilsson and Kowalewski, J. Magn. Reson. 146, 345 (2000) ] for inner-sphere relaxation. It is demonstrated that the outer-sphere relaxation rate can be expressed as an integral of a product of a translational diffusion correlation function and a function analogous to the inner-sphere spectral density. A numerical implementation of the theory is described and applied to a large number of realistic parameter sets for S = 7/2 and S = 1, which may correspond to Gd(III) and Ni(II) systems. It is shown that the outer-sphere contribution is relevant and should be included into the analysis of nuclear magnetic relaxation dispersion relaxation profiles, especially for slow relative translational diffusion and fast molecular tumbling

  • 45. Kubica, A.
    et al.
    Kowalewski, Jozef
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kruk, D.
    Odelius, Michel
    Stockholm University, Faculty of Science, Department of Physics.
    Zero-field splitting in nickel(II) complexes: A comparison of DFT and multi-configurational wavefunction calculations2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 6, p. 064304-Article in journal (Refereed)
    Abstract [en]

    The zero-field splitting (ZFS) is an important quantity in the electron spin Hamiltonian for S = 1 or higher. We report calculations of the ZFS in some six- and five-coordinated nickel(II) complexes (S = 1), using different levels of theory within the framework of the ORCA program package [F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2, 73 (2012)]10.1002/wcms.81. We compare the high-end ab initio calculations (complete active space self-consistent field and n-electron valence state perturbation theory), making use of both the second-order perturbation theory and the quasi-degenerate perturbation approach, with density functional theory (DFT) methods using different functionals. The pattern of results obtained at the ab initio levels is quite consistent and in reasonable agreement with experimental data. The DFT methods used to calculate the ZFS give very strongly functional-dependent results and do not seem to function well for our systems.

  • 46.
    Larson, Åsa
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Fonseca dos Santos, Samantha
    Orel, Ann E.
    Dissociative recombination of HCl+2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 8, article id 084304Article in journal (Refereed)
    Abstract [en]

    The dissociative recombination of HCl+, including both the direct and indirect mechanisms, is studied. For the direct process, the relevant electronic states are calculated ab initio by combining electron scattering calculations to obtain resonance positions and autoionization widths with multi-reference configuration interaction calculations of the ion and Rydberg states. The cross section for the direct dissociation along electronic resonant states is computed by solution of the time-dependent Schrodinger equation. For the indirect process, an upper bound value for the cross section is obtained using a vibrational frame transformation of the elements of the scattering matrix at energies just above the ionization threshold. Vibrational excitations of the ionic core from the ground vibrational state, v = 0, to the first three excited vibrational states, v = 1, v = 2, and v = 3, are considered. Autoionization is neglected and the effect of the spin-orbit splitting of the ionic potential energy upon the indirect dissociative recombination cross section is considered. The calculated cross sections are compared to measurements. Published by AIP Publishing.

  • 47. Lechner, B. A. J.
    et al.
    de Wijn, A. S.
    Stockholm University, Faculty of Science, Department of Physics.
    Hedgeland, H.
    Jardine, A. P.
    Hinch, B. J.
    Allison, W.
    Ellis, J.
    Atomic scale friction of molecular adsorbates during diffusion2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 19, p. 194710-Article in journal (Refereed)
    Abstract [en]

    Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.

  • 48.
    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

  • 49.
    Leetmaa, Mikael
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Wikfeldt, Kjartan Thor
    Stockholm University, Faculty of Science, Department of Physics.
    Ljungberg, Mathias P.
    Stockholm University, Faculty of Science, Department of Physics.
    Odelius, Michael
    Stockholm University, Faculty of Science, Department of Physics.
    Swenson, Jan
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Pettersson, Lars G. M.
    Stockholm University, Faculty of Science, Department of Physics.
    Diffraction and IR/Raman Data do not Prove Tetrahedral Water2008In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 8, p. 084502-Article in journal (Refereed)
    Abstract [en]

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

  • 50. Lemke, Sonja
    et al.
    Handle, Philip H.
    Plaga, Lucie J.
    Stern, Josef N.
    Seidl, Markus
    Fuentes-Landete, Violeta
    Amann-Winkel, Katrin
    Stockholm University, Faculty of Science, Department of Physics. University of Innsbruck, Austria.
    Koester, Karsten W.
    Gainaru, Catalin
    Loerting, Thomas
    Boehmer, Roland
    Relaxation dynamics and transformation kinetics of deeply supercooled water: Temperature, pressure, doping, and proton/deuteron isotope effects2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 3, article id 034506Article in journal (Refereed)
    Abstract [en]

    Above its glass transition, the equilibrated high-density amorphous ice (HDA) transforms to the low-density pendant (LDA). The temperature dependence of the transformation is monitored at ambient pressure using dielectric spectroscopy and at elevated pressures using dilatometry. It is found that near the glass transition temperature of deuterated samples, the transformation kinetics is 300 times slower than the structural relaxation, while for protonated samples, the time scale separation is at least 30 000 and insensitive to doping. The kinetics of the HDA to LDA transformation lacks a proton/deuteron isotope effect, revealing that this process is dominated by the restructuring of the oxygen network. The x-ray diffraction experiments performed on samples at intermediate transition stages reflect a linear combination of the LDA and HDA patterns implying a macroscopic phase separation, instead of a local intermixing of the two amorphous states.

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