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Theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and surface adsorbates
Stockholm University, Faculty of Science, Department of Physics. (Kvantkemi)
2010 (English)Doctoral 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).

Abstract [sv]

Den här avhandlingen presenterar resultat av teoretisk modellering av röntgen- och vibrationella spektroskopier applicerade på flytande vatten och på CO adsorberat på en Ni(100) -yta. Reverse Monte Carlo-metoden används till att söka efter vattenstrukturer som reproducerar diffraktion, IR/Raman, röntgenabsorption och emission genom att anpassa strukturerna till experimentella data samt till pålagda restriktionsvillkor. Vissa av strukturerna är skapade så att de har en stor andel brutna vätebindningar eftersom nya röntgenabsorptions- och emissionsexperiment har setts stödja förekomsten av sådana strukturer. I anpassningsprocessen används en metod för att snabbt beräkna IR/Raman-spektrum för en isolerad OH-stretch, där frekvensen representeras av det elektriska fältet projicerat i stretch-koordinatens riktning.Vi utvärderar kritiskt denna metod genom att jämföra den med kvantkemiska klusterberäkningar. Vidare undersöks vattens röntgenemissionsspektrum, vars modellering kompliceras av nödvändigheten att inkludera vibrationella effekter i spektrumberäkningarna på grund av ett dissociativt intermediärt tillstånd. Baserat på Kramers-Heisenbergformeln utvecklas en ny semiklassisk metod som inkluderar vibrationella effekter. Metoden visar sig fungera mycket väl för ett endimensionellt testsystem. Dessutom implementerar vi röntgenabsorption och emission i en periodisk Täthetsfunktionalteorikod som vi sedan applicerar  på is och på ett ytadsorbatsystem: CO på Ni(100).

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2010. , 61 p.
Keyword [en]
water, XAS, XES, IR, Raman
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-38868ISBN: 978-91-7447-096-3 (print)OAI: oai:DiVA.org:su-38868DiVA: diva2:318103
Public defence
2010-06-04, sal FR4, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.Available from: 2010-05-11 Created: 2010-05-03 Last updated: 2011-11-14Bibliographically approved
List of papers
1. Are Recent Water Models Obtained by Fitting Diffraction Data Consistent with IR/Raman and X-ray Absorption Spectra?
Open this publication in new window or tab >>Are Recent Water Models Obtained by Fitting Diffraction Data Consistent with IR/Raman and X-ray Absorption Spectra?
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2006 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, 244510- p.Article in journal (Refereed) Published
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

Identifiers
urn:nbn:se:su:diva-12384 (URN)10.1063/1.2408419 (DOI)
Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2010-05-07Bibliographically approved
2. Diffraction and IR/Raman Data do not Prove Tetrahedral Water
Open this publication in new window or tab >>Diffraction and IR/Raman Data do not Prove Tetrahedral Water
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2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 129, no 8, 084502- p.Article in journal (Refereed) Published
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

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-14617 (URN)10.1063/1.2968550 (DOI)000259008900022 ()
Available from: 2008-12-05 Created: 2008-12-05 Last updated: 2011-04-26Bibliographically approved
3. Assessing the electric-field approximation to IR and Raman spectra of dilute HOD in D2O
Open this publication in new window or tab >>Assessing the electric-field approximation to IR and Raman spectra of dilute HOD in D2O
2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, 034501- p.Article in journal (Refereed) Published
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).

Identifiers
urn:nbn:se:su:diva-31948 (URN)10.1063/1.3154144 (DOI)000268206800035 ()
Available from: 2009-12-01 Created: 2009-12-01 Last updated: 2010-05-07Bibliographically approved
4. A simple picture of x-ray emission of condensed phase water
Open this publication in new window or tab >>A simple picture of x-ray emission of condensed phase water
(English)Manuscript (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.

Identifiers
urn:nbn:se:su:diva-39054 (URN)
Available from: 2010-05-06 Created: 2010-05-06 Last updated: 2010-05-07Bibliographically approved
5. Semi-classical description of nuclear dynamics in x-ray emission of water
Open this publication in new window or tab >>Semi-classical description of nuclear dynamics in x-ray emission of water
2010 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 24, 245115- p.Article in journal (Refereed) Published
Abstract [en]

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

National Category
Other Chemistry Topics
Research subject
Quantum Chemistry
Identifiers
urn:nbn:se:su:diva-47120 (URN)10.1103/PhysRevB.82.245115 (DOI)
Available from: 2010-11-29 Created: 2010-11-29 Last updated: 2011-11-14Bibliographically approved
6. An implementation of core level spectroscopies in a real space Projector Augmented Wave code
Open this publication in new window or tab >>An implementation of core level spectroscopies in a real space Projector Augmented Wave code
(English)Manuscript (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.

Identifiers
urn:nbn:se:su:diva-39056 (URN)
Available from: 2010-05-06 Created: 2010-05-06 Last updated: 2010-05-07Bibliographically approved
7. Theoretical Approximations to X-ray Absorption Spectroscopy of Liquid Water and Ice
Open this publication in new window or tab >>Theoretical Approximations to X-ray Absorption Spectroscopy of Liquid Water and Ice
Show others...
2010 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, Vol. 177, no 2-3, 135-157 p.Article in journal (Refereed) Published
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.

Keyword
X-ray absorption spectroscopy; DFT; NEXAFS; Water structure; Transition-potential
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-31447 (URN)10.1016/j.elspec.2010.02.004 (DOI)000277949100007 ()
Available from: 2009-11-13 Created: 2009-11-13 Last updated: 2011-11-22Bibliographically approved

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