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  • 1. Besharat, Zahra
    et al.
    Halldin Stenlid, Joakim
    Soldemo, Markus
    Marks, Kess
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Önsten, Anneli
    Johnson, Magnus
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Weissenrieder, Jonas
    Brinck, Tore
    Göthelid, Mats
    Dehydrogenation of methanol on Cu2O(100) and (111)2017Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 146, nr 24, artikel-id 244702Artikel i tidskrift (Refereegranskat)
    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.

  • 2. Beye, M.
    et al.
    Anniyev, T.
    Coffee, R.
    Dell'Angela, M.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katayama, T.
    Kaya, S.
    Krupin, O.
    Mogelhoj, A.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Norskov, J. K.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Schlotter, W. F.
    Sellberg, Jonas
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wolf, M.
    Wurth, W.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)2013Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, nr 18, artikel-id 186101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

  • 3. Beye, Martin
    et al.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Xin, Hongliang
    Dakovski, Georgi L.
    Dell'Angela, Martina
    Föhlisch, Alexander
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Hantschmann, Markus
    Hieke, Florian
    Kaya, Sarp
    Kuehn, Danilo
    LaRue, Jerry
    Mercurio, Giuseppe
    Minitti, Michael P.
    Mitra, Ankush
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Moeller, Stefan P.
    Ng, May Ling
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, United States.
    Nordlund, Dennis
    Norskov, Jens
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, Hirohito
    Persson, Mats
    Schlotter, William F.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. KTH Royal Institute of Technology, Sweden; SLAC National Accelerator Laboratory, United States.
    Wolf, Martin
    Abild-Pedersen, Frank
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Wurth, Wilfried
    Chemical Bond Activation Observed with an X-ray Laser2016Ingår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, nr 18, s. 3647-3651Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free electron laser to directly observe the decreased bonding antibonding splitting following bond-activation using an ultrashort optical laser pulse.

  • 4. Dell'Angela, M.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Katayama, T.
    Kaya, S.
    Krupin, O.
    LaRue, J.
    Mogelhoj, A.
    Nordlund, D.
    Norskov, J. K.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wolf, M.
    Wurth, W.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Real-Time Observation of Surface Bond Breaking with an X-ray Laser2013Ingår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 339, nr 6125, s. 1302-1305Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.

  • 5. Dell'Angela, M.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kaya, S.
    Katayama, T.
    Krupin, O.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SUNCAT Center for Interface Science and Catalysis, USA; SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SUNCAT Center for Interface Science and Catalysis, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Wolf, M.
    Wurth, W.
    Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer2015Ingår i: structural dynamics us, ISSN 2329-7778, Vol. 2, nr 2, artikel-id 025101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse.

  • 6.
    Gladh, Jörgen
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Hansson, Tony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)2013Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 615, s. 65-71Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3-4 higher desorption yield at comparable fluence when desorption is induced via 400 nm light, compared to 800 nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

  • 7.
    Gladh, Jörgen
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Li, Jibiao
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ljungberg, M. P.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Matsuda, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stanford Synchrotron Radiation Lightsource, USA.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    X-ray emission spectroscopy and density functional study of CO/Fe(100)2012Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, nr 3, artikel-id 034702Artikel i tidskrift (Refereegranskat)
    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.

  • 8.
    Gladh, Jörgen
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Detection of adsorbate overlayer structural transitions using sum-frequency generation spectroscopy2015Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 633, s. 77-81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 9. Katayama, T.
    et al.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Dell'Angela, M.
    Foehlisch, A.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kaya, S.
    Krupin, O.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Nordlund, D.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wurth, W.
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser2013Ingår i: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 187, s. 9-14Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface.

  • 10.
    LaRue, Jerry
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA; Chapman University, USA; Fritz-Haber Institute of the Max-Planck-Society, Germany.
    Krejčí, O.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Charles University in Prague, Czech Republic; Institute of Physics of the Czech Academy of Sciences, Czech Republic.
    Yu, L.
    Beye, M.
    Ng, M. L.
    Öberg, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Xin, H.
    Mercurio, G.
    Moeller, S.
    Turner, J. J.
    Nordlund, D.
    Coffee, R.
    Minitti, M. P.
    Wurth, W.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Abild-Pedersen, F.
    Ogasawara, H.
    Real-Time Elucidation of Catalytic Pathways in CO Hydrogenation on Ru2017Ingår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, nr 16, s. 3820-3825Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 11. LaRue, Jerry L.
    et al.
    Katayama, Tetsuo
    Lindenberg, Aaron
    Fisher, Alan S.
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Ogasawara, Hirohito
    THz-Pulse-Induced Selective Catalytic CO Oxidation on Ru2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, nr 3, artikel-id 036103Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 12.
    Marks, Kess
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Besharat, Zahra
    Soldemo, Markus
    Önsten, Anneli
    Weissenrieder, Jonas
    Halldin Stenlid, Joakim
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Göthelid, Mats
    Adsorption and decoposition of ethanol on Cu2O(111) and (100)2019Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, nr 33, s. 20384-20392Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ethanol dehydrogenation on metal oxides such as Cu2O is an important reaction for the production of renewable energy by fuel cells both via the production of H2 fuel and applied in direct alcohol fuel cells. To better understand this reaction we studied the adsorption, dissociation and desorption of ethanol on Cu2O(111) and (100) surfaces using high-resolution photoelectron spectroscopy (PES), vibrational sum frequency generation spectroscopy (SFG), and temperature programmed desorption (TPD) accompanied by density functional theory (DFT) calculations. On Cu2O(100) the first layer consists primarily of dissociatively adsorbed ethoxy. Second and third layers of ethanol physisorb at low temperature and desorb below 200 K. On the Cu2O(111) surface, adsorption is mixed as ethoxy, ethanol and the products following C-C cleavage, CHx and OCHx, are found in the first layer. Upon heating, products following both C-C and C-O bond breaking are observed on both surfaces and continued heating accentuates the molecular cracking. C-O cleavage occurs more on the (100) surface, whereas on the Cu2O(111) C-C cleavage dominates and occurs at lower temperatures than on the (100) surface. The increased ability of Cu2O(111) to crack ethanol is explained by the varied surface structure including both surface oxygen, electron rich O-vacancies and Cu.

  • 13.
    Marks, Kess
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Yazdi, Milad Ghadami
    Hansson, Tony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Engvall, Klas
    Harding, Dan J.
    Göthelid, Mats
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Adsorption and decomposition of naphthalene on oxygen pre-covered Ni(111)Manuskript (preprint) (Övrigt vetenskapligt)
  • 14.
    Marks, Kess
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Yazdi, Milad Ghadami
    Piskorz, Witold
    Simonov, Konstantin
    Stefanuik, Robert
    Sostina, Daria
    Guarnaccio, Ambra
    Ovsyannikov, Ruslan
    Giangrisostomi, Erika
    Sassa, Yasmine
    Bachellier, Nicolas
    Muntwiler, Matthias
    Johansson, Fredrik O. L.
    Lindblad, Andreas
    Hansson, Tony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kotarba, Andrzej
    Engvall, Klas
    Göthelid, Mats
    Harding, Dan J.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)2019Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, nr 24, artikel-id 244704Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H-2 gas, as well as the formation of carbidic and graphitic surface carbon. Published under license by AIP Publishing.

  • 15.
    Nilsson, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    LaRue, J.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Dell'Angela, M.
    Beye, M.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nørskov, J. K.
    Wurth, W.
    Abild-Pedersen, F.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Catalysis in real time using X-ray lasers2017Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 675, s. 145-173Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We describe how the unique temporal and spectral characteristics of X-ray free-electron lasers (XFEL) can be utilized to follow chemical transformations in heterogeneous catalysis in real time. We highlight the systematic study of CO oxidation on Ru(0001), which we initiate either using a femtosecond pulse from an optical laser or by activating only the oxygen atoms using a THz pulse. We find that CO is promoted into an entropy-controlled precursor state prior to desorbing when the surface is heated in the absence of oxygen, whereas in the presence of oxygen, CO desorbs directly into the gas phase. We monitor the activation of atomic oxygen explicitly by the reduced split between bonding and antibonding orbitals as the oxygen comes out of the strongly bound hollow position. Applying these novel XFEL techniques to the full oxidation reaction resulted in the surprising observation of a significant fraction of the reactants at the transition state through the electronic signature of the new bond formation.

  • 16.
    Nilsson, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nordlund, D.
    Waluyo, I.
    Huang, N.
    Ogasawara, H.
    Kaya, S.
    Bergmann, U.
    Naeslund, L. -A
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Wernet, Ph.
    Andersson, K. J.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Schiros, Theanne
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    X-ray absorption spectroscopy and X-ray Raman scattering of water and ice; an experimental view2010Ingår i: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 177, nr 03-feb, s. 99-129Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Here we present a review of X-ray absorption spectroscopy and X-ray Raman scattering with the perspective to understand the spectra of water including changes with temperature, mass of the water molecule and presence of monovalent ions. The different detection schemes are discussed and it is concluded that transmission X-ray absorption measurements, using a small area where the thickness is uniform, and X-ray Raman scattering give the most reliable spectra. Different model systems are discussed such as the surface and bulk of ice and various adsorbed monolayer structures on metal surfaces.

  • 17.
    Nilsson, Anders
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nordlund, Dennis
    Stanford.
    Waluyo, Ira
    Stanford.
    Huang, Ningdong
    Stanford.
    Ogasawara, Hirohito
    Stanford.
    Kaya, Sarp
    Stanford.
    Bergmann, Uwe
    Stanford.
    Näslund, Lars-Åke
    Stanford.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Wernet, Philippe
    Helmholtz-Zentrum Berlin, Germany.
    Andersson, Klas J.
    Haldor-Topsoe.
    Schiros, Theanne
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G.M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    X-ray Absorption Spectroscopy and X-ray Raman Scattering of Water : an Experimental View2010Ingår i: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 177, s. 99-129Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Here we present a review of X-ray absorption spectroscopy and X-ray Raman scattering with the perspective to understand the spectra of water including changes with temperature, mass of the water molecule and presence of monovalent ions. The different detection schemes are discussed and it is concluded that transmission X-ray absorption measurements, using a small area where the thickness is uniform, and X-ray Raman scattering give the most reliable spectra. Different model systems are discussed such as the surface and bulk of ice and various adsorbed monolayer structures on metal surfaces

  • 18.
    Schiros, Theanne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory .
    Andersson, K. J.
    MacNaughton, J.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Matsuda, Akitaka
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Nagoya University .
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Takahashi, O.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory .
    Ogasawara, H.
    Unique water-water coordination tailored by a metal surface2013Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, nr 23, s. 234708-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    At low coverage of water on Cu(110), substrate-mediated electrostatics lead to zigzagging chains along [001] as observed with STM [T. Yamada, S. Tamamori, H. Okuyama, and T. Aruga, Anisotropic water chain growth on Cu(110) observed with scanning tunneling microscopy Phys. Rev. Lett. 96, 036105 (2006)]. Using x-ray absorption spectroscopy we find an anomalous low-energy resonance at similar to 533.1 eV which, based on density functional theory spectrum simulations, we assign to an unexpected configuration of water units whose uncoordinated O-H bonds directly face those of their neighbors; this interaction repeats over trough sites with enhanced electron density and is analogous to the case of a hydrated electron.

  • 19.
    Schiros, Theanne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Haq, Sam
    Ogasawara, Hirohito
    Takahashi, Osamu
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Andersson, Klas
    Pettersson, Lars G.M.
    Hodgson, Andrew
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Structure of water adsorbed on the open Cu(110) surface: H-up, H-down, or both?2006Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 429, nr 4-6, s. 415-419Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigated the structure of the water monolayer on an open surface, Cu(1 1 0), at low temperature. We found that water adsorbs molecularly, adopting a 2:1 ratio of H-down and H-up configurations. This behavior of water on an open surface is quite different to the behavior on close-packed surfaces, such as Pt(1 1 1) and Ru(0 0 0 1), where water adsorbs primarily H-down, but can be understood on the basis of a range of different water adsorption sites across the observed (7 × 8) unit cell.

  • 20.
    Schiros, Theanne
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Takahashi, Osamu
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Andersson, Klas J.
    Haldor-Topsoe.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G.M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, Hirohito
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    The Role of Substrate Electrons in the Wetting of a Metal Surface2010Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 132, nr 9, s. 094701-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We address how the electronic and geometric structures of metal surfaces determine water-metal bonding by affecting the balance between Pauli repulsion and electrostatic attraction. We show how the rigid d-electrons and the softer s-electrons utilize different mechanisms for the redistribution of charge that enables surface wetting. On open d-shell Pt(111), the ligand field of water alters the distribution of metal d-electrons to reduce the repulsion. The closed-shell Cu d10 configuration of isostructural Cu(111), however, does not afford this mechanism, resulting in a hydrophobic surface and three-dimensional ice cluster formation. On the geometrically corrugated Cu(110) surface, however, charge depletion involving the mobile sp-electrons at atomic rows reduces the exchange repulsion sufficiently such that formation of a two-dimensional wetting layer is still favored in spite of the d10 electronic configuration

  • 21. Xin, H.
    et al.
    LaRue, J.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Beye, M.
    Dell'Angela, M.
    Turner, J. J.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ng, M. L.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kaya, S.
    Mercurio, G.
    Hieke, F.
    Nordlund, D.
    Schlotter, W. F.
    Dakovski, G. L.
    Minitti, M. P.
    Foehlisch, A.
    Wolf, M.
    Wurth, W.
    Ogasawara, H.
    Norskov, J. K.
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Abild-Pedersen, E.
    Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations2015Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, nr 15, artikel-id 156101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5 sigma and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

  • 22. Yazdi, Milad Ghadami
    et al.
    Moud, Pouya. H.
    Marks, Kess
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Piskorz, Witold
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Hansson, Tony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Kotarba, Andrzej
    Engvall, Klas
    Göthelid, Mats
    Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation, and Carbon Passivation2017Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, nr 40, s. 22199-22207Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit that it has been in use for a long time in fossil feedstock catalytic steam reforming, understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature-programmed desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may coexist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation, and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.

  • 23.
    Öberg, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Dell'Angela, M.
    Anniyev, T.
    Beye, M.
    Coffee, R.
    Föhlisch, A.
    Katayama, T.
    Kaya, S.
    LaRue, J.
    Mögelhöj, A.
    Nordlund, D.
    Ogasawara, H.
    Schlotter, W. F.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Sorgenfrei, F.
    Turner, J. J.
    Wolf, M.
    Wurth, W.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Nörskov, J. K.
    Pettersson, L. G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state2015Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 640, s. 80-88Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate.

  • 24.
    Öberg, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Marks, Kess
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Östrom, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Indication of non-thermal contribution to visible femtosecond laser-induced CO oxidation on Ru(0001)2015Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 143, nr 7, artikel-id 074701Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We studied CO oxidation on Ru(0001) induced by 400 nm and 800 nm femtosecond laser pulses where we find a branching ratio between CO oxidation and desorption of 1: 9 and 1: 31, respectively, showing higher selectivity towards CO oxidation for the shorter wavelength excitation. Activation energies computed with density functional theory show discrepancies with values extracted from the experiments, indicating both a mixture between different adsorbed phases and importance of non-adiabatic effects on the effective barrier for oxidation. We simulated the reactions using kinetic modeling based on the two-temperature model of laser-induced energy transfer in the substrate combined with a friction model for the coupling to adsorbate vibrations. This model gives an overall good agreement with experiment except for the substantial difference in yield ratio between CO oxidation and desorption at 400 nm and 800 nm. However, including also the initial, non-thermal effect of electrons transiently excited into antibonding states of the O-Ru bond yielded good agreement with all experimental results.

  • 25.
    Öberg, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G M
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    CO oxidation on Ru(0001) modeled from first-principles and femtosecond laser measurementsManuskript (preprint) (Övrigt vetenskapligt)
  • 26.
    Öberg, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nestsiarenka, Yuliya
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Matsuda, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Hansson, Tony
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, Lars G.M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öström, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface2012Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, nr 17, s. 9550-9560Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The kinetics of acetylene adsorption and cyclotrimerization was studied by vibrational sum-frequency generation spectroscopy (SFG) and density functional theory (DFT) calculations. At low temperature, SFG shows two resonances corresponding to acetylene adsorbed in two different sites. Upon heating, two new vibrational resonances appear. We interpret these resonances as being due to C2H2 island formation and adsorbed C4H4, which is the intermediate in the subsequent cyclotrimerization reaction to form benzene. A kinetic model is applied, which allows determination of the relevant activation barriers. The barrier for C2H2 diffusion is determined to be 43 +/- 1 kJ/mol. The activation barrier for formation of the C4H4 intermediate is found to be 84 +/- 6 kJ/mol and the barrier for benzene formation 5 +/- 3 kJ/mol lower. Barriers to diffusion and formation of C4H4 and C6H6 obtained from DFT calculations are in quantitative agreement with the experiments once the locally high coverage in C2H2 islands is included.

  • 27.
    Östrom, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Free University of Berlin .
    Krenz, M.
    Radu, I.
    Bovensiepen, U.
    Wolf, M.
    Frischkorn, C.
    Coupling of spin and vibrational degrees of freedom of adsorbates at metal surfaces probed by vibrational sum-frequency generation2013Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, nr 13, s. 132403-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vibrational spectroscopy using sum-frequency generation has been used to investigate the coupling between a ferromagnetic thin film and adsorbed molecules, here CO on Ni/Cu(100). The CO stretching vibration exhibits a strong magnetic contrast with a pronounced temperature dependence, underlining the high sensitivity of this adsorbate-specific spectroscopy method. Our results indicate that the strong temperature dependence is caused by dynamical changes in the surface chemical bond when the CO stretch vibration is coupled to thermally excited external vibrational modes.

  • 28.
    Östrom, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Öberg, Henrik
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Xin, H.
    Larue, J.
    Beye, M.
    Dell'Angela, M.
    Gladh, Jörgen
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ng, M. L.
    Sellberg, Jonas A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Kaya, S.
    Mercurio, G.
    Nordlund, D.
    Hantschmann, M.
    Hieke, F.
    Kuehn, D.
    Schlotter, W. F.
    Dakovski, G. L.
    Turner, J. J.
    Minitti, M. P.
    Mitra, A.
    Moeller, S. P.
    Foehlisch, A.
    Wolf, M.
    Wurth, W.
    Persson, M.
    Norskov, J. K.
    Abild-Pedersen, F.
    Ogasawara, H.
    Pettersson, Lars G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. SLAC National Accelerator Laboratory, USA.
    Probing the transition state region in catalytic CO oxidation on Ru2015Ingår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 347, nr 6225, s. 978-982Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Femtosecond x-ray laser pulses are used to probe the carbon monoxide (CO) oxidation reaction on ruthenium (Ru) initiated by an optical laser pulse. On a time scale of a few hundred femtoseconds, the optical laser pulse excites motions of CO and oxygen (O) on the surface, allowing the reactants to collide, and, with a transient close to a picosecond (ps), new electronic states appear in the OK-edge x-ray absorption spectrum. Density functional theory calculations indicate that these result from changes in the adsorption site and bond formation between CO and O with a distribution of OC-O bond lengths close to the transition state (TS). After 1 ps, 10% of the CO populate the TS region, which is consistent with predictions based on a quantum oscillator model.

  • 29.
    Öström, H.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Triguero, L.
    KTH Syd, Campus Haninge.
    Weiss, K.
    Department of Physics, Uppsala University.
    Ogasawara, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Garnier, M. G.
    Institut für Physik, Universität Basel.
    Nordlund, D.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nyberg, M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, L. G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Orbital rehybridization in n-octane adsorbed on Cu(110)2003Ingår i: Journal of Chemical Physics, ISSN 0021-9606, Vol. 118, nr 8, s. 3782-3789Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have investigated the local electronic structure of n-octane adsorbed on the Cu(110) surface using symmetry-resolved x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) in combination with density functional theory (DFT) spectrum calculations. We found new adsorption-induced states in the XE spectra, which we assign to interaction between the bonding CH orbitals and the metal surface. By performing a systematic investigation of the influence of different structural parameters on the XA and XE spectra, we conclude that the molecular geometry is significantly distorted relative to the gas-phase structure. The bonding to the surface leads to a strengthening of the carbon–carbon bonds and a weakening of the carbon–hydrogen bonds, consistent with a rehybridization of the carbons from sp3 to sp2.8. ©2003 American Institute of Physics.

  • 30.
    Öström, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Föhlisch, A.
    Institut für Experimentalphysik, Universität Hamburg.
    Nyberg, M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Weinelt, M.
    Lehrstuhl für Festkörperphysik, Universität Erlangen-Nürnberg.
    Heske, C.
    Department of Chemistry, University of Nevada.
    Pettersson, L. G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ethylene on Cu(110) and Ni(110): Electronic structure and bonding derived from x-ray spectroscopy and theory2004Ingår i: Surface Science, ISSN 0039-6028, Vol. 559, nr 2-3, s. 85-93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The bonding of ethylene to Cu(1 1 0) and Ni(1 1 0) is analyzed in detail using symmetry-resolved X-ray absorption (XAS) and emission (XES) spectroscopies in conjunction with density functional theory (DFT) calculations of geometric structure and spectra. XES, which probes the occupied valence states, reveals the formation of bonding and non-bonding orbitals of π-3d as well as π*-3d character. Additional mixing of σ and π states indicates rehybridization upon adsorption. The anti-bonding π-3d and π*-3d combinations are unoccupied and seen in XAS. A lower intensity of the π* transition for Ni is evidence of larger π* occupancy upon bonding. The position of the σ* shape-resonance indicates a 0.02 Å longer C–C bond on Ni than on Cu, in good agreement with the DFT structure optimizations. The XE spectra are well-reproduced both by specific spectrum calculations based on cluster models and by the carbon p-density of states calculated using periodic boundary conditions. The contribution of both π and π* levels to the new, surface-induced occupied states close to the Fermi-level lends support to the traditional Dewar–Chatt–Duncanson picture of the bonding. Theoretical charge-density difference plots support an alternative view of ethylene bonding in terms of the specific involvement of the excited molecular triplet state. Based on the variation in XE intensities the main difference between ethylene bonding to Cu and Ni is found to be an about two times larger occupancy of the π* orbital upon chemisorption on the transition metal, which comes along with C–C bond elongation and stronger σ–π rehybridization.

  • 31.
    Öström, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nordlund, D.
    Ogasawara, H.
    Stanford Synchrotron Radiation Laboratory,.
    Weiss, K.
    Department of Physics, Uppsala University.
    Triguero, L.
    KTH Syd, Campus Haninge.
    Pettersson, L. G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Geometric structure and chemical bonding of acetylene adsorbed on Cu(110)2004Ingår i: Surface Science, ISSN 0039-6028, Vol. 565, nr 2-3, s. 206-222Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The chemical bonding and geometric structure of acetylene adsorbed on Cu(1 1 0) is analyzed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) in combination with density functional theory (DFT) total energy geometry optimizations and spectral calculations. XPS reveals two different adsorbed species at liquid nitrogen temperature. The molecular alignment is deduced from angle-resolved XAS, revealing that in one site the molecules are aligned with the C–C axis along the [0 0 1] direction and in the other site with an average angle of 35° to the Cu rows. The position of the shape resonance is used to deduce a C–C bond length of 1.35 Å, which is close to the values obtained from the DFT geometry optimizations. XES reveals strong σ–π mixing and new occupied states close to the Fermi level, originating from the out-of-plane π* orbital, which becomes occupied upon adsorption in agreement with the Dewar–Chatt–Duncanson model of the bonding.

  • 32.
    Öström, Henrik
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Triguero, L.
    KTH Syd, Campus Haninge.
    Nyberg, M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Ogasawara, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Pettersson, L. G. M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Nilsson, A.
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Bonding of Saturated Hydrocarbons to Metal Surfaces2003Ingår i: Physical Review Letters, ISSN 0031-9007, Vol. 91, nr 4, s. 046102-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The adsorption of octane on Cu(110) was studied by x-ray absorption and x-ray emission spectroscopy, in combination with spectrum calculations in the framework of density functional theory, as a model system for alkane adsorption on transition metals. Significant electron sharing between the adsorbate and metal surface and involvement of both bonding and antibonding C-H molecular orbitals in the molecule-metal bond was found. The calculations were extended to the case of octane adsorbed on Ni(110), and the position of the metal d band was found to be important for the bonding. The results were generalized to show that this is important for the efficiency as an alkane dehydrogenation catalyst.

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