| 1. |
- Beye, M., et al.
(author)
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Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001)
- 2013
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:18
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Journal article (peer-reviewed)abstract
- 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.
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| 2. |
- Dell'Angela, M., et al.
(author)
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Real-Time Observation of Surface Bond Breaking with an X-ray Laser
- 2013
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1302-1305
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Gladh, Jörgen, et al.
(author)
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X-ray emission spectroscopy and density functional study of CO/Fe(100)
- 2012
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 136:3
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Öberg, Henrik, et al.
(author)
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Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface
- 2012
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:17, s. 9550-9560
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Gladh, Jörgen, 1964-
(author)
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Bonding and Desorption Mechanismsof CO on Metal Surfaces
- 2012
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Licentiate thesis (other academic/artistic)abstract
- I have investigated two different systems CO/Fe(100) and CO/Ru(0001), toobtain new information on the binding and desorption processes. The twodifferent systems have served as a model system, one for a static examination,CO on iron, and for the dynamic case, CO on ruthenium. To perform theseinvestigations, several types of techniques have been used such as, X-rayabsorption spectroscopy, X-ray emission spectroscopy, and femtosecond laserinduced desorption techniques such as two-pulse correlation.For the CO/Fe(100) system, we found that the on-top CO “1 phase” canbe described by the Blyholder-Nilsson-Pettersson model. The pre-dissociativephase of CO bound at hollow sites, “3 phase”, can be described in a Dewar-Chatt-Duncanson like picture.For the CO/Ru(0001) system, it was found that all our data could be fitted from an empirical friction heat bath model. Moreover, it turned out, thatthere is a strong frictional coupling to the substrate electrons and phonons.
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| 6. |
- Gladh, Jörgen, et al.
(author)
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Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)
- 2013
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In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 615, s. 65-71
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Journal article (peer-reviewed)abstract
- 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.
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| 7. |
- Nilsson, Anders, et al.
(author)
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X-ray absorption spectroscopy and X-ray Raman scattering of water and ice; an experimental view
- 2010
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In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 177:03-feb, s. 99-129
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Journal article (peer-reviewed)abstract
- 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.
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| 8. |
- Schiros, Theanne, et al.
(author)
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The Role of Substrate Electrons in the Wetting of a Metal Surface
- 2010
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 132:9, s. 094701-
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Journal article (peer-reviewed)abstract
- 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
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