| 1. |
- Chakarov, D V, et al.
(författare)
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Photos induced desorption and intercalation of potassium atoms deposited on graphite(0001)
- 1996
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Ingår i: Applied Surface Science. - 0169-4332 .- 1873-5584. ; 106, s. 186-192
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Tidskriftsartikel (refereegranskat)abstract
- In addition to the photodesorption phenomenon previously observed for single K atoms from a graphite surface covered with a monolayer of potassium we present new results related to the photon stimulated interaction of potassium with graphite, which concerns alternative routes for energy relaxation of the photo excited K adatoms: photoinduced intercalation. The desorption yield has a threshold at h omega approximate to 3 eV and a maximum at h omega(max) approximate to 4.9 eV, Polarization measurements indicate a substrate-mediated mechanism. The coverage dependence suggests that only the ionic 2D, K-phase is photo active. The proposed mechanism includes attachment of photo-generated hot electrons to the K 4s adsorbate resonance of energy E(res). Assuming an analogous excitation process we discuss different mechanisms for the K photo intercalation and possible applications of the photon stimulated doping of carboneous materials at low temperature.
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| 2. |
- CHAKAROV, DV, et al.
(författare)
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WATER-ADSORPTION AND COADSORPTION WITH POTASSIUM ON GRAPHITE(0001)
- 1995
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 11:4, s. 1201-1214
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Tidskriftsartikel (refereegranskat)abstract
- Water and water coadsorbed with potassium on the basal plane of graphite were studied with thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS) in the temperature range 85-900 K. Water alone adsorbs nondissociatively on the clean graphite surface at 85 K, forming hydrogen bonded aggregates. Its structure depends both on the coverage and on substrate temperature. With increasing coverage at 85 K(0.5-1.0 monolayer (ML)) the libration mode at similar to 86 meV shows a rapid upward shift, indicating a phase transition from a 2D to a 3D structure. The transition can also be induced by annealing the low coverage structure. Water coadsorption with potassium is nonreactive or reactive, depending on temperature and potassium coverage. The nonreactive coadsorption at T-s = 85 K occurs only below a critical potassium coverage of BK less than or equal to 0.3 ML. It is characterized by substantial symmetry changes of the adsorbed water molecules, compared to the pure water adsorption, and is attributed to formation of hydrated-ion species on the surface. The surface solvation number at the lowest K coverage is three to four H2O molecules per potassium atom. K and H2O react at submonolayer coverages at 120-160 K to form surface KOH, KH, KxOy, and volatile products. The surface species gradually transforms/decomposes at elevated temperatures (200-500 K) to first form potassium-oxygen complexes that then serve as precursors to graphite oxidation to CO2 at similar to 750 K.
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| 3. |
- CHAKAROV, DV, et al.
(författare)
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WATER-ADSORPTION ON GRAPHITE(0001)
- 1995
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Ingår i: Vacuum. - 0042-207X .- 1879-2715. ; 46:8-10, s. 1109-1112
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Tidskriftsartikel (refereegranskat)abstract
- Wafer adsorption on the clean graphite (0001) surface has been studied by high-resolution electron-energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). At 85 K H2O adsorbs non-dissociatively forming hydrogen-bonded aggregates. The structure and the growth mode of water clusters depend on the substrate temperature and the coverage. At all coverages, above a few per cent of a monolayer (ML), the desorption is characterized by zero-order kinetics, while the HREEL spectra reveal a threshold coverage approximately 1 ML when the average co-ordination of the H2O molecules changes. Isothermal measurements of the desorption rate and HREELS measurements at elevated temperatures suggest an irreversible phase transition from amorphous to crystalline ice at approximately 135 K.
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| 4. |
- Hellsing, B, et al.
(författare)
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Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite
- 1997
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 106:3, s. 982-1002
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Tidskriftsartikel (refereegranskat)abstract
- We present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3-6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2 x 10(14) atoms cm(-2)), the dependence of the cross section on photon energy has a threshold at (h) over bar omega approximate to 3.0 eV and rises up to a maximum of 1.8 +/- 0.4 x 10(-20) cm(2) at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respectively. The observed photodesorption is a single-photon, nonthermal event, consistent with a substrate-mediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of the photoyield vs photon energy. Fitting the model parameters to the experimental data, we determine (i) the energy and slope of the excited state potential energy curve, and (ii) the position and width of the potassium-induced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K+ and K-0. The calculated cross sections for s- and p-polarized Light are in qualitative agreement with the measurements. (C) 1997 American Institute of Physics.
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| 5. |
- Hellsing, B, et al.
(författare)
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Photoinduced desorption of potassium atoms from graphite
- 1996
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 363:1-3, s. 247-251
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Tidskriftsartikel (refereegranskat)abstract
- We present an experimental and theoretical investigation of desorption of potassium atoms from the basal plane of graphite induced by photons with energies from 2 to 6 eV. The intensity of the photon flux employed in the measurements is low, and the observed photodesorption is a single-photon, non-thermal event. At monolayer coverage the photon-energy dependence of the cross section has a maximum at 4.8 eV. The experimental observations are interpreted in terms of a hot carrier mechanism, which involves attachment of optically excited substrate hot electrons to the empty 4s state of ionized potassium, and then desorption. The theory predicts a Gaussian line shape of the photoyield versus photon energy. Fitting the model parameters to the experimental data, we determine: (i) the potential energy for Gr + K+ and Gr + K-0; and (ii) the position (2.4 eV above the Fermi level) and width (0.15 eV) of the potassium 4s resonance, which is in good agreement with independent experimental observations.
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| 6. |
- Österlund, Lars, 1967-, et al.
(författare)
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Dissociative sticking of O-2 on Al(111)
- 1997
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 55:23, s. 15452-15455
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Tidskriftsartikel (refereegranskat)abstract
- The dissociative sticking probability, S-0, of O-2 on Al(111) has been measured as a function of incident translational and vibrational energy. S-0 rises from approximate to 10(-2) for energies approximate to 30 meV to near unity in the range 0.6-2.0 eV, demonstrating an activated dissociation event. Vibrational excitation enhances the sticking. Surface temperature has no effect on S-0. A peculiar dependence on polar angle is observed. The results are used to discuss possible mechanisms for the recent observation [H. Brune et al. Phys. Rev. Lett. 68, 624 (1992)] of widely separated O atoms after O-2 dissociation on Al(111).
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| 7. |
- Österlund, Lars, 1967
(författare)
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Elementary Surface Processes on Graphite and Aluminium
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Molecular processes on solid surfaces are of fundamental and technological importance. This work focuses on various reactive processes on a carbon (graphite(0001)) surface, and on the initial oxygen interaction with an aluminium (Al(111)) surface. In the former case, the interaction of pure K and H2O overlayers, and coadsorbed K+H2O overlayers have been addressed, while in the latter case the energy dependent dissociation probability of O2 has been investigated. The methods employed in these studies include high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), work function mea-surements, and molecular beam techniques. At low cover-ages and low temperatures (T~83 K), potassium is found to adsorb in a two-dimensional overlayer that consists of mobile, ionic K adatoms. At higher coverages, K is forced to condense into 2x2 islands with metallic-like character at .THÄTA.#126;0.3. The bind-ing energy is estimated to be between 1-1.3 eV for the different K submonolayer phases. The data analysis is complicated by intercalation, even at low temperatures. Photon irradiation (hv>3 eV) of the K/graphite(0001) system leads to desorption and intercalation of K atoms. The wavelength, photon power and polarization dependencies indicate a substrate mediated des-orption process. The coverage dependence of the photodesorption shows that only the ionic (but not the metallic) K adatoms are photoactive. A model has been developed, which explains the observed K photodes-orption quantitatively. The key ingredients include photoexcitation of electrons in the graphite bulk by a pi.-pi.* interband transition. The primary photoexcited electrons are elastically or weakly inelastically scattered to-wards the surface and attach to the K 4s-derived resonance, which results in a neutralization of the K ions. The neutral K atoms are thereby repelled from the surface and may, during the excited state motion escape from the surface as neutrals, before they decay back to the ionic ground state. Recaptured K atoms may intercalate before the excitation energy is dissipated. Water adsorbs as low coordinated, two-dimensional clusters on graphite at low temperatures and low cov-erages, but transforms into three-dimensional structures by mild annealing. The bind-ing energy of the H2O molecules is 0.45 eV per molecule, close to the sublimation energy of ice. Coadsorption of H2O and K on graphite(0001) at low temperatures (T~83K) and low K coverages leads to disruption of the two-dimensional H2O clusters, and to formation of hydrated-ion like complexes. Water dissociates at a K cov-erage threshold at .THÄTA.#126;0.3, which corresponds to the critical coverage for K condensation to a metallic phase on the bare graphite surface. At elevated temperatures, H2O and K react to yield KH, KOH, and K-O com-plexes on the sur-face. H2 and H2O is released into vacuum, and successively more oxygen-rich surface complexes are formed, as identified by HREELS. The graphite surface itself gasifies at T?750 K, to yield carbon dioxide. The initial steps of aluminium oxidation has been studied in a newly constructed molecular beam apparatus. The O2 sticking on Al(111) is found to be an activated event, which does not depend on the surface tem-perature. By preparing O2 molecules with high translational energy, or in vibrationally excited states, an enhanced sticking is ob-served. The sticking coefficient shows a maximum at an incident angle of #126;25degree;. The results are in-terpreted as a direct sticking mechanism (no precursor), which depends sensitively on molecule-surface impact parameters and surface corrugation.
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| 8. |
- Österlund, Lars, 1967, et al.
(författare)
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Model studies of NOx storage
- 1999
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Ingår i: 8th European Conference on Applications of Surface and Interface Analysis (ECASIA’99) in Sevilla, Spain, October, 4-8, 1999.
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Konferensbidrag (refereegranskat)
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| 9. |
- Österlund, Lars, 1967-, et al.
(författare)
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Potassium adsorption on graphite(0001)
- 1999
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Ingår i: Surface Science. - 0039-6028 .- 1879-2758. ; 420:2-3, s. 174-189
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Tidskriftsartikel (refereegranskat)abstract
- Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the It adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage theta(c) (theta(c) = 0.2-0.3), the K adatoms are dispersed and (partially) ionized, whereas at theta > theta(c) islands of a metallic 2 x 2 K phase develop that coexist with the dispersed K adatoms up to theta = 1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E-b = 0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed. (C) 1999 Elsevier Science B.V. All rights reserved.
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| 10. |
- Österlund, Lars, 1967-, et al.
(författare)
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Quantitative determination of adsorbate-adsorbate interactions
- 1999
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 83:23, s. 4812-4815
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Tidskriftsartikel (refereegranskat)abstract
- A new concept called configuration distribution analysis is introduced to extract quantitative information on adsorbate-adsorbate interactions from high-resolution scanning tunneling microscopy data. From atom-resolved nitrogen island configurations on an Fe(100) surface we show that the propensity to form small, compact c(2 X 2) nitrogen islands is due to a nearest neighbor repulsion and next-nearest neighbor attraction. We demonstrate the importance of including many-body terms and elastic interactions to account for the detailed description of the island distribution.
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