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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Langmuir 7 (1991), S. 3019-3026 
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Soft-x-ray photoelectron spectroscopy (SXPS) is utilized to study the coadsorption of K and PF3 on Ru(0001) at 90 and 300 K. In the absence of K, PF3 adsorbs molecularly at both temperatures. In the presence of a fractional monolayer of K, initially PF3 completely dissociates resulting in the formation of adsorbed KF and P species. As the surface is further exposed to PF3, some of the PF3 molecules adsorb via partial dissociation, resulting in the formation of PF and PF2. This process continues until all the K has reacted. At 300 K, a fraction of the incoming PF3 molecules react with the adsorbed KF and form a species which is tentatively identified as KPF6. The data show that surface chemistry is different at the two temperatures, as some of the chemical reaction channels occurring at 300 K are blocked at 90 K. The reduced surface mobility of the incident PF3 molecules at 90 K adversely affects the probability of PF3 and KF interactions, which, in turn, causes the concentration of adsorbed PF3 relative to P to be larger at lower temperatures.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5928-5944 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution electron energy loss spectroscopy (EELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) have been combined to study CO chemisorption on the Cr(110) surface. At 120 K, two molecular binding modes are sequentially populated. The first, α1CO, is a novel binding mode for a clean metal surface in which the molecules are bound with the C–O axis nearly parallel to the Cr(110) surface. These α1CO molecules exhibit a νCO of 1150–1330 cm−1, no detectable ion emission in ESDIAD, and form a c(4×2) α1CO overlayer at saturation (θCO(approximate)0.25). A model is proposed in which the α1CO are π bonded to the Cr(110) surface at twofold symmetric surface hollow sites. Annealing experiments demonstrate that the α1 binding mode is the precursor to dissociation on Cr(110). At higher CO coverages, 0.25〈θCO〈0.35, the α1CO overlayer is disordered by the addition of the second binding mode, α2CO. Three new vibrations, at 495, 1865, and 1975 cm−1, are observed by EELS while O+ ion emission along the [110] direction is seen in ESDIAD. These α2CO results are indicative of terminally bound molecules adsorbed at atop and bridge sites and aligned approximately perpendicular to the surface. Annealing studies of CO and O/CO adlayers provide evidence for a surface reaction in addition to direct dissociation above 170 K. In contrast, adsorption at 300 K leads to direct dissociative chemisorption with no detectable intermediate molecular adsorbate. The implications of these results to catalytic reactions of CO are considered and comparisons to CO adsorption on clean and "promoted'' transition metals are made.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1572-879X
    Keywords: n-butane hydrogenolysis ; W(111) ; Pt ; bimetallic
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The Relationship between surface structure and reactivity is investigated by means ofn-butane hydrogenolysis, a known structure sensitive reaction, for planar and faceted Pt/ W(111) surfaces. The W(111) surface reconstructs to form pyramidal facets with [211] orientation upon vapor deposition of Pt (〉1.3 ML) and annealing above 750 K. The hydrogenolysis kinetics over the planar and the faceted surface are found to be quite different. The planar surface has a higher selectivity towards ethane formation and a higher reaction rate. The apparent activation energies are found to be 33 ± 4 kJ/mol for the planar surface and 76 ± 6 kJ/mol for a surface covered with ∼ 20 nm facets. There appears to be a correlation between the concentration of fourfold coordination (C4) sites on the surface and the amount of ethane produced. The C4 concentration is altered by changing the facet size (annealing temperature). The results indicate the presence of a different intermediate on the C4 sites as evidenced by the differences in the apparent activation energy, the reaction rate and the overall selectivity.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Catalysis letters 20 (1993), S. 269-274 
    ISSN: 1572-879X
    Keywords: Iron ; titanium dioxide ; ultrathin films ; SMSI
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We report evidence in this letter for the encapsulation of Fe clusters supported by single crystal TiO2. After thermal annealing in ultrahigh vacuum (UHV) at 500–700 ° C for several hours, Fe clusters are almost completely covered by Ti suboxides TiOx while Fe remains mainly in the metallic state. The phenomenon can be clearly measured using LETS (low energy ion scattering) and XPS (X-ray photoelectron spectroscopy).
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2202-2215 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the transmission of low energy (〈10 eV) 16O+ ions through ultrathin films of condensed molecular solids, NH3 and H218O, in order to address the fundamental scattering processes that occur in the desorption of ions from below the surface of solids. 16O+ ions with a peak energy of ∼7 eV and a narrow angular distribution [full-width at half-maximum (FWHM) ∼15°] are generated by means of electron stimulated desorption (ESD) from an 16O oxidized W(100) surface and their yield, energy and angular distribution are measured with a digital ESDIAD (ESD ion angular distribution) detector. Ultrathin NH3 and H218O films of known thickness are condensed on the oxidized surface at 25 K and changes in the ion yield, energy and angular distribution are observed as a function of coverage. We find that adsorption of only 0.5 monolayer of H218O is enough to suppress the 16O+ ion emission by a factor of 100, while three monolayers of NH3 are necessary for equivalent suppression of the 16O+ ion emission. The angular distribution of the ions increases slightly with increasing overlayer coverage. We also find that a small percentage of H218O dissociates upon adsorption. We suggest that one electron charge transfer between 16O+ and H218O, and between 16O+ and the dissociation product OH are the main reasons for the strong attenuation of 16O+ ions by only a fraction of a monolayer of H218O. Charge transfer is also believed to be the main process that causes suppression of 16O+ ions by ultrathin NH3 films. Other elastic and inelastic processes are not believed to contribute significantly to 16O+ attenuation in NH3 or H218O films. © 1995 American Institute of Physics.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Cl− yield produced by dissociative electron attachment of a submonolayer of CF2Cl2 is enhanced by factors of 102 and 104 when CF2Cl2 is coadsorbed with water ice and ammonia ice, respectively, on a surface at ∼25 K. Moreover, the magnitude of Cl− enhancement increases strongly with decreasing CF2Cl2 concentration. This enhancement is attributed to dissociation of CF2Cl2 by capture of electrons self-trapped in polar water or ammonia molecules. This process may be an unrecognized sink for chlorofluorocarbons in the atmosphere. Cl− ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone. © 1999 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 2801-2810 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report on the transmission of F+ and F− ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F+ and F− ions are produced by ESD (electron stimulated desorption) of a PF3 monolayer adsorbed on a Ru(0001) surface (PF3/Ru(0001) surface). We find two surprising results: (a) the off-normal F+ signal is attenuated to ∼1% by only ∼2.5 monolayer (ML) of H2O, while a much thicker layer, ∼10 ML of H2O, is necessary for equivalent attenuation of the F− ion emission, and (b) 1 ML of H2O increases the emission of F− ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F− ions transmitted through condensed H2O films indicate that elastic scattering is an important process in determining the attenuation of F− by H2O. No direct evidence for any kind of ion–molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F+ without substantial changes in angular distribution suggests that charge transfer processes are important in limiting the transmission of F+ ions. Our quantum mechanical calculations indicate that the increase in F− emission upon adsorption of≤1 ML of H2O is mainly due to a decrease in the neutralization probability of F− with the substrate, by a dielectric screening mechanism. The calculations also show that the increase in the F− survival probability saturates after a water bilayer (1 ML H2O) is formed, which is in excellent agreement with the experiment. Our measurements show no evidence for diffusion of H2O on the PF3/Ru(0001) surface between 20 and 60 K; the lack of diffusion, together with exponential attenuation of F+ and F− with H2O thickness, indicates that H2O vapor condensed on the PF3/Ru(0001) surface at 20 K grows statistically. © 1997 American Institute of Physics.
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