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Quantitative description of Methane adsorption on ethylidyne-covered Pt(111) is possible using a kinetic model. This model forces adsorbing molecules through an extrinsic precursor, from which they may desorb or migrate to binding sites on the surface.

The dynamics of ethylene adsorption on Pt(111) into both the di-σ- and π-bonded states were investigated at 95 and 40 K, respectively, using supersonic molecular beam techniques. The angular dependence of ethylene adsorption into both states is similar to the angular dependence for ethane adsorption, which has a much weaker bond to the surface in its final state. In contrast to ethane, high adsorption probabilities for ethylene prevail to high incident kinetic energies, suggesting that the strong interaction of ethylene with the surface influences adsorption. The initial adsorption probability of ethylene is approximately independent of surface temperature between 40 and 450 K, suggesting that there is no reversible, thermalized intrinsic precursor to adsorption. At 40 K, the adsorption probability increases with coverage (in the π-bonded state). However, at 95 K, the adsorption probability of ethylene remains constant with increasing self-coverage (in the di-σ- bonded state) for trajectories incident with low parallel momentum, but decreases with coverage at high parallel momentum. High parallel momentum may contribute to an increased overall scattering probability from a “hot” extrinsic precursor, resulting in a decreased net adsorption probability at higher ethylene coverage in the rigidly bound di-σ state. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The kinetics of krypton adsorption on Pt(111) were investigated using supersonic molecular beam techniques. Krypton adsorbs at defects via an intrinsic precursor below a surface temperature of 85 K. The difference in activation energies for desorption and migration of a Kr atom on the terrace seeking a defect site is 10.7 kJ/mol, indicating that at 80 K, a Kr atom makes about 10[sup 7] site hops before desorbing or finding a binding site. Below 60 K stable adsorption occurs on terraces, where the initial adsorption probability is independent of surface temperature. The activation energy for zero-order desorption from Pt(111) terraces is 12.9 kJ/mol; the activation energy for Kr migration on the terraces is then calculated to be ≤2.2 kJ/mol. Krypton adsorption proceeds at nonzero coverages via an extrinsic precursor. The adsorption probability of Kr increases with self-coverage, and is described by the modified Kisliuk model [H. C. Kang, C. B. Mullins, and W. H. Weinberg, J. Chem. Phys. 92, 1397 (1990); C. R. Arumainayagam, M. C. McMaster, and R. J. Madix, J. Phys. Chem. 95, 2461 (1991)]. The Kr overlayer on terraces compresses between 60 and 42 K, then forms a second state before forming stable multilayers at 34 K. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The dynamics of methane trapping on CO-covered Pt(111) in low coverage, c(&sqrt;3x5)rect, and c(4x2) structures was investigated using supersonic molecular beam techniques at a surface temperature of 50 K; at this temperature methane was stably adsorbed on the clean (A. F. Carlsson and R. J. Madix, to be published) surface, but not in multilayers (A. F. Carlsson and R. J. Madix, to be published), and thus trapped amidst adsorbed CO molecules. Molecular trapping was enhanced to greater degrees with increasing CO coverage, and the methane uptake decreased with increasing CO coverage, as would be expected. The trapping probability further increased as methane covered the Pt(111)-CO surface; the modified Kisliuk model [J. Chem. Phys. 92, 1397 (1990); J. Phys. Chem. 95, 2461 (1991)] describes the coverage-dependent trapping probability. Methane adsorption may occur directly on the surface, or via two entrance channels into an extrinsic precursor, where the trapping probability is higher. The angular dependence of methane trapping on the CO-covered surface suggests an increasing corrugation in the gas-surface potential as the CO coverage increases; the corrugation may also contribute to the enhancement of the overall trapping probability by facilitating interconversion of perpendicular to parallel momentum during the gas-surface collision. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The orientation, absolute coverage and core-electron binding energies of furan (C4H4O) and 2,5-dihydrofuran (C4H6O) on Ag(110) have been measured using near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy. Both furan and 2,5-dihydrofuran (2,5-DHF) are tilted 22±7° from the plane of the surface for both submonolayer and monolayer coverages. The saturation monolayer coverages on the Ag(110) surface of 0.45±0.07 ML for furan and 0.41±0.06 ML for 2,5-DHF are consistent with expectations based on van der Waals radii. The C(1s) binding energies for a monolayer of furan on Ag(110) are 284.8 and 286.2 eV, while the C(1s) binding energies for 2,5-DHF are 285.1 and 286.6 eV. The O(1s) binding energies are 532.8 and 533.8 eV for furan and 2,5-DHF monolayers, respectively. The onset of excitations from C(1s) levels to the continuum lies well above the Fermi level due to the weak bonding interactions with the surface. [ABSTRACT FROM AUTHOR]

The π bond orientation of ethylene and propylene on the clean and atomic oxygen covered Ag(110) surfaces has been determined with near-edge x-ray absorption fine-structure (NEXAFS) measurements. The π system of ethylene is tilted 20±5° and 18±5° from the surface normal for ethylene on the clean and atomic oxygen covered surfaces, respectively. The tilt angle of the π system is 20±5° on the clean surface and 31±5° on the atomic oxygen covered surface. On both the clean and atomic oxygen covered surface, the carbon–carbon double bond in ethylene and propylene appears to form a π-donor type bond with the Ag(110) surface. [ABSTRACT FROM AUTHOR]

Carbon K edge near edge x-ray absorption fine structure (NEXAFS) spectra of adsorbed acetonitrile and reactive intermediates derived therefrom on Ag(110) were studied in order to determine their orientation and bonding to the surface. For both the multilayer and the adsorbed monolayer there is excellent agreement between the spectra of the adsorbed acetonitrile and the gas phase species. For the multilayer coverage the orientation of the acetonitrile molecules is not completely random; the molecules prefer an orientation with their bond axes closer to parallel than perpendicular to the surface. At monolayer coverage acetonitrile bound to the surface is unrehybridized. It is bound with its linear molecular skeleton parallel to the surface and randomly oriented azimuthally. Annealing molecular CH3 CN to 325 K in the presence of coadsorbed atomic oxygen produces a linear CH2 CN surface intermediate, the molecular axis of which is parallel to the surface and perpendicular to the close-packed direction. The C–C and C–N bond lengths are essentially unperturbed from their values in gas phase CH3 CN. Bonding appears to occur between the methylene carbon and the metal, as expected. A broadening of the π* resonance for transition into the π* orbital perpendicular to the surface is interpreted in terms of π-donor bonding. [ABSTRACT FROM AUTHOR]

The orientation of allyl alcohol (CH2=CHCH2OH), propargyl alcohol (CH 3/4 CCH2OH), and their surface alkoxy groups has been examined on Ag(110) using near edge x-ray absorption fine structure (NEXAFS) measurements. The planes containing the π* orbitals of allyl alcohol and its alkoxy species, allyloxy (CH2=CHCH2O-), are tilted 26±4° and 32±5° from the surface normal, respectively. Allyl alcohol itself shows no azimuthal ordering, but the π* orbital in allyloxy is azimuthally oriented 33±15° from the close-packed direction ([110] azimuth). Propargyl alcohol is randomly oriented on the Ag(110) surface. Its alkoxy group, propargyloxy (CH 3/4 CCH2O-), is strongly oriented with the triple bond axis parallel to the plane of the surface and directed along the [001] azimuth, perpendicular to the troughs. The position of the σ* resonances indicate that no loss of the allylic hydrogen has occurred and that the double and triple bonds are slightly, but not significantly, perturbed from their normal gas phase bond lengths. [ABSTRACT FROM AUTHOR]

A two-step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions. [ABSTRACT FROM AUTHOR]

The dissociative adsorption of C1–C4 alkanes on Ir(110)–(1×2) was investigated using supersonic molecular beam techniques. Three regimes of reactivity were observed. At incident kinetic energies less than 100 kJ/mol the dissociative adsorption probability for propane and butane increased to about 0.6 with both decreasing surface temperature and decreasing translational energy of the incident molecule suggesting that activation occurred via trapping of the alkane on the surface. At kinetic energies below 110 kJ/mol for butane, 90 kJ/mol for propane, 60 kJ/mol for ethane, 40 kJ/mol for methane, and at elevated surface temperatures the reaction probabilities for alkanes were independent of surface temperature and translational energy, but increased significantly with the molecular weight or carbon chain length of the incident alkane. This behavior indicates an unactivated reaction channel for each species except methane. At kinetic energies above 110 kJ/mol for butane, 90 kJ/mol for propane, 60 kJ/mol for ethane, and 40 kJ/mol for methane the initial dissociative sticking probability increased with increasing translational energy and was independent of surface temperature, indicating direct translational activation of the incident alkane. The translational energy required to activate the incident alkane via this channel increased with carbon number or molecular weight, indicative of energy transfer processes that dissipate energy in the reactive collision. The dissipative transfer of translational energy out of the reactive channel via a hard cube collision with the surface accounts qualitatively for the increase in the apparent activation barrier with increasing molecular weight in this activated channel. [ABSTRACT FROM AUTHOR]

Molecular beam techniques have been used to study the dissociative chemisorption of nitrogen on W(110). Chemisorption probabilities have been measured as a function of incidence angle θi and kinetic energy Ei surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling approximately with Ei, rather than with the velocity component normal to the surface. This probability is ≤3×10-3 for Ei≤30 kJ mol-1, and rises by more than a factor of 100 by ∼100 kJ mol-1, where it levels off at ∼0.35. It is argued that this behavior arises due to a strong chemical interaction prior to the barrier to dissociation. Angular scattering distributions revealed predominately quasispecular scattering with evidence as well for a diffuse component at low energies. The sticking probability falls steadily with increasing surface coverage and a saturation coverage of ∼0.25 atomic ML is observed for Ei∼10 kJ mol-1. At higher incidence kinetic energies, this saturation coverage increases to ∼0.5 ML at 200 kJ mol-1. LEED structures are also reported, corresponding to coverages of 0.25, 0.3, 0.5, and 0.52 ML. The 0.25 and 0.5 ML structures are identified as p(2×2) and c(4×2), respectively, for which structure models are proposed. [ABSTRACT FROM AUTHOR]

The adsorption structure of carbon monoxide on W(100)-(5×1)–C has been characterized using high-resolution electron energy loss spectroscopy and near edge x-ray absorption fine structure measurements. A single C–O stretch of 2100 cm-1 is observed at a surface temperature of 130 K. The C–O bond vector is determined to be normal to the surface on the basis of near edge x-ray absorption fine structure measurements. The position in energy of the C(1s)→σ* resonance in CO was measured to be 304.2 eV. The empirical relation between the σ* resonance energy and bond length yields a bond length of 1.12 Å. All spectroscopic results are consistent with CO bound atop a single tungsten atom, normal to the surface plane with minimal C–O bond weakening. These results correlate with the known difference in the reactivity of the W(100)-(5×1)–C surface compared to clean W(100). The increased barrier for CO dissociation on the W(100)-(5×1)–C surface is explained on the basis of reduced backdonation of electron density from the tungsten d band to the CO 2π* orbitals. Tight-binding slab calculations for CO on W(100) and W(100)-(5×1)–C which support this contention are discussed. [ABSTRACT FROM AUTHOR]