摘要:

The atomic geometries of the charge neutral surfaces of several oxides exhibiting different crystal structures and varying participation of O(2p) electrons in the chemical bonding have been calculated using tight‐binding total energy models. Surface structures have been computed for exemplary cubic (MgO), wurtzite (ZnO), β‐tridymite, and ideal β‐cristobalite (SiO2) oxides. The cubic oxide exhibits a minimum energy structure involving small outward relaxations of the oxygens and inward relaxations of the cations. For the cleavage faces of wurtzite ZnO, large bond‐length‐conserving relaxations occur because the surface atoms can relax without appreciable distortion of the local bond lengths. The charge neutral faces of β‐tridymite and ideal β‐cristobalite SiO2also undergo bond‐length‐conserving relaxations. Thus the mechanism for the surface relaxation of tetrahedrally coordinated oxides is significantly different from that of the cubic oxides as is the role of the oxygenpelectrons in the surface chemical bonding. Most importantly, the surface structural chemistry of charge neutral oxide surfaces reflects the interplay of both surface topology (via the ability to undergo bond‐length‐conserving relaxations) and local coordination chemistry (via the participation ofpelectrons in bonding versus nonbonding surface state bands).

ISSN:0734-2101    

DOI:10.1116/1.577474

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The structure of the Cu(110)(2×3)–N phase has been investigated by parallel studies using 3‐keV Li+and 2‐keV He+ion scattering and scanned energy mode normal emission N 1sphotoelectron diffraction. The Li+ion scattering results provide clear evidence for an adsorbate‐induced reconstruction of the Cu surface and, in particular, indicate ashorteningof the Cu–Cu atomic spacing along the 〈100〉 azimuth to a value of 2.7±0.1 Å, similar to the 〈110〉 spacing of 2.55 Å. A model of the surface consistent with this result involves a pseudo‐square Cu top atom layer with N atoms occupying alternate hollow sites leading to a local structure which is a 6% distorted Cu(100)c(2×2)–N phase. He+ion scattering data provides support for the N hollow adsorption sites. Scanned energy mode photoelectron diffraction is insensitive to the nature of the reconstruction but provides information on the local adsorption sites and, hence, on the lateral registry of the reconstructed overlayer and the underlying substrate.

ISSN:0734-2101    

DOI:10.1116/1.577475

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

X‐ray diffraction is used to study the phase behavior and structure of the Au(001) surface between 300 K and the triple point. ForT>1170 K, the x‐ray reflectivity is consistent with a thin, disordered surface film. AtT=1170 K, there is a reversible transition to an incommensurate, hexagonal overlayer that resides on top of the square substrate. At 970 K, the orientation of the hexagonal layer rotates discontinuously by 0.81 ° with no observable change in incommensurability.

ISSN:0734-2101    

DOI:10.1116/1.577476

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Scanning tunneling microscope (STM) and high resolution low‐energy electron diffraction studies of slowly cooled Si(111) surfaces misoriented by 1.3° towards the [1̄1̄2] direction show the existence of a preferred displacement of the (7×7) reconstruction across single‐layer steps. STM results show that this correlation is quite strong: it occurred in 42 of the 46 steps observed. The correlation leads to a quantization of the terrace widths to units of the (7×7) unit cell. Less extensive results on the 6° misoriented surface suggest that the (7×7) reconstruction is also correlated across triple‐layer steps.

ISSN:0734-2101    

DOI:10.1116/1.577536

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Intensities of positrons specularly diffracted from Cu(111) were measured at the Brandeis positron beam facility and analyzed in the energy range 8 eV

ISSN:0734-2101    

DOI:10.1116/1.577535

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Recently, there has been some controversy about the mechanism of methanol decomposition on the (111) faces of platinum and palladium. Here, the decomposition of methanol on (1×1) and (2×1) Pt(110) is examined by temperature‐programmed desorption and electron‐energy‐loss spectroscopy. It is found that on the (1×1) reconstruction of Pt(110), the methanol adsorbs molecularly at 100 K. Upon heating to 140 K, the C–O bond breaks to yield water and a mixture of CHx adspecies. The CHx adspecies then further react to form CH4, Had, Cad, and some acetylenic complexes. By comparison, if the sample preparation procedure is changed to produce a (2×1) reconstruction, no water, methane, carbon, or higher hydrocarbons are detected. Instead, the methanol decomposes via a methoxy intermediate to yield CO and hydrogen. These results demonstrate that the mechanism of methanol decomposition on platinum is strongly affected by small changes in the sample preparation procedure and surface structure of the sample. This may explain some of the apparent discrepancies in the literature.

ISSN:0734-2101    

DOI:10.1116/1.577537

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The interaction of gas phase methyl radicals, •CH3, with a well characterized surface in ultrahigh vacuum has been studied for the first time. Methyl radicals were produced by low pressure thermal decomposition of azomethane flowing through a hot alumina tube. The sticking coefficient and surface reactions of •CH3interacting with a Mo(100) surface covered by one monolayer of atomic oxygen have been investigated using temperature programmed desorption and x‐ray photoelectron spectroscopy. The sticking coefficient of •CH3on this surface is remarkably low, ∼2×10−3. This implies there exists a barrier to adsorption associated with either the energy necessary to distort the molecule from planar to tetrahedral geometry or possibly a restriction imposed by electron spin conservation. •CH3reacts on the surface to form methane and hydrogen. Residual carbon eventually combines with the surface oxygen present to form carbon monoxide.

ISSN:0734-2101    

DOI:10.1116/1.577538

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Measurements using modulated molecular beam relaxation spectroscopy identify ODaconsumption as the rate‐limiting step for D2O formation on Pt(111). In the limit of low surface coverage, D2O is produced via two parallel pathways, Da+ODa→D2Oawith activation energyEa≊16 kcal/mol, and 2ODa→D2Oa+OawithEa≊18 kcal/mol. From these results the reactions Da+Oa→ODa, Da+ODa→D2Oa, and D2Oa+Oa→2ODaare found to be exothermic with enthalpy changes ΔH≊−16, −9, and −8 kcal/mol, respectively, and ΔHOHa≊−61 kcal/mol is obtained for the heat of adsorption of hydroxyl on platinum. The general features of the reaction mechanism cause complex kinetic behavior, manifest as changes in the apparent order of the rate function and the effective activation energy as surface temperature and reactant pressures are varied. The implications for transient and steady‐state rate measurements are discussed.

ISSN:0734-2101    

DOI:10.1116/1.577539

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Transition metals supported on alumina are widely used as catalysts, and over the last few decades, a large amount of experimental work has been done to characterize such systems. While it is generally believed that the catalytic reactions occur on the surface and near the surface of the materials, knowledge of the reactions and the adsorption sites on the surface and related energetics is seriously lacking, especially at the atomic structure level. We have used both the film linearized muffin‐tin orbital (FLMTO) method and anabinitiomolecular quantum cluster method to model the chemisorption of SO2on CuO/γ‐Al2O3. This is of interest in connection with the use of this system as a regenerable adsorbent of SO2for flue‐gas clean up. The modeling and calculations with the FLMTO method show that the copper atoms are adsorbed at the vacant surface octahedral sites (i.e., with respect to oxygen coordination). The modeling and calculations with theabinitiocluster method rule out the adsorption of SO2molecules with sulfur–copper bonding and indicate that SO2molecules are adsorbed via oxygen–copper bonding.

ISSN:0734-2101    

DOI:10.1116/1.577540

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have recently completed the first generation scanning photoemission microscope at beamline X1A at the National Synchrotron Light Source (NSLS), the X1‐SPEM. The instrument is designed to use the soft x‐ray undulator at the NSLS as a high brightness source to illuminate a Fresnel zone plate, thus forming a finely focused probe, ≤0.4 μm in size, on the specimen surface. A grating monochromator selects the photon energy in the 400–800 eV range with an energy resolution of 0.8–2 eV. The flux in the zone plate focus is in the 108–109photons/s range. A single pass cylindrical mirror analyzer (CMA) is used to record photoemission spectra, or to form an image within a fixed electron energy bandwidth as the specimen is mechanically scanned. The best one‐dimensional resolution achieved so far is 0.1 μm, as judged by the 25%–75% intensity rise across an edge. This is close to the diffraction limit of the zone plate used. Due to astigmatism introduced by the beamline optics, the two‐dimensional resolution in the plane of least confusion is presently limited to 0.4 μm. Images of a microelectronic device are presented, along with some representative spectra, to demonstrate the present capabilities. With an upgrade of the beamline optics, and a new generation of zone plates, the two‐dimensional spatial resolution in a second generation instrument is anticipated to be about 50 nm.

ISSN:0734-2101    

DOI:10.1116/1.577541

出版商:American Vacuum Society    

年代:1991

数据来源: AIP