The adsorption of CO on the (110) plane of tungsten; LEED,XPS, and Auger measurements

Adsorption of CO on W(110) at 100 K produces a number of ordered LEED patterns as coverage increases, culminating in a p(5 × 1) pattern for a full virgin CO layer. The beta-1 layer obtained by heating a virgin layer to 400 K has a p(2 × 1) structure. Absolute coverages, obtained by comparison of XPS intensities (and Auger intensities where feasible) with those of oxygen on tungsten at O/W = 0.5 indicate that CO/W ? 0.8 for the full virgin layer and ? 0.3 for beta-1. These results, together with the LEED data, indicate that low temperature adsorption of virgin CO is not very site specific, and that beta-1 must be dissociated with C and O lying along alternate closepacked rows of W. XPS results for the oxygen 1s peak show that the latter shifts in beta and beta-1 from its position in virgin CO to an energy equal to that seen for pure oxygen on tungsten. A number of electron impact desorption results are also presented, and the nature of the various binding states of CO on this plane is discussed.     

Structure and vibrations of CO adsorbed on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces

Chemisorption of CO on the Ni(100)p(2 × 2)O and c(2 × 2)O surfaces has been investigated by high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). At 175 K CO adsorption on Ni(100)p(2 × 2)O saturates at about 1 L exposure in a structure interpreted to be Ni(100)p(2 × 2)O—p(2 × 2)CO. The CO layer is stable at 175 K but desorbs readily around 300 K. The EEL spectrum for p(2 × 2)CO shows vibrational losses at 46 meV and 245 meV interpreted to be due to excitations of the Ni-C and C-O stretching vibrations of CO molecules bridge bonded to two nearest neighbour Ni atoms. This interpretation is also supported by the LEED observations. For the preceeding dilute CO layer the vibrational loss spectrum reveals CO adsorption both to Ni bridge sites and hollow sites. At 175 K CO does only adsorb stationary on p(2 × 2)O defects in the Ni(100)c(2 × 2)O surface and not at all on epitaxially grown NiO(111) and (100) surfaces.     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

Chemisorption of CO on the unreconstructed Pt(100) surface

The chemisorption of CO on the clean, unreconstructed Pt(100)-1 × 1 surface was investigated by LEED and XPS. Three LEED patterns, c(2 × 2), (√2 × 3√2) R45° and c(4 × 2), were observed with increasing CO exposure and structure models corresponding to these LEED patterns were proposed. The absolute coverage of CO was determined by combining the O(1s) XPS data with coverage information derived from LEED. The maximum CO coverage thus obtained was θ = 0.75 and the initial sticking coefficient was determined to be s₀ = 0.6. This coverage calibration can also be utilized for other oxygen containing molecules by comparing the corresponding O(1s)it peak intensities.     

Ionic implantation of N2+ in Ni(110) at 300 K

The present work gives results of a preliminary investigation, carried out by SES, AES, LEED and ELS, on the implantation of nitrogen ions in Ni(110) as a function of ion dose and subsequent surface heat treatment at different temperatures. The fine structure in the SES spectrum is the most sensitive to implantation: peaks at 9, 17.5 and 31.5 eV are shifted towards lower energies by E = 1 eV for the first two peaks and 2.8 eV for the last. At high nitrogen doses a disordered layer is observed by LEED. The p(2 × 3) structure is obtained when the crystal is heated to 750 K. The two electron loss peaks of 4.8 and 10 eV arise from an induced electron N_2p level situated 4.8 eV below the Fermi level.     

Dynamical behaviour of PtRh(100) alloy surface during dissociative adsorption of NO and reaction of NO with H2

When a clean Pt-Rh(100) alloy surface was exposed to NO at T > 440 K, the LEED pattern changed sequentially as p(1 × 1) → c(2 × 2) → c(2 × 2) + p(3 × 1) → p(3 × 1), where the c(2 × 2) pattern appeared immediately after the exposure to NO. In contrast to this, the appearance time for the p(3 × 1) depends strongly on the initial Rh concentration on the surface adjusted by annealing. When the p(3 × 1) surface was exposed to H₂ by mixing H₂ into NO gas, the AES intensity of O(a) decreased and of N(a) increased markedly and the LEED pattern changed from p(3 × 1) to c(2 × 2). These results suggest that N(a) has equal affinity to Pt and Rh atoms so that the N(a) does not distinguish the Pt and Rh sites on the alloy surface. On the other hand, O(a) makes a stronger bond with Rh atoms so that Rh atom segregation onto the surface is induced. By reacting randomly distributed Rh atoms on the Pt-Rh(100) surface with oxygen, a surface compound in a p(3 × 1) arrangement is built on the surface.     

Vibrational properties of the adsorbed monolayer on face-centered cubic crystals

Calculations of mean-square displacements 〈u²〉 of the atoms in adsorbed monolayers on fcc crystals are presented and compared with LEED experimental results. This text is restricted to the case of a C(2 × 2) adsorbed layer on a (100) surface [experimental case of Ni(100) with adsorbed sulfur, sodium, cesium or oxygen]. 〈u²〉's perpendicular to and parallel to a (100) surface are calculated for the adsorbed atoms and the atoms of the first surface layer of the crystal. The values obtained are compared with those for a clean (100) surface and the volume of the crystal. Every possibility for force constants between adsorbate and substrate atoms is examined. It is shown that the measurement of 〈u²〉 perpendicular to the (100) surface yields the adsorbate-substrate force constants and that 〈u²〉 parallel to the (100) surface yields the adsorbate-adsorbate force constants.     

Determination of the Co(001)c(2 × 2)-S superstructure by comparison of LEED intensity data

A cross-comparison of LEED intensity data for Co(001)c(2 × 2)-S and Ni(001)c(2 × 2)-S gives evidence that the sulfur atoms are chemisorbed on Co(001) in a fourfold site, at 1.3 Å from the first substrate layer.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

LEED studies of several structural models of the Si{1 0 0} surface

Dynamical LEED calculations are performed for the Poppendieck c(4 × 2) and the Chadi (2 × 1), c(2 × 2), p(2 × 2) and c(4 × 2) structures. All the non-(2 × 1) structures give forbidden beams that are far too intense, and comparison with experiment shows that none of the structures individually, nor any combination of the Chadi structures, can represent the Si{100} surface.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Reconstruction of W(110) induced by exposure to NO and by subsequent heat treatment

The (110) face of a tungsten single crystal was found to be partially reconstructed after an exposure, at 300 K, of 300 L of nitric oxide. This surface liberated N₂ when heated to 975 K, after which the reconstruction appeared to have been completed. At this stage a well developed c(11 × 5) LEED pattern was observed and a surface oxide, W₃O₂, is proposed for this reconstructed surface. The above mentioned surface reconstructs again after further heat treatment and is characterised by a weak p(2 × 2) LEED pattern. Work function measurements and the thermal stability of this surface structure indicate that the latter is not the same as that produced by oxygen adsorption on W(110).     

Phase transitions on clean Si(110) surfaces

The properties of the structure of clean Si(110) surfaces have been investigated by LEED. The phase transitions between surface structures Si(110)?(4 × 5), Si(110)?(2 × 1) and Si(110)(5 × 1) take place at about 600 and 750°C. The time of reconstruction from the high temperature phase to the low temperature phase may exceed the time of the sample cooling. That explains why the Si(110)?(2 × 1) and the Si(110)?(5 × 1) superstructures may be seen at room temperature. Surface defects favour the retaining of high temperature phases on the surface at room temperature. The transition from the Si(110)?(5 × 1) structure to the Si(110)?(2× 1) structure and conversely in the temperature range of 720–750°C apparently occurs through formation of the intermediate structures Si(110)?(7 × 1) and Si(110)?(9 × 1). The models are given of superstructures observed by LEED.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Author index volumes 111–120

Adsorption of monolayer amounts of bismuth on the 100 surface plane of tungsten has been studied by probe hole field emission microscopy and electron spectroscopy. Sub-monolayer bismuth forms a relatively strongly bound layer of bismuth-tungsten dipoles with dipole moment μ = (0.18 ± 0.02) × 10^?30C m and polarizability α = (6.3 ± 1.3) × 10^?40J V^?2m². Changes in work function and their dependence on temperature closely parallel those produced by adsorbed lead which was shown by LEED to form c(2 × 2), p(2 × 2) and (1 × 1) structures. Bismuth is thought to behave in a similar way, but, unlike lead, forms a second monolayer which replicates the first. Electron spectroscopy shows that sub-monolayer bismuth removes the surface state (Swanson) peak and at monolayer coverage a new peak emerges and shifts with increasing coverage. Using Gadzuk's theory, this peak is tentatively attributed to the ²P level in bismuth adatoms which form a p(2 × 2) structure in the first and second monolayers. Its shift with coverage is ascribed to an increase in the local surface field. There remains the difficulty of reconciling the proposed occupation of the ²P level with the observed small positive charge on the bismuth adatom.     

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI₂ on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.     

氮离子轰击钼(001)和钼(110)表面形成的有序结构

用能量为1千电子伏,束流为6微安的氮离子轰击含有痕量碳和氧的钼(001)和钼(110)表面10至15分钟,在俄歇能谱中出现了很强的氮的俄歇峰。从室温直到350℃退火,低能电子衍射观察表明,表面是无序层。样品加热到530℃和650℃之间,在钼(001)表面上得到c(2×2)-氮,p(2×2)-氮和(4(2^1/2)×2^1/2)R45°-氮、氧三种结构的低能电子衍射图;在密堆的钼(110)面得到单一结构的c(7×3)-氮的低能电子衍射图。低能电子衍射图与热脱附密切相关 关键词：     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

On the contraction of the W(001)-(1 × 1) surface using LEED intensity analysis

Three recent independent attempts at deducing the W(001)-(1 × 1) surface structure by LEED beam intensity analysis have yielded contractions of the topmost layer spacing of 6 ± 6%, 11 ± 2%, 4.4 ± 3% normal to the surface plane. We investigate possible reasons for the discrepancies by comparing published experimental and theoretical profiles of these workers as well as our own. Our main conclusions are that the direct comparison of experimental data of different investigators shows deviations which are comparable to the changes in the calculated profiles for various surface contractions. Also the deviations between calculated intensity profiles using different (but still realistic) assumed scatteting potentials are comparable to the changes in the calculated profiles for various surface contractions. The main uncertainty in the scattering potential is the choice between the Slater free electron exchange-correlation term (coefficient α = 1) or the Kohn-Sham version ($\text{α =}\text{2}\text{3}$) or a value in between. For tungsten the corrections due to relativistic atomic scattering must also be considered. These uncertainties in the calculated and experimental profiles lead to the conclusion that the surface layer contraction of W(001)-(1 × 1) is not known at the present time. To assess the potential of LEED in deducing surface structures of this type further comprehensive analyses are required where the uncertainties in the theoretical scattering potential are also examined.     

Phase transitions on stepped and disordered surfaces: Xe adsorbed on Cu and NaCl single crystal surfaces

The influence of imperfections of single crystal surfaces on two-dimensional phase transitions is studied by LEED and AES. On a smooth, well-ordered (100)Cu or NaCl face, the first adsorbed layer of xenon undergoes a 2D gas-incommensurable solid first order transition. This transition broadens and becomes continuous on the disordered (100)Cu, the stepped (610)Cu and the disordered (610)NaCl surfaces. The corresponding small variation (±3%) of the chemical potential along the surface can be explained by a defect induced variation of adsorption energy on the substrate for the Cu surfaces or by a limitation of the size of 2D crystals by surface imperfections for the (610)NaCl substrate. Heterogeneities do not modify the lateral structure of 2D solid xenon which remains hexagonal close-packed, but they reduce the size of 2D crystallites in the case of the disordered (610)NaCl substrate. Other spectacular effects are observed on the (610)Cu surface having periodic monoatomic steps: (i) There is a 45° rotation of the 2D crystal with respect to its position on the smooth (100)Cu surface. Hence, the orientational ordering of the xenon overlayer changes drastically, (ii) The LEED pattern at T = 84 K is interpreted as due to a pseudo hexagonal close-packed xenon overlayer with a (2 × 6) coincidence mesh, orientated along the step direction. Moreover, the step edge roughness and the xenon atom size induce a static transverse distortion wave whose wavelength varies with temperature.