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The surface geometric and electronic structure of the FeAl(110) intermetallic alloy has been investigated by scanning tunnelling microscopy and angle-resolved photoemission spectroscopy (ARPES). Preferential sputtering results in depletion of Al in the surface region and subsequent annealing promotes surface segregation of Al and gives rise to new reconstructed phases. A bulk terminated surface structure is obtained after annealing the surface to 400 °C. However, an incommensurate phase develops above 800 °C with a stoichiometry consistent with an FeAl2 structure in the topmost layer. The ARPES measurements confirm the Al segregation with increased density of states (DOS) near the Fermi level. The increased DOS is believed to be due to hybridization between the Fe d and Al sp states. The increased intensity of the Al 2p core level for the incommensurate phase also confirms the higher Al surface concentration for this phase.

The current carrying capabilities of RABiTS are connected to the crystalline quality of the seed buffer layer and the stability of the metal/seed layer interface. Our study shows that the epitaxial growth of commonly used seed layers on textured Ni is mediated by a sulfur superstructure present on the metal surface. Many structural defects generated during seed layer growth (secondary orientations, in-plane rotation, incomplete cube texture) can be related to the S surface concentration and superstructure coverage. More generally, our results indicate that the epitaxial deposition of several classes of oxides (fluorite, perovskite, RE/sub 2/O/sub 3/) on several {100} fcc metals depends, in addition to chemical stability and lattice match, on the existence and optimization of S superstructures on the metal surface. On these bases, we discuss issues related to the growth of different oxides on Ni, Ni-alloys, and Pd surfaces having different chemical and structural properties.

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate's surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

Low-energy electron diffraction (LEED) I – V has been utilized to determine the surface structure of Cu(2 1 0). The surface structure is found to exhibit multilayer relaxation following the trend − − + (− is contraction and + is expansion in the interlayer spacing). The magnitude of interplanar relaxation is found to be damped, where | Δd 12 |>| Δd 23 |>| Δd 34 |, etc. This kind of behavior is quite different from that observed on Al(2 1 0) [Phys. Rev. B 38 (1988) 7913], where the magnitude of interplanar relaxation in the third interlayer is larger than that in the second interlayer (i.e., | Δd 23 | Δd 34 |). The difference of damped multilayer relaxation behavior of Cu(2 1 0) and Al(2 1 0) could be related to a charge density oscillation perpendicular to the surface.

The surface structure of Ru(0 0 0 1) has been studied between 300 and 1870 K using X-ray diffraction. At 300 K, the outermost layer is contracted by 2.2(4)%, in agreement with the value determined by low-energy electron diffraction studies, but about half that predicted by ab initio calculations. H-adsorption appears to introduce disorder through an increased surface Debye–Waller factor and an increased step density, but does not significantly alter the interlayer spacing. Temperature-dependent studies of the clean surface structure indicate a thermal expansion of the first three interlayer distances that is slightly smaller than that of the bulk, accompanied by increasing vibrational motion comparable to that of the bulk.

The structure of strained Cu films deposited on Ru(0001) surfaces at 720 K was investigated by x-ray diffraction techniques. Our analysis shows that a single Cu monolayer adopts a pseudomorphic structure with the first three interlayer spacings significantly relaxed. The two-layer structure consists of a commensurate, uniaxially modulated stripe-phase reconstruction. Modeling the diffraction intensities through application of simulated annealing techniques, together with least-squares refinement, has led to a crystallographic description of this structure in terms of a set of three-dimensional modulation functions. We find that the reconstruction persists through both Cu layers, and leads to relaxation of the Cu and Ru interlayer spacings. We also calculate the average strain distribution as a function of position across the unit cell.

The growth morphology and electronic structure of Ni on Ag(100) has been studied with scanning tunneling microscopy (STM) and synchrotron based angle resolved photoemission spectroscopy. At deposition temperatures at or below 300 K, STM reveals Ni cluster growth on the surface along with some subsurface growth. Upon annealing to 420 K, virtually all Ni segregates into the subsurface region forming embedded nanoclusters. The electronic structure of Ni d bands in the unannealed surface shows dispersion only perpendicular to the surface whereas the annealed surface has Ni d bands that exhibit a three-dimensional-like structure. This is a result of the increased Ni d–Ag sp hybridization bonding and increased coordination of the embedded Ni nanoclusters.

We present X-ray scattering studies of the structure and growth of the heteroepitaxial interface Cu–Ru(0001) between 300 and 870 K. For Cu depositions of between one and two monolayers (ML), a bilayer stripe-phase coexists with domains pseudomorphic to the Ru substrate. At the completion of 2 ML, the stripe-phase wavevector abruptly locks to that of the substrate with a commensurability δ=1/16 or 1/18, depending on the temperature. Above 2 ML, a cooperative growth process leads to three-phase coexistence, including the bilayer stripe-phase and rotated and unrotated bulk-like Cu islands.

Electron-energy-loss spectroscopy (EELS) data are presented to illustrate line shape changes that occur as a result of oxygen interaction with metal surfaces. The metals were aluminum, beryllium and nickel. Core-level EELS data were taken for excitations from Al(2p), Be(1s), Ni(3p/3s) and O(1s) levels to the conduction band (CB) density of states (DOS) of the materials. The primary beam energies for the spectra were 300, 450, 300, and 1135 eV, respectively. The data are presented in both the (as measured) first-derivative and the integral forms. The integral spectra were corrected for coherent background losses and analyzed for CB DOS information. These spectra were found to be in qualitative agreement with published experimental and theoretical studies of these materials. One peak in the spectra for Al oxide is analyzed for its correlation with excitonic screening of the Al(2p) core hole. Similar evidence for exciton formation is found in the Ni(3p) spectra for Ni oxide. Data are also presented showing o...

We present the results of X-ray scattering studies of the structure and phase behavior of the Ir(001) surface between room temperature and T=2000 K. Two distinct phases have been identified. For temperatures above T=1950 K, the Ir(001) surface is rough, similar to what has been observed for Pt(001) at elevated temperatures. Below about T=1950 K, the surface reconstructs to form a quasi-hexagonal unit cell, which is commensurate with the substrate lying beneath it. The measured X-ray intensities, both inplane and along the specular rod, are consistent with a buckled, (5×1) description of the structure, including a 15% expansion of the top layer spacing and an enhanced surface Debye–Waller factor. A detailed comparison is made of the phase behavior determined here for Ir(001) to that found earlier for Pt and Au(001).

The segregation and ordering behavior of the (100), (110), (210), (310) and (111) surfaces of the ordered intermetallic compound FeAl were investigated by means of AES and LEED. Starting from preferentially sputtered, i.e. Al-depleted surfaces sequential annealing leads to segregation of aluminum for all five surfaces. For intermediate annealing temperatures several ordered superstructures are found for the low index surfaces, which can partly be attributed to metastable Fe 3 Al films developing at the surfaces. All well-annealed surfaces exhibit Al concentrations beyond bulk stoichiometry. The (100) surface is found to be Al terminated, additionally the second layer is enriched in Al. For all other FeAl surfaces no 1×1 diffraction patterns are observed in the whole temperature range up to 1000°C, indicating the absence of bulk terminated surfaces, in contrast to the equivalent surfaces of NiAl. An incommensurate surface alloy of FeAl 2 stoichiometry finally forms on the most densely packed FeAl(110) surface. The other more open surfaces are characterized by multilayer segregation of Al. While (111) and (210) surfaces develop long-range surface reconstructions (3×3 and 1×3), the (310) surface is unstable and facets. The differences in the segregation behavior between NiAl and FeAl can qualitatively be explained by a combination of different segregation models using the heat of formation of the alloy.

The effects of adsorbed H on the Mo1−xRex(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ∼0.25 ML. A (2×2) pattern is observed for H coverages around Θ∼0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.

Information about the occupied portion of the surface density of states of materials can be derived from electron-excited Auger electron spectroscopy (AES), which is a standard experimental technique in most surface science laboratories. Surface sensitive experimental techniques that provide information regarding the unoccupied portion of the surface density of states are often not standard and are not so readily available. Here we explore the possibility of utilizing the same experimental equipment as in AES to derive information about the unoccupied portion of the surface density of states from a characteristic loss spectroscopy, in particular, from core-level inelastic electron-scattering spectroscopy (CLIESS). An important application of this technique is in comparative studies. CLIESS spectra from clean surfaces of aluminum, beryllium and nickel are presented. These data were taken in the first-derivative mode using the reflection of monoenergetic primary beams of 450 eV energy for Be, and 300 eV for...

We report an X-ray scattering study of the structure and morphology of a stepped Au(001) surface between room temperature and bulk melting. At high temperatures, between 1170 and 1220 K, the vicinal Au(001) surface appears rough. Below T0 = 1170 K, there is a facetting transition: hexagonally reconstructed (001) facets appear in coexistence with unreconstructed, stepped domains. The tilt angle of the stepped domains increases with decreasing temperature, exhibiting a power-law behavior versus reduced temperature. The phase behavior of the hexagonally reconstructed domains follows that observed on nominally unstepped Au(001) surfaces. At T1 = 992 K, there is a second facetting transition at which the stepped domains reconstruct to form a rotated and corrugated, quasi-hexagonal structure. The corrugation is threefold commensurate with the average terrace width, consistent with a 2° magic vicinal phase. The orientational phase diagram of vicinal Au(001) is deduced and found to be qualitatively similar to that of vicinal Pt(001), although with intriguing differences.

The oxidation of the (100), (110) and (111) surfaces of the intermetallic compound FeAl has been investigated using LEED and XPS. On all three surfaces, oxidation at room temperature leads to the formation of an amorphous oxide film on top of an Al-depleted interlayer. The film growth can be divided into two regions of differing kinetics, i.e. the initial formation of a closed oxide film and a subsequent thickening. In the first region, the oxygen-uptake rate varies significantly with surface orientation, while in the thickening regime the uptake is the same for all surfaces. The maximum thickness as well as the composition of the oxide films were found to depend on the initial oxidation rate. At higher oxidation temperatures, ordered oxide films of around 5–8 A in thickness are formed, very similar to those observed on NiAl. Photoemission spectra from these ordered phases showed evidence for Al atoms in two different chemical environments, i.e. the well-known oxide species in the interior of the film and an additional species present at the oxide/alloy interface.

The effects of hydrogen in Ni(100) submonolayer homoepitaxy have been investigated by classical-potential total-energy calculations with semiclassical hydrogen zero-point-energy corrections. The fast diffusion of H on a flat Ni(100) surface is found to persist even when the surface is decorated with various intrinsic defects such as Ni adatoms, islands, and steps created during epitaxial growth. On the other hand, the presence of H significantly enhances the mobility of both Ni adatoms and Ni islands, resulting in a decrease in the Ni island density and a corresponding increase in the average island size. These results suggest that hydrogen will function as an antisurfactant in Ni(100) homoepitaxy. Connections to available experiments are made. {copyright} {ital 1997} {ital The American Physical Society}

The adsorption of hydrogen on the Mo(110) surface has been investigated by both low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). Results fall into three regimes. For low coverages, the LEED pattern observed remains (1 × 1), similar to the clean surface. At hydrogen coverages near 0.5 ML, a substrate reconstruction alters the observed pattern to (2 × 2). For higher coverages, fractional-order diffraction beams are extinguished, leaving a (1 × 1) pattern. Two vibrational peaks are observed in HREELS spectra for both low- and high-coverage regimes at loss energies of 88 and 136 meV, and at 99 and 152 meV, respectively. A quasi-trigonal adsorption site is consistent with the vibrational energies and diffraction patterns observed at low and saturation coverages. At intermediate coverages, a vibrational peak with differing symmetry indicates the occupation of a long bridge-site during the transition from (2 × 2) to the hydrogen-saturated (1 × 1). No evidence is found for the hydrogen “liquid” phase reported by Balden et al. [Phys. Rev. Lett. 73 (1994) 854] for H W (110) ; consequently this phase is not responsible for the giant phonon anomalies observed in both systems. Published by Elsevier Science B.V.

As a consequence of bulk truncation, the atomic structure and composition within the surface region of an ordered or random alloy may differ significantly from that of the bulk. Specifically, the response to the rearrangement resulting from the reduction of surface energy may cause elemental redistribution within the outermost layers and two- or three-dimensional atomic reordering. Furthermore, the presence of adsorbates may have a pronounced effect on these properties and cause significant changes. These properties for some surfaces of transition-metal aluminides, transition-metal carbides, and random alloys will be presented.

Clean random alloy surfaces usually exhibit non-bulklike concentration profiles. Adsorption at low temperatures does not modify these profiles as there is thermal activation for interdiffusion of surface atoms required. For strongly chemisorbed species like oxygen or carbon, however, the activation barrier can be overcome prior to desorption leading to a new equilibrium distribution of the surface constituents. This paper summarizes the results of different LEED-I(V) studies on the adsorption of hydrogen, oxygen and carbon on the (100) face of the random alloy Mo 0.75 Re 0.25. Hydrogen, which desorbs near room temperature, is found to affect the interlayer spacings only, whilst the concentration profile remains completely unchanged. In contrast, enhanced molybdenum segregation at temperatures above 600° C is shown to happen with oxygen and carbon adsorption leading to exclusive Mo-C and Mo-O bonds, respectively. The surface structures formed for high coverages differ substantially from each other. Oxygen induces a missing row structure and is bound in a “quasi” three-fold coordinated site. In contrast, carbon is adsorbed in five-fold hollow sites at the surface and in six-fold coordinated octahedral subsurface sites.

The effect of adsorbed H on the Mo 0.95 Re 0.05 (110) surface has been investigated. Results obtained from low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and angle-resolved ultraviolet photoemission spectroscopy (ARUPS) are presented. A (2 × 2) LEED pattern is observed for H coverages around Θ ∼ 0.5 ML and is attributed to reconstruction of the substrate. At higher coverages, a (1 × 1) pattern is observed. Two peaks are observed at loss energies of 99 and 153 meV in the HREELS spectra for the H-saturated Mo 0.95 Re 0.05 (110) surface consistent with a quasi-trigonal adsorption site. Both peaks show an isotopic shift, confirming that they are due to hydrogen vibrational modes. A two-dimensional Fermi surface was determined using ARUPS for the H-saturated Mo 0.95 Re 0.05 (110) surface. Fermi-surface nesting vectors were observed at the momentum predicted by theoretical calculations for H-saturated Mo(110) where a phonon anomaly is observed.

The absolute coverages of atomic deuterium adsorbed on the Cu(111) surface at a temperature of 100 K have been determined using nuclear reaction analysis (NRA), and correlated with thermal desorption spectra (TDS) and deuterium-induced low energy electron diffraction (LEED) patterns for Cu(111). It was found that the adsorption of deuterium saturates at a coverage of 0.67 ± 0.03 ML, significantly lower than that observed for fcc(111) transition metal surfaces. The D coverage for a (2 × 2) LEED pattern was measured to be 0.5 ML, in common with those of H Ni (111) and H(D) Ag (111) . No bulk absorption was observed. The low saturation coverage of D on Cu(111) is attributed to the combination of a low heat of adsorption and D-D repulsion on the surface.

The temperature dependence of the equilibrium surface morphology of a stepped Pt(111) surface, miscut by 1.8° towards [112], has been studied by X-ray scattering techniques between 300 and 1900 K. Below Tc = 1389 K, the surface is found to be uniformly stepped and unreconstructed. Above Tc, the surface transforms into a periodic array of wide terraces and narrow terraces. Unlike macroscopic facets, the wide terraces are mesoscopic in extent, ranging from 230 A at 1389 K to 94 A at 1900 K. Reflectivity measurements suggest that multiple-height steps are not prevalent. The wave vector characterizing the array (δ) follows a power law versus reduced temperature [t = (T - T c ) T c ] : δ = δ 0 t \ gb with \ gb = 0.66 ± 0.05 .

We report on measurements of scattered projectile angular and charge state distributions for 3.75 keV/amu O{sup {ital q}+} (3{le}{ital q}{le}8) ions surface channeled along the {l_angle}1{bar 1}0{r_angle} direction of a Au(110) single crystal target. Projectile trajectory calculations indicate that the projectiles spend less than 30 fs within 2 A of the topmost Au surface layer. Nevertheless, almost complete charge equilibration is observed in the scattered projectile charge state distributions. It is concluded that, {ital already} {ital at} {ital the} {ital target} {ital interface}, strong screening effects are present which make possible direct capture to the projectile {ital L} and {ital M} shells in relatively large impact parameter collisions.

The structure and composition of the (100) and (110) oriented surfaces of the ordered alloy FeAl were investigated by LEED and AES. Following sputtering and annealing, c(2×2) and (1×1) LEED patterns are observed on the (100) surface. The c (2 × 2) is attributed to an ordered arrangement of Al atoms on top of a complete Fe layer, while the ( 1 × 1 ) corresponds to the Al terminated surface, as determined previously. On the ( 110) oriented surface, a ( 2 × 1 ) diffraction pattern, an incommensurate superstructure, and severe streaking are observed with LEED upon annealing. For the (2 × 1) structure, a missing-row model is compatible with a stoichiometry of Fe2Al. The incommensurate superstructure and streak phase are associated with a near-surface aluminum concentration > 0.5, indicating segregation of Al. In contrast to the NiAl (110) surface, no (1 × 1) , which would indicate the formation of the bulk-like terminated surface, is observed with LEED. Annealing both surfaces over 800°C leads to evaporation of Al.

The electronic band structure of the monolayer graphite/TaC(111) system was examined using XPS, AES, and ARUPS and the results were compared with predictions of the rigid-band model to determine its validity. The measured graphite σ1 band energy relative to the Fermi energy is −23.0 eV at \gG and the π1 band energy is −10.3 eV at \gG and −3.4 eV at K K. By comparing these values to the results of a linearized augmented-plane-wave calculation of the electronic structure of an isolated graphite monolayer, the qualitative picture of charge transfer from the substrate into the graphite overlayer is confirmed, but the rigidband model is not sufficient to explain the observed binding energy differences between experiment and theory. The degree of hybridization between the electronic bands of TaC and the graphite monolayer is therefore not negligible and should be included in descriptions of the electronic band structures of such systems.

We have determined absolute coverages of deuterium on Cu(100) using the reaction D(3He, p)4He and found a saturation coverage of one deuterium atom per surface copper atom. LEED observations reveal no superlattice structures below $ 0.45 ML and four different superlattice structures above this coverage.

We present and apply an extension of the perturbation method tensor LEED in order to handle surface stoichiometry, i.e. the substitution of surface atoms by chemically different species. This avoids the need for new full dynamical calculations for surfaces of different chemical composition and allows fast access to their LEED intensities. This “chemical tensor LEED” is applied to the chemically disordered alloy Mo x Re 1− x (100) for x = 0.75, 0.85 and 0.95 for which LEED intensities were measured. Oscillatory layer dependent segregation is found in agreement with conventional LEED analyses. Even small Re concentrations strongly modify the structure of the pure Mo(100) surface, i.e. the surface reconstruction is lifted and multilayer relaxation effects are increased.

A simple model is established to describe diffraction phenomena (LEED) from a unidirectionally reconstructed TaC(110) surface. By comparing diffraction data with results of detailed simulations, the reconstruction is found to consist of alternate (100) and (010) facets propagating periodically along the [110] direction like an irregular ridge-and-valley grating, with (110) terraces completely absent. The irregularity is due to variation in facet sizes, and the mean separation is determined to be L = 6 lattice spacings. The validity of this model is supported by STM imaging results which confirm the structure obtained from the LEED-data analysis.

The structure and phase behavior of the clean Pt(111) surface has been studied between 300 K and 0.92 T m via x-ray scattering. The surface is unreconstructed at low temperatures (T

The specular reflectivity from the (110) surface of a Cu single crystal has been measured for temperatures between 300 and 1100 K. At lower temperatures, the reflectivity is well described by a model which yields a contraction of the spacing between the first and the second surface layers by 8% relative to the bulk layer separation. The fits also show large surface-normal vibrational amplitudes within the first few atomic layers. The line shape of the scattering transverse to the specular direction has two components below 1000 K, and evolves with increasing temperature. However, our data do not preclude the existence of a surface-roughening transition.

The structure of Cu(110)-(2[times]3)N has been investigated with x-ray reflectivity measurements of nine independent surface rods. Observed reflectivities ranging over five orders of magnitude were compared with calculations based on structural models. A pseudo-(100) reconstruction consisting of four [1[bar 1]0] copper rows in the outermost layer for every three of the bulk is favored both by symmetry and by an [ital R]-factor analysis. An excellent fit, with an [ital R] factor of 0.0051, is obtained for this model by including a large expansion of the interlayer spacing (57% greater than bulk) and large corrugation amplitude (50% of the bulk interlayer spacing) in the outermost layer. These values can be understood by considering a hard-sphere model of the surface structure. The corrugation results in two raised and two lowered rows of copper atoms in the outermost layer and extends to several layers into the substrate (amplitudes from 9% to 22% of the bulk interlayer spacing). Vibrational amplitudes in the reconstructed layer are anisotropically enhanced by a factor of 2.8 normal to the surface and by 1.3 within the surface plane over bulk values. Nitrogen atoms are located 0.75 A above the copper layer, and form a nearly ([radical]2 [times] [radical]2 )more » [ital R]45[degree] structure with respect to the reconstructed copper. This model is shown to be consistent with most previous experimental observations.« less

We summarize the results of x-ray scattering studies of the Au(001) and Pt(001) surfaces between 300 K and their respective bulk melting temperatures (T{sub m}). Both surfaces exhibit three distinct structural phases. At high temperatures (0--88 T{sub m} < T < T{sub m} ) both surfaces are disordered. At a temperature of T/T{sub m} {minus} 0.88 there are reversible phase transformations to incommensurate, corrugated-hexagonal phases. Below T/T{sub m} {minus} 0.8 hexagonal domains rotate with respect to the substrate orientation. In Pt, the rotational transformation is continuous and shows mean-field behavior. In Au, the rotational transformation is mainly discontinuous.

The geometric structure of the TaC(310) surface has been investigated using high-resolution low-energy electron diffraction. An ideally truncated TaC(310) surface has a staircase geometry where each unit consists of a (100) terrace, three row spacings wide, separated by a single-height step. However, it is determined that the surface is reconstructed with both terrace width and step height tripled on average while the surface still retains the (310) orientation. In addition, a small number of double-height steps are found to coexist with the triple-height steps. Possible physical origins of these observations are discussed.

Using medium‐energy ion scattering with channeling and blocking, we have investigated the (1×2) reconstructions of Ni(110) and Au(113) (stabilized by submonolayer amounts of K and Ca, respectively). Our data contain evidence that the two surface structures correspond to missing‐row type reconstructions. Monte Carlo simulations show that the first interlayer spacing of the (1×2)‐Au(113) surface undergoes a large contraction (−18.6%), while the first interlayer spacing of (1×2)‐Ni(110) undergoes a small, unexpected expansion (+3%). Although the symmetry of Au(113) allows a lateral registry shift, none is found. Our structural models are discussed in terms of the energy balance between competing surface electronic charge densities.

The structure of the clean Pt(111) surface has been studied between 300 K and 0.92${\mathit{T}}_{\mathit{m}}$ (${\mathit{T}}_{\mathit{m}}$=2045 K) via x-ray scattering. The surface is unreconstructed for temperatures less than 0.65${\mathit{T}}_{\mathit{m}}$, but reconstructs at higher temperatures to form a layer isotropically compressed and incommensurate with the underlying bulk (111) planes. A disordered arrangement of discommensurations separates regions with ideal fcc stacking from regions with faulted stacking. With increasing temperature, both the compression of the surface layer and the orientational order of the discommensurations increase.

The projectile {ital K} Auger electron emission observed during grazing collisions of 60-keV N{sup 6+} ions with Au(110) is characterized by two components with strikingly different dependences on perpendicular velocity. A fast'' component dominates and is ascribed to subsurface'' emission. This component was satisfactorily modeled using a Monte Carlo simulation of the projectile trajectories {ital after} surface penetration. A slow'' component appears at very small angles and has a time dependence characteristic of the neutralization-deexcitation cascade predicted to occur above the surface {ital prior} to bulk penetration.

We present N7VV (4f7/25d5d) Auger spectra from TaC(111) taken in time coincidence with photoemitted bulk and surface‐shifted 4f7/2 core‐level electrons, and compare these data with self‐convolutions of calculated valence band d densities of states (SCDOS) for the surface (first) and bulk (exponential sum of second, fourth, and sixth subsurface) atomic layers of a 13‐layer slab. The Ta 4f levels of TaC(111) and Ta(100) both exhibit large surface core level shifts, but of opposite sign. Therefore, we compare the TaC(111) data and calculations with analogous N7VV Auger coincidence spectra and calculated SCDOS that we have previously published for Ta(100). The bulk Ta(100) and surface TaC(111) spectra show favorable agreement with the calculations; the surface Ta(100) and bulk TaC(111) spectra, on the other hand, exhibit marked disagreement with the calculations. These discrepancies are consistent with and provide added support for the presence of our proposed1 surface‐to‐bulk (bulk‐to‐surface) interatomic Au...

Different submonolayer phases of bismuth on copper (100) have been investigated with low‐energy electron diffraction and Auger electron spectroscopy. The room temperature observations, made after depositing Bi onto well annealed Cu, agreed with those reported previously. Below room temperature, a low coverage c(2×4) phase was observed. With increasing Bi coverage, this phase was observed to coexist with the c(2×2) phase, until finally, only the c(2×2) phase was observed. Restructuring of the Cu substrate into {210} facets, at 26°34’ angle with the original (100) surface, was obtained by depositing Bi onto roughened Cu(100), and subsequently annealing. A possible explanation for the nucleation and growth of these facets is given.

X‐ray diffraction is used to study the phase behavior and structure of the Au(001) surface between 300 K and the triple point. For T>1170 K, the x‐ray reflectivity is consistent with a thin, disordered surface film. At T=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.

Recent measurements of electron spectra for slow multicharged N ion-surface collisions are presented. The emphasis is on potential emission, i.e., the electron emission related to the neutralization of the ions. When using N ions that carry a K shell vacancy into the collision, characteristic K Auger electron emission from the projectiles is observed as well as, for specific surfaces, target atom Auger transitions (resulting from vacancy transfer). Measurements of the intensity of these Auger transitions as a function of the time the ions spend above the surface can serve as a useful probe of the timescales characterizing the relevant neutralization processes. This technique is elucidated with the help of some computer simulations. It is shown that neutralization timescales required in the atomic ladder picture, in which neutralization takes place by resonant capture followed by purely intra-atomic Auger transitions, are too long to explain our experimental results. The introduction of additional neutralization/de-excitation mechanisms in the simulations leads to much better agreement with the experiments.

Using angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation, the absolute work functions of four (100) surfaces having estimated compositions TaC1.0, TaC0.5, HfC1.0, and HfC0.6 were determined to be 4.38, 4.73, 4.63, and 3.87 eV, respectively. The results are interpreted qualitatively by considering the work function as a sum of two terms, one which arises from the presence of a surface dipole layer and another which is associated with the Fermi level. The difference in work-function behavior (its increasing with carbon content x for HfCx but decreasing with x for TaCx) is shown to be associated with a difference in Fermi-level behavior.

Chemisorption of nitrogen on Cu(110) has been studied using XPS, LEED, AES, and TDS. Gas phase nitrogen was activated and accelerated to 200 eV to promote interaction with the surface. Coverages were obtained from XPS N 1s areas and calibrated using a p(2 × 1)O structure. After exposures at 300 K, nitrogen coverages saturated near 1 ML. The width of the N 1s core indicates that only one site is occupied for coverages below 2 3 ML, but additional site(s) are occupied at higher coverages. Annealing the crystal to 650 K produced a stable (2 × 3) structure consisting of 2 3 ML of nitrogen in a single site. The lineshape of the nitrogen Auger signal is independent of coverage or surface structure. Annealing the crystal to 800 K removes all nitrogen from the surface, by a combination of desorption and absorption into the bulk.

Core-level binding energies (BE's) and valence-band structures, determined with x-ray photoelectron spectroscopy, and C {ital KVV} Auger spectra were obtained for TaC{sub {ital x}} (0.5{approx lt}{ital x}{approx lt}1.0) and HfC{sub {ital x}} (0.6{approx lt}{ital x}{approx lt}1.0). In TaC{sub {ital x}} the metal 4{ital f} BE, C 1{ital s} BE, and the BE of the main {ital p}-{ital d} valence-band peak decreased (moved toward the Fermi level) as {ital x} decreased; in HfC{sub {ital x}} these BE's increased as {ital x} decreased. In TaC{sub {ital x}} the largest BE shift with changing {ital x} was for the metal 4{ital f} BE; in HfC{sub {ital x}} it was for the C 1{ital s} BE. For TaC{sub {ital x}} the relative intensity in the valence-band spectra between 0 and 2 eV BE changed significantly as {ital x} decreased, eventually becoming the dominant intensity of the spectrum; for HfC{sub {ital x}}, deviations from {ital x}{congruent}1 did not change the shape of the valence-band spectra appreciably. The data from both sets of materials are explained in terms of changes in the charge distributions accompanying changes in {ital x}. And although the two sets of spectroscopic behavior are quite different, the changes in the charge distributionsmore » we are proposing are nevertheless quite similar. In particular, it is proposed that, as {ital x} decreases for either HfC{sub {ital x}} or TaC{sub {ital x}}, the electron charge increases in the vicinity of metal ions and decreases in the vicinity of carbon ions. An analysis of the C {ital KVV} Auger spectra is consistent with this interpretation in that the analysis suggests a reduction in C 2{ital p}-electron occupancy per carbon atom with decreasing {ital x}. The 4{ital f} BE's of Ta and Hf in the elemental metals also were measured and are used to discuss changes in the charge distributions that occur upon formation of HfC{sub {ital x}} and TaC{sub {ital x}}.« less

We present an x-ray scattering study of the structure and strain distribution of the orthorhombic (20X20) reconstruction of a one monolayer Ag film grown on Ru(0001) at T=690 K. The application of very-fast simulated annealing algorithms to the measured x-ray intensities leads to a model in which the Ag film forms a two-dimensionally modulated hexagonal network of dislocations separating smaller patches of ideal hcp and faulted fcc adlayer stacking. Calculations of the in-plane strain distribution based on the model reveal a simple stripelike structure with a 4.5-nm periodicity, reminiscent of what is observed on Cu/Ru(0001).

Using scanning tunneling microscopy, we study the step configuration on TaC(910) which is vicinal to the (100) plane, miscut 6.34{sup o} toward the [010] direction. After annealing at {approx}2100{sup o}C, the surface is dominated by double-height steps which are bunched between relatively long intervening (100) terraces. The step-separation distribution is very skewed and sharply peaked. Monte Carlo simulations show that a strong, medium-range, repulsive step interaction A/x{sup 2} must be combined with a weak, long-range, attractive interaction, -B/x{sup 1.0{+-}0.5} to fit the measured distribution. Possible physical origins for these interactions are discussed.

We present a study of the {100} biaxially textured Ni (001) surface and oxide seed layer nucleation by in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations revealed the existence of a c(2×2) superstructure on the textured Ni surface due to segregation of sulfur contained in the bulk metal. The sulfur superstructure promotes the epitaxial (002) nucleation of seed layers such as Y2O3-stabilized ZrO2 (YSZ) and CeO2 on the metal and optimizes the biaxial texture necessary for high Jc superconductors on RABiTS.

Upon sputtering and subsequent annealing, the subsurface region of the (100) surface of FeAl undergoes a transition to a practically stable and ordered Fe{sub 3}Al slab. It is capped by an Al top layer forming a rather complex interface to the bulk FeAl. This shows that routine surface preparation procedures with sputtering involved can lead to substantial restructuring extending deep into the surface. {copyright} {ital 1996 The American Physical Society.}

We have made systematic measurements of scattered projectile charge-state distributions for multicharged He, N, Ne, Na, and Ar ions grazingly incident on a Au(110) single-crystal target. For projectiles whose neutral binding energies Lie below the valence band (N, Ar, He, and Ne), observed 1+ charge fractions were small at low velocities and steeply increased above well-defined threshold velocities. However, for Na projectiles with neutral binding energy above the Fermi level, a large (about 84%) 1+ charge fraction was found. The velocity dependence of the Na neutral fraction shows a ''kinematic resonance'' due to the virtual population of electronic states above the Fermi level in the projectile rest frame. The measured sample temperature dependence of the scattered 1+ charge fraction for Ne9+ projectiles incident along the [110] surface channeling direction reveals a significant decrease in projectile neutralization once the (2X1)-(1X1) phase transition temperature of the Au(110) surface at 650 K has been reached.

We have studied the morphologies of the TaC(310), (210), and (110) surfaces using scanning tunneling microscopy. Heating the crystals to high temperatures activates a faceting of these surfaces into a hill-and-valley structure consisting of enlarged (100) terraces and (010) step walls. Step-separation distributions obtained from these surfaces can be well fit by sharp Gaussians and are much narrower than predicted for the noninteracting terrace-step-kink model, indicating a strong repulsive interaction exists between steps on the faceted surfaces. This faceting is suggested to be driven by a decrease in the total step repulsive energy through a reduction of the total number of step pairs. {copyright} {ital 1996 The American Physical Society.}