Low-energy Electron Diffraction Structure Research Articles

On the coverage and structure of the [formula omitted]-R30 surface phase

The study addresses the nature and coverage of the (√3×√3) surface phase of the Ag Si(1 1 1) interface that has remained unresolved in literature. Ag is adsorbed in ultra-high vacuum on to the (7 × 7) reconstructed Si(1 1 1) surface with a deposition rate of 3.3 × 10 12 atoms cm −2s −1. The initial stages of growth and intermediate equilibrium phase formation are determined by using Low Energy Electron Diffraction (LEED), Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). For depositions carried out at higher temperatures (> RT), several plateaus in the Auger uptake curve at 0.33, 0.66 and 1.0 ML coverages of Ag with the (√3×√3)-R-30° LEED structures are formed. It is observed that a minimum coverage (θ) of 0.33 ML is required for the formation of the (√3×√3) phase. The (√3×√3) phase obtained at higher substrate temperatures causes the reappearance of the p-electron related states, that were quenched by 1.0 ML adsorption at RT. The EELS obtained at these (√3×√3) plateaus are observed to be different, indicating different local structural surface phases for the same long range order.

The epitaxial growth of Ag on Si(111)-(7 x 7) surface and its (√3x√3)-R30 surface phase transformation

Ag is adsorbed in ultra-high vacuum on to the (7x7) reconstructed Si(111) surface with submonolayer coverage control with a deposition rate of 3-3 x 1012 atoms/cm2/sec. The initial stages of growth and intermediate equilibrium phase formation are determined by using low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) for structural information, and auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS) for composition and interaction analyses. Room temperature (RT) adsorption results in the nearly epitaxial (1 x 1) surface phase growth in the simultaneous multi-layer growth mode. The quenching of the dangling bond states during adsorption is observed by monitoring thep-character of the Si LVV auger peak. For depositions carried out at high temperatures (HT), several plateaus in the auger uptake curve with the (√3 x √3)-R30° LEED structures are formed. It is observed that a minimum coverage of 0–33 monolayer (ML) is required for the formation of the (√3 x √3) phase and this phase causes the reappearance of thep-electron-related states that were quenched by 1.0 ML adsorption at RT. However the (√3 x √3) is observed for higher coverages (0.66 and 1.0 ML) also. The polar angle anisotropy of Si(2p) emission in XPD indicates the rearrangement of substrate Si atoms for the formation of the (√3 x √3) phase. The EELS data also shows relevant changes due to adsorption of Ag at RT and upon annealing. The results suggest the importance of controlled deposition parameters, the lack of which may have kept the determination of the nature and coverage of the (√3 x √3) surface phase unresolved in literature.

The growth of magnesium chloride monolayer and multilayer structures on different transition metal (Pt, Pd, Rh) single crystals with varied orientations

The growth of magnesium chloride (MgCl 2) monolayer and multilayer structures on a variety of transition metal single crystal surfaces [Pd(111) and (100), Pt(111) and (100)-hex and Rh(111)] has been studied. Low energy electron diffraction (LEED) has been employed as a probe of the long range order at both monolayer and multilayer coverages while temperature programmed desorption (TPD) provided information on the strength of adsorbate-substrate interactions. LEED results indicated that MgCl 2 growth at both monolayer and multilayer coverages is dependent upon the magnitude of the monolayer adsorbate-substrate interactions which are themselves a function of the underlying single crystal template. LEED results indicated that, on each transition metal surface studied, the MgCl 2 monolayer structures act as a template for subsequent multilayer structures. On Pt(111) and Pd(111), identical, well ordered LEED structures at both monolayer and multilayer coverages, were produced. The commensurate (4 × 4) and (√13 × √13)R13.9° monolayer structures were associated with expansions and contractions in the MgCl 2 unit cell dimension, respectively. These structural changes were correlated with the strength of the monolayer adsorbate-substrate interaction, which on these surfaces lead to distinct monolayer TPD features. In contrast, MgCl 2 growth on Pt(100)-hex and Pd(100) exhibited more complex LEED patterns associated with various degrees of ordered and rotationally disordered domains. This was attributed to a reduction in the magnitude of monolayer adsorbate-substrate interactions, correlated by TPD measurements. Experiments on Rh(111) indicate that the factors responsible for ordered MgCl 2 growth on this surface differ from those observed on Pt and Pd single crystals. In general, better matching between the substrate and adsorbate unit cell dimensions leads to enhanced adsorbate-substrate interactions which are responsible for the degree of ordering at monolayer and subsequent multilayer coverages.

Carbon nucleation on Si(100) using a negative carbon ion beam

The initial nucleation stages of carbon film on Si(100) substrates were investigated by low energy electron diffraction (LEED) and Auger electron spectroscopy. In the initial stages, energetic carbon ions lead to a phase transformation from a two domain p(2×1) to a (1×1) LEED structure pattern after about 6×1015 carbon ion/cm2 dose (3 atomic layers). At room temperature, the energetic carbon ions are deposited as silicon carbide (SiC) up to a thickness of about 10 atomic layers. As the deposition temperature increases to 300 °C, the thickness of the SiC interlayer increases to 30 atomic layers. An increased carbon ion dose leads to the formation of sp3 or sp2 rich carbon film depending on the carbon ion energy and the deposition temperature. Higher energy (150 eV) C− ion beams and lower deposition temperatures (room temperature) produce sp3 rich carbon films. At 150 eV ion energy, the transition temperature from sp3 to sp2 rich carbon film during deposition is about 150 °C. Lower energy (<50 eV) C− ion beams at room temperature produce sp2 rich carbon films. During postdeposition annealing, the sp3 rich carbon film deposited at room temperature was converted to an sp2 rich carbon film above 740 °C.

Phase transition and atomic structure of an Fe 3Si(100) single crystal surface

The surface of Fe 3Si(100) was investigated by low-energy electron diffraction (LEED) and Auger electron spectroscopy. Three different phases were observed depending on the sample annealing temperature. Two (1×1) structures with the full 2D symmetry of the DO 3-type crystal surface can be prepared at temperatures T≤400°C and T≥680°C. The two phases exhibit different LEED spot intensities, indicating that their structures are different. In the temperature regime between 525 and 625°C only a subset of diffraction spots is visible, corresponding to a c(1×1) periodicity. In this phase obviously the chemical order of the DO 3 structure is lost in the surface region. For the low temperature (1×1) phase the LEED structure analysis favours a model consisting of a bulk-like layer stacking sequence with an additional silicon layer on top of a mixed FeSi layer.

Surface atomic geometry of Si(001)-(2×1): A low-energy electron-diffraction structure analysis

The reconstruction of the Si(001)-2×1 surface consists of asymmetric and buckled Si dimers. The vertical separation between the up and the down atom within the dimer is about 0.72±0.05 A and the dimer bond length of 2.24±0.08 A has been found to be slightly smaller than the Si-Si distance in the bulk. The tilt of the dimer is 19±2°. The formation of Si dimers induces pronounced distortions in the substrate that were detectable down to the fifth Si layer. The structure determination is based on two independent low-energy electron-diffraction data sets taken in two different laboratories. The structural results agree well within the error limits, though noticeable differences occur between the experimental data sets. These differences in the experimental data can possibly be attributed to different preparation procedures.

Interaction of Hydrogen and Methane with Inp100) and GaAs(100) Surfaces

The interaction of thermally activated hydrogen atoms and methane molecules with Inp100) and GaAs(100) surfaces was studied by X-ray and UV-induced photoelectron spectroscopy (XPS, UPS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and desorption spectroscopy (DS). In most cases the interaction causes a strong decomposition of the surface due to a preferential loss of the group-V and an enrichment of the group-III elements. Hydrogenation of the clean Inp100) 4×2 surface can be divided into three stages. First, there is a saturation of dangling bonds, where the 4×2 reconstruction is preserved. Then, there is a breaking of the In dimers present at the surface, resulting in a 4×1 LEED structure. Finally, a loss of phosphorus and the build-up of metallic indium droplets follows. Bombardment of Inp100) surfaces with methane ions results in the formation of In–C and P–C species. Hydrogen exposure of GaAs(100) surfaces is more effective, since it changes the surface structures already at the initial stages of interaction. This is corroborated by the HREELS and DS data, which give strong evidence for a preferential loss of arsenic. Desorption spectra were taken during the hydrogen exposure and they show for low hydrogen pressure directly the desorption of AsH3. For high hydrogen pressure (p ≥ 1×10−4 Torr) GaH3 is detected in addition. The intensity ratio of desorbing species (AsH3/GaH3) decreases with increasing hydrogen pressure. After extrapolation a value of one results at a pressure of p = 1×10−3 Torr. Models for the interaction of hydrogen and methane with Inp100) and GaAs(100) are discussed in detail.

Dimer bond geometry in D/Ge(100)-(2 x 1): A low-energy electron-diffraction structure analysis.

The asymmetry of the Ge dimer in the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction of Ge(100) is removed upon adsorption of deuterium D. The $R$-factor analysis indicates a slight remaining asymmetry which is attributed to the coexistence of bare and D-covered dimers. The Ge-Ge bond length of 2.4(2) \AA{} in the dimer does not change within the error limits when compared to the clean surface. The D atoms bond on top of the Ge atoms, exhibiting a Ge-D bond length of 1.6(2) \AA{}.

LEED structure determination of the (100)-surface of a CoSi 2 crystal and a CoSi 2 film grown epitaxially on Si(100)

The surface structures of CoSi 2(100) for a bulk crystal and a 12 Å thin film grown epitaxially on Si(100) are compared by means of quantitative low energy electron diffraction (LEED). Similar LEED patterns are observed for both systems corresponding to a c(2 × 2) periodicity with respect to the Si(100) surface. Also, intensity versus energy curves are very similar for the film and bulk samples. Consequently, we derive practically the same best-fit model when fitting the experimental data with spectra obtained from full dynamical LEED calculations: the CoSi 2 surface is arranged in CaF 2 structure, terminated by one full Si layer with an additional c(2 × 2) Si overlayer of half monolayer coverage. The top silicon adatoms reside in four-fold symmetric hollow sites. The unfavourable energetics of this rather peculiar bonding geometry are obviously overcompensated by the energy gain from the saturation of dangling bonds on the surface. The geometry found is consistent with the chemical passivity of the surface.

The growth of aluminum on a rhenium (101̄0) surface

Al thin films were deposited at 873 K on a Re(101̄0) surface and examined by means of low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and work function (ΔΦ) measurements. In the submonolayer range (0< Θ Al<0.6), we find two Al LEED structures, a p(3 × 1) at Θ Al ≈ 0.3, and a p(2 × 1) at Θ Al ≈ 0.5. In the same coverage range, a single TD state denoted as β 3 appears having a desorption energy of 440 kJ/mol. Close to Θ Al ≈ 1 another (2 × 1) LEED phase and a second TD state ( β 2) develop which exhibits a desorption energy of ∼ 375 kJ/mol. This (2 × 1) phase is then replaced up to Θ Al ≈ 2 by a clear c(2 × 2) phase, followed by a p(2 × 2) and another c(2 × 2) structure at still higher Al coverages. In this multilayer range, the TD spectra are dominated by a single desorption state denoted as β 1 which reflects a fractional-order kinetics and yields an activation energy of ∼ 345 kJ/mol. The deposition of Al reduces the Re work function by ∼ 1.2 eV. A plot of the Re versus Al Auger intensity reveals a clear break at Θ Al ≈ 1 and a less pronounced break around Θ Al ≈ 2, while for higher Al coverages the Re AES intensity decays unspecifically, which suggests deviations from a plain layer-by-layer growth. Up to ∼ 8 deposited monolayers the structure of the Al film is determined by the crystallography of the Re(101̄0) surface and by relaxation processes to overcome the misfit.

Changes of the Pt 3Co(100) surface stoichiometry with NO adsorption

An ordered alloy surface of Pt 3Co(100) has been studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and temperature programmed reaction (TPR). The clean surface exhibited a c(2 × 2) LEED structure, when the sample was carefully annealed at temperatures lower than the critical temperature for order-disorder transition of the bulk. With NO adsorption, the surface structure was reconstructed to a (1 × 1) LEED structure and was changed back to the c(2 × 2) structure by reaction with hydrogen at 500 K. From the results of LEED and AES studies combined with the H 2 + D 2 → 2HD reaction, it was deduced that the reconstructed (1 × 1) surface is terminated by a Pt-enriched layer, and that the clean c(2 × 2) surface reveals an ordered PtCo outmost layer. Furthermore, compared with the reaction of N 2O and the adsorption of nitrogen activated by high frequency discharge, it was shown that the surface reconstruction originated from adsorbed surface nitrogen and not from surface oxygen.

Incommensurate structures and epitaxial growth of Li on Ru(0001): A quantitative low-energy electron-diffraction study.

The structural properties of Li films on a Ru(0001) surface during initial growth were determined employing the technique of low-energy electron diffraction (LEED). For submonolayer coverages between ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{Li}}$=0.61 and ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{LI}}$=0.78, incommensurate structures oriented along high-symmetry directions of the substrate were observed. Fully dynamical LEED calculations of a (5\ifmmode\times\else\texttimes\fi{}5) structure modeling the Li overlayer at ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{Li}}$=0.64 revealed a Li-Ru interlayer spacing of 2.17\ifmmode\pm\else\textpm\fi{}0.10 \AA{}. The computational effort for solving this structure could be significantly reduced by using a (5\ifmmode\times\else\texttimes\fi{}1) unit mesh instead of the (5\ifmmode\times\else\texttimes\fi{}5) unit cell without losing structural information. The first Li layer is completed at a coverage of 0.78. In the multilayer regime, the Ru(0001)-Li system exhibits also long-range order as indicated by adsorbate-induced hexagonal LEED patterns. LEED structure analyses of Li films with thicknesses of two and three layers revealed that the Li film is grown fcc(111)-like on Ru(0001). The interlayer spacings are ${\mathit{D}}_{\mathrm{Li}\mathrm{\ensuremath{-}}\mathrm{Li}}$=2.35\ifmmode\pm\else\textpm\fi{}0.10 \AA{} and ${\mathit{D}}_{\mathrm{Li}\mathrm{\ensuremath{-}}\mathrm{Ru}}$=2.35\ifmmode\pm\else\textpm\fi{}0.10 \AA{}; the Li hard-sphere radius turns out to be 1.52\ifmmode\pm\else\textpm\fi{}0.05 \AA{}, close to the metallic Pauling radius.

Adsorption of sodium and potassium on a gold(100) surface: An example of alkali-metal-induced deconstruction.

We have studied the interaction of Na and K with a (5\ifmmode\times\else\texttimes\fi{}20) reconstructed Au(100) surface between 130 and 1150 K using low-energy-electron-diffraction (LEED), x-ray-photoelectron spectroscopy (XPS), work-function change (\ensuremath{\Delta}\ensuremath{\Phi}), and thermal-desorption spectroscopy (TDS) measurements. Our results indicate a strong temperature dependence of the interaction phenomena: At 300 K, we find for both Na and K adsorption the lifting of the inherent (5\ifmmode\times\else\texttimes\fi{}20) reconstruction at a critical coverage ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{crit}}$\ensuremath{\approxeq}0.2 ML, and, at somewhat larger \ensuremath{\Theta}, a thermally activated transformation to a (likewise reconstructed) (1\ifmmode\times\else\texttimes\fi{}2) phase of the missing-row (MR) type. Higher coverages lead to different LEED structures depending on the alkali metal and the deposition temperature. All structural changes are sensitively mirrored in the \ensuremath{\Delta}\ensuremath{\Phi}-\ensuremath{\Theta} relations, which reveal intermediate work-function maxima right at ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{crit}}$. These maxima are superimposed on the typical descending \ensuremath{\Delta}\ensuremath{\Phi}-\ensuremath{\Theta} curves usually observed with alkali-metal adsorption on metal surfaces (\ensuremath{\Delta}${\mathrm{\ensuremath{\Phi}}}_{\mathrm{m}\mathrm{a}\mathrm{x},\mathrm{N}\mathrm{a}}$=-1.9 eV; \ensuremath{\Delta}${\mathrm{\ensuremath{\Phi}}}_{\mathrm{m}\mathrm{a}\mathrm{x},\mathrm{K}}$=-2.6 eV). At 130 K, the formation of the (1\ifmmode\times\else\texttimes\fi{}2) MR reconstructed phases cannot be observed anymore, and accordingly no intermediate \ensuremath{\Delta}\ensuremath{\Phi} maxima appear.

Observations of coverage- and temperature-dependent surface structures formed upon Li deposition on Cu(110).

A systematic study of surface structures of Li-covered Cu(110) was performed by means of low-energy electron-diffraction (LEED) and Auger electron spectroscopy techniques. The experiments were carried out with increasing deposition of Li both at low temperature (90 K) and room temperature (300 K). At 90 K a series of LEED patterns, (3\ifmmode\times\else\texttimes\fi{}1), (2\ifmmode\times\else\texttimes\fi{}1), (3\ifmmode\times\else\texttimes\fi{}1), and successive complicated structures were observed with increasing Li coverage. The sequence of the LEED patterns was interpreted as simple overlayer structures of Li in terms of a continuous increase of the packing density of deposited Li atoms sitting in the trough along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction of the Cu(110) surface. Marked differences in the sequence of the LEED structures were observed for observations at 300 K; the sequence of the structures was (1\ifmmode\times\else\texttimes\fi{}2), (1\ifmmode\times\else\texttimes\fi{}1), (4\ifmmode\times\else\texttimes\fi{}1), (5\ifmmode\times\else\texttimes\fi{}1), and (n\ifmmode\times\else\texttimes\fi{}1) where n8 with an increase of Li coverage. The first (1\ifmmode\times\else\texttimes\fi{}2) structure was assigned to a missing-row-type restructuring of the substrate Cu atoms. The structure models for the (4\ifmmode\times\else\texttimes\fi{}1), (5\ifmmode\times\else\texttimes\fi{}1), and so on were discussed in terms of the formation of Li-Cu surface alloys proposed previously by Tochihara and Mizuno for the Li/Cu(100) system.

Nonpseudomorphic and surface-reconstructed ultrathin epitaxial fcc Fe films on Cu(100).

A high-precision low-energy electron-diffraction (LEED) structure analysis for an eight-monolayer ultrathin fcc Fe film epitaxially grown on Cu(100) is presented. The film is free of strain, i.e., its lattice coincides with that of equilibrium fcc iron rather than with that given by the Cu substrate. The top layer is reconstructed and its distance to the second layer is expanded. Similar features hold also for films in the range 5--11 monolayers. The theory-experiment fit is the best ever achieved in a LEED structure determination with a Pendry R factor of 0.09.

LEED structure determination of tetragonal MnNi films on Ni(100).

The structure of compositionally ordered MnNi films epitaxially grown on Ni(100) by deposition of 3--4 monolayers Mn above 550 K is characterized. Using a quantitative low-energy-electron-diffraction (LEED) intensity analysis, the first two interlayer spacings have been determined to be ${\mathit{d}}_{12}$=1.76\ifmmode\pm\else\textpm\fi{}0.02 and ${\mathit{d}}_{23}$=1.86\ifmmode\pm\else\textpm\fi{}0.03 \AA{}. Third and deeper interlayer spacings of 1.87\ifmmode\pm\else\textpm\fi{}0.04 and 1.88\ifmmode\pm\else\textpm\fi{}0.04 \AA{}, respectively, have been found. A pronounced corrugation is observed for the first layer only, where the Mn atoms are displaced outwards by 0.30\ifmmode\pm\else\textpm\fi{}0.02 \AA{}. This surface layer has a composition of 50% Mn and 50% Ni. Deeper layers have a similar composition, but due to the relative insensitivity to the composition in deeper layers other concentrations cannot be excluded by the LEED analysis. The resulting film structure is indicative of the formation of tetragonal MnNi films.

A surface spectroscopic study of ultra-thin Ni films on Mo(110)

The morphology, structures and CO chemisorptive properties of ultra-thin Ni films on a Mo(110) surface have been studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS). It is found that Ni grows layer-by-layer on Mo(110) at a sample temperature of 115 K; however, upon annealing to > 600 K, Ni multilayers form three-dimensional clusters. The Ni TPD spectra show two peaks corresponding to desorption from Ni 3D clusters and a 2D uniform monolayer, respectively. Two LEED structures, (8 × 2) and (7 × 2), are observed for Ni/No(110) and are attributed to Ni overlayers with distorted Ni(111) lattices. The (8 × 2) phase is found to transform to the (7 × 2) phase at a Ni coverage of ~ 1 monolayer. This phase transition is also observable in the CO IR spectra. CO adsorbed on the (8 × 2) and (7 × 2) Ni structures shows characteristic stretching frequencies at 2087 and 2065 cm −1, respectively. In addition, the formation of the Ni(7 × 2) structure from disordered Ni is also reflected in the CO IR spectra. The enthalpy difference between the (7 × 2) and the disordered phases is determined to be 3.5 kcal/mol with the (7 × 2) phase being more stable. Upon the deposition of Ni onto CO/Mo(110) at 80 K, CO is found to spill-over from the Mo substrate onto the Ni overlayer resulting in a CO/Ni/Mo(110) structure. Finally, IR spectra of CO on Ni/Mo(110) surfaces as a function of CO coverage and annealing temperature are presented and discussed in terms of surface structure and CO mobility.

Preparation and structure of 1–8 monolayer thick epitaxial iron oxide films grown on Pt(111)

Well ordered epitaxial iron oxide layers with FeO stoichiometry were grown onto Pt(111) single crystal substrates and characterized afterwards with low-energy electron diffraction (LEED) and Auger electron spectroscopy. They were prepared by first depositing iron from the vapor onto the substrate and then oxidizing it in 10−6 Torr of oxygen. At 1 monolayer coverage, a very stable structure with a hexagonal unit cell 15% larger than the platinum (1×1) substrate unit cell is formed, exhibiting a threefold symmetric LEED pattern. This pattern is consistent with a (8×8) superstructure, as 8 FeO(111)-(1×1) lattice spacings fit onto nine lattice spacings on the Pt(111)-(1×1) substrate surface due to the lattice mismatch. Above 2 monolayer coverage a hexagonal (2×2) structure with respect to the FeO(111)-(1×1) unit cell forms, which shows a sixfold symmetric LEED pattern revealing a lattice constant which is in agreement with that of the sodium chloride structure of bulk FeO. LEED beam intensity spectra as a function of the electron energy of these two LEED structures have been measured and are discussed with respect to their symmetry and their changes as a function of the overlayer thickness. The step structure on the surface of an 8 monolayer thick iron oxide film with a FeO(111)-(2×2) LEED pattern was studied by analyzing the LEED beam profiles. We observe steps one iron–oxygen bilayer high as well as one Pt(111) layer distance high. This indicates that only one type of atom termination exists on the FeO(111)-(2×2) surface.

LEED structure determination of two ordered surface alloys: Cu(100)-c(2 × 2)Mn and Ni(100)-c(2 × 2)Mn

The structure of the Cu(100)-c(2 × 2)Mn and Ni(100)-c(2 × 2)Mn phase was determined by a quantitative low-energy electron diffraction (LEED) intensity analysis. These structures are ordered surface alloys which form after deposition of 0.5 ML Mn on the clean Cu(100) and Ni(100) surface above 270 K. The alloy layer shows a pronounced corrugation in which Mn atoms in the first layer are displaced outwards by 0.30 ± 0.02 A ̊ on Cu(100) and 0.25 ± 0.02 A ̊ on Ni(100). The interlayer distance from the Cu (Ni) atoms to second layer Cu (Ni) atoms is close to the Cu(100)-[Ni(100)] bulk interlayer spacing of 1.807 Å (1.76 Å). Deeper interlayer distances are also very similar to the respective bulk value.

Ethylidyne on Pt(111): Determination of adsorption site, substrate relaxation and coverage by automated tensor LEED

The adsorption of ethylidyne on Pt(111) is investigated by low-energy electron diffraction (LEED). An automated search program for LEED structure analysis is used to determine the adsorption geometry from comparison of experiment and theory. The fcc threefold hollow site is preferred by the fit. The carbon-carbon bond distance is 1.49 Å and the carbon-platinum bond length 1.91 Å. A buckling within the top two substrate layers is detected which corresponds to a local expansion of the first-to-second layer distance underneath the adsorbate by 0.08 Å as compared to the bulk value. The three substrate atoms forming the hollow site are closer to one another than in the bulk structure by 0.11 Å. A tilt of the carbon-carbon bond by 6° seems to occur and could be indicative of a wagging vibration of this bond. The hydrogen atoms do not contribute to the diffraction that yields the ordered pattern as indicated by the absence of hydrogen related features in the experimental I– V curves, suggesting relatively free rotation of the methyl group about the carbon-carbon axis. Additionally, a distinction between a p(2 × 2) and an average of three domains of a p(2 × 1) structure, which both give rise to the same (2 × 2) LEED pattern, is possible. The p(2 × 2) is favored in the structural analysis, giving an ethylidyne coverage of 0.25 monolayers.