The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

Determination of the c(2 × 2) structure of sulfur on Fe{001} by low-energy electron diffraction

In the early stages of reaction with sulfur, a clean Fe{001} surface develops a c(2 × 2) superstructure. A low-energy electron diffraction analysis of this structure leads to a model in which the S atoms lie in the four-fold symmetrical sites on the Fe{001} surface, the S-Fe interplanar spacing being $\text{1.09 ± 0.05}\text{A}\text{?}$ and corresponding to an effective radius of 1.06 Å for the chemisorbed S atoms. In contrast to Fe{001} 1 × 1-O, the first interlayer spacing of the substrate here is not significantly expanded.     

Displacive surface phases formed by hydrogen chemisorption on W {001}

A detailed LEED study is reported of the surface phases stabilised by hydrogen chemisorption on W {001}, over the temperature range 170 to 400 K, correlated with absolute determinations of surface coverages and sticking probabilities. The saturation coverage at 300 K is 19(± 3) × 10¹⁴ atoms cm^?2, corresponding to a surface stoichiometry of WH₂, and the initial sticking probability for both H₂ and D₂ is 0.60 ± 0.03, independent of substrate temperature down to 170 K. Over the range 170 to 300 K six coverage-dependent temperature-independent phases are identified, and the transition coverages determined. As with the clean surface $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ displacive phase, the c(2 × 2)-H phase is inhibited by the presence of steps and impurities over large distances (～20 Å), again strongly indicative of CDW-PLD mechanisms for the formation of the H-stabilised phases. These phases are significantly more temperature stable than the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$, the most stable being a c(2 × 2)-H split half-order phase which is formed at domain stoichiometries between WH_0.3 and WH_0.5. LEED symmetry analysis, the dependence of half-order intensity and half-width on coverage, and I-V spectra indicate that the c(2 × 2)-H phase is a different displacive structure from that determined by Debe and King for the clean $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$. LEED I-V spectra are consistent with an expansion of the surface-bulk interlayer spacing from 1.48 to 1.51 Å as the hydrogen coverage increases to ～4 × 10¹⁴ atoms cm^?2. The transition from the split half-order to a streaked half-order phase is found to be correlated with changes in a range of other physical properties previously reported for this system. As the surface stoichiometry increases from WH to WH₂ a gradual transition occurs between a phase devoid of long-range order to well-ordered (1 × 1)-H. Displacive structures are proposed for the various phases formed, based on the hypothesis that at any coverage the most stable phase is determined by the gain in stability produced by a combination of chemical bonding to form a local surface complex and electron-phonon coupling to produce a periodic lattice distortion. The sequence of commensurate, incommensurate and disordered structures are consistent with the wealth of data now available for this system. Finally, a simple structural model is suggested for the peak-splitting observed in desorption spectra.     

The preparation and surface structure of clean V(110)

The first quantitative determination of the surface structure of a group VB metal is reported. A procedure is described for the preparation of a clean, well-ordered surface of V(110), free from the major bulk impurity, oxygen. The clean surface exhibits the two-dimensional periodicity of the corresponding bulk plane. Experimental LEED intensity measurements are compared to the results of multiple-scattering model calculations, with very good agreement being obtained for a value of the first interlayer spacing d, close to the bulk value. Minimization of the r factor for the comparison of experimental and calculated intensity spectra leads to a value of $\text{d = 2.12 ± 0.02}\text{A}\text{?}$, compared to the bulk value of 2.14 Å.     

Surface structure determination of W(001)p(1 × 1)-2H: Chemisorption induced substrate relaxation

Surface structural parameters for the full coverage W(001)p(1 × 1)-2H system have been determined using new LEED measurements and model calculations for bridge-bonded hydrogen. The technique is shown to be clearly sensitive to hydrogen position at the surface with the H-W layer spacing determined to be $\text{1.17 ± 0.04}\text{A}\text{?}$ resulting in a H-W bond length of 1.97 Å. The distance between the top two tungsten layers has been determined to be less than 2% contracted relative to bulk termination which is less contraction than measured the for clean W(001) surface by other studies.     

Hydrogen-induced reconstruction on a high step density W(001) surface

The hydrogen-induced reconstruction on a high step density W(001) crystal, ($\text{2}\text{×}\text{2}$)R45°-H, with steps oriented parallel to the [110] and ～ 28 Å average terrace width has been investigated using LEED symmetry, beam shape analyses, and EELS. The symmetry of the LEED pattern is observed to change from p2mg for the ($\text{2}\text{×}\text{2}$)R45° clean surface reconstruction to c2mm for the commensurate phase ($\text{2}\text{×}\text{2}$)R45°-H reconstruction. Correspondingly, the shapes of the half-order beams indicate that the hydrogen-induced reconstruction domains are much less elongated than the clean surface domains. A splitting of each half-order beam into four beams at higher exposures indicates the existence of two domains of the incommensurate phase. A commensurate phase v₁ vibrational loss peak centered at 160 meV in the EELS spectrum broadens on the low-energy side during the incommensurate phase and then shifts toward 130 meV and narrows as the (1×1)-H saturation structure develops. These observations imply that there is no long-range inhibition ( ～ 20 Å) to the formation of either commensurate or incommensurate phase; hydrogen induces a switching of the atomic displacements from 〈110〉 directions on a clean surface to 〈100〉 directions, even with steps oriented parallel to the [110]; and in the incommensurate phase there is a distribution of hydrogen site geometries with the most probable geometry more like the commensurate phase geometry than the saturation phase geometry.     

The surface structure of V(100)

The structure of the clean V(100)?(1×1) surface is determined, based on an r-factor comparison of experimental LEED intensity-energy spectra with the results of multiple-scattering model calculations. Minimization of the r-factor with respect to the calculational variables leads to optimum values of the first and second interlayer spacings of $\text{d}{}_1\text{=1.41 ± 0.01}\text{A}\text{?}$ and $\text{d}{}_2\text{=1.53 ±0.01}\text{A}\text{?}$, corresponding respectively to a contraction of 7% and an expansion of 1% with respect to the bulk value of $\text{d}{}_{\mathrm{<mtext>B</mtext>}}\text{=1.5141}\text{A}\text{?}$. Preliminary studies of the adsorption of O₂ and CO confirm that the V(100)?(5×1) structure observed during the process of cleaning the crystal is not characteristic of the clean surface, as suggested recently by Davies and Lambert (Surface Science 107 (1981) 391), but rather is associated with the presence of a significant concentration of oxygen in the surface region.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

The effect of oxygen coverage on surface relaxation of Ni(110) measured by medium energy ion scattering

Changes have been observed in the upper layer spacing of a clean and an oxygen covered Ni(110) single crystal by employing medium energy ion scattering, combined with channeling and blocking. We find a contraction of 4% for the clean surface and a minor expansion of 1% for a surface with $\text{1}\text{3}$ monolayer of adsorbed oxygen.     

Structural studies of the reconstructed W(001) surface with MeV ion scattering

The structure of the clean, reconstructed W(001) surface has been studied by MeV ion back-scattering channeling. We extract quantitative information on the number of displaced surface atoms (～ $\text{1}\text{2}$ monolayer) and the magnitude of the lateral component of their displacement (～ 0.23 Å) by measuring the energy dependence of the surface peak.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Xe monolayer adsorption on Ag(111) I. Structural properties

The monolayer of adsorbed xenon on the (

1) surface of silver was studied using low- energy electron diffraction. The structure is an hexagonal lattice aligned with but out of registry with the substrate. The Xe-Xe spacing at 25 K is 4.44 ± 0.01 Å in the full monolayer and 4.47 ± at less than full coverage. The Xe-Ag spacing is 3.5 ± 0.1 Å. Here $\text{〈u}{}_{\mathrm{\perp }}\text{〉}{}^2\text{T}$, the meansquare vibrational amplitude normal to the surface divided by the temperature is (1.83 ± 0.4 × 10^?4 A²/K. The heat of adsorption is 0.28 ± 0.03 eV/atom.     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

The role of impurities in the stability of ZnO surfaces

Both “as-grown” and “real” etched prism and (000$\text{1}$) oxygen surfaces have been studied by LEED and Auger electron spectroscopy. Heat treatment up to 800 K was sufficient to remove impurities other than calcium on all surfaces and potassium on the polar “real” surface. These could only be removed by ion bombardment. The Ca was associated with a (3 × 1) superstructure on the prism surface and a $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the polar surface. On the “as-grown” polar surface it was also possible to see (3 × 3) structure associated with reduced amounts of Ca. The especially strong binding of the electropositive elements on the negative oxygen polar surface is due to charge transfer, i.e. impurity stabilisation, this in turn can lead to chemical shifts in some of the Zn Auger transitions and to changes in the oxygen peak shape.     

The surface structure of Si(100) surfaces using averaged LEED: II. The (2×1) clean surface structure

Constant momentum transfer averaged LEED data from a Si(100) (2 × 1) clean surface structure have been produced for the $\text{(00), (10), (11), (}\text{1}\text{2}\text{0)}$ and $\text{(1}\text{1}\text{2}\text{)}$ beams. All averages show strong features in addition to those attributable to single scattering from the bulk. If this structure is assumed to also originate from single scattering, the surface reconstruction must be deep (>^～4 layers). Alternatively this structure can be ascribed to multiple scattering. As data from the Si(100) (1 × 1)H surface structure produced “good” averaging over an identical range of data, this latter conclusion has considerable bearing on the future usefulness of this averaging approach to surface structure analysis by LEED.     

Adsorption of ethylene on the Pt(100) surface

Clean Pt(100) surfaces with bulk-like 1×1 structure, or the stable, reconstructed 5×20 structure and held at 200 or 330 K were exposed to ethylene. Ultraviolet photoemission spectroscopy identified the nature of the adsorbed species which depends on the structure and temperature of the clean surface and the amount adsorbed. It is ethylene on the 5 × 20 structure at 200 K, a vinyl radical on the same surface at 300 K up to half a monolayer, the remainder being added as acetylene; it is acetylene on the 1 × 1 surface at 330 K and a mixture of acetylene, vinyl and ethylene on the 1 × 1 surface at 200 K. Whatever the nature of the adsorbate, the surface coverage θ increased with exposure ? as $\text{(1 ? θ = C?}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{)}$. By contrast, on a surface covered with any C₂ hydrocarbon acetylene adsorbs with Langmuir kinetics. The kinetics are explained in terms of the relationship between the attraction an approaching molecule experiences from the bare surface and its Van der Waals repulsion from preadsorbed molecules.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

A kinetic study of the initial oxidation of the Ni(001) surface by RHEED and x-ray emission

Nickel (001) surfaces were prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction. The oxidation of this surface in pure C₂ to form NiO was studied by reflection high energy electron diffraction (RHEED) and X-ray emission. On exposure to oxygen the “clean” surface was found to chemisorb oxygen to produce a coverage of 0.014 $\text{micro}\text{g}\text{cm}{}^2$ in an ordered c(2 × 2) structure. Within this film there appears to exist a number of nucleation sites dependent on temperature and step density. Growth is by oxygen capture at the periphery of these sites to produce oxide islands approximately three oxygen planes thick which spread to cover the surface. At room temperature this film does not thicken with additional oxygen exposure.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.     

The condensation of gold onto tantalum (100) single crystal surfaces: LEED,AES analysis

The condensation of gold onto clean and contaminated, single crystal, tantalum (100) surfaces has been followed by using LEED and AES. On a contaminated surface gold condenses as crystallites in a (211) surface orientation with some degree of preferred, azimuthal orientation. On a clean surface gold condenses in an ordered overlayer. Up to approximately $\text{3}\text{4}$ monolayer the structure conforms to the (1 × 1) tantalum surface. Beyond this, the observed LEED structure may be interpreted initially in terms of a TaAu overlayer made up of 90° rotated domains with (001)TaAu//(100)Ta and 〈 10 〉 TaAu// 〈 11 〉 Ta, and then in terms of a gold overlayer in a “distorted (111)” orientation. Annealing of these gold films always results in the formation of a (1 × 1) TaAu overlayer of small crystallite size.