The sorption of CO and O on Ni (111) studied by leed and iss

The adsorption of CO and O on Ni (111) was studied by low-energy ion scattering (ISS) and low-energy electron diffraction (LEED). For the ordered (√7/2) × (√7/2) R19.1° CO layer ion scattering gives a coverage greater than $\text{1}\text{2}$ monolayer, and for the (2 × 2) O layer a coverage of $\text{1}\text{4}$ monolayer. The CO is non-dissociatively adsorbed, with the C bound to the Ni. The molecules are oriented parallel to the surface normal. Island formation at lower CO coverages is possible.     

Recoil implantation from a thin source: I. Underlying theory and numerical results

Recoil implantation from a monolayer source, for example an adsorbed layer or a layer of altered stoichiometry, is considered. We introduce the incident ion current I, the fractional surface coverage characterizing the recoil source θ, the differential scattering cross-section dσ, and the integral distribution function F(x, ψ) for recoil-source atoms entering the target at angle ψ and stopping beyond x. The number of atoms implanted beyond x follows as $\text{H(x) = IθλN? σ F(x,ψ)}$. The receding of the target surface at velocity υ due to sputtering can be allowed for by integrating over $\text{x: H(x, t) = (}\text{1}\text{υ}\text{) ∫ H(x′)dx′}$. For very high doses the result is a steady-state situation in which the number of implanted atoms is given by H(x, ∞). Numerical results of four kinds are presented: H(x, H(x, ∞), the half-time for the build-up of the steady-state situation, and the half-depth of the implanted atoms, in all cases for oxygen on the surface of Be, Al, Mo and W.     

Study of low energy noble gas ion reflection from monocrystalline surfaces; Influence of thermal vibrations of the surface atoms: II. Calculational evaluation of the sensitivity for model parameters and possibilities for estimation

The sensitivity of the characteristics of low energy noble gas ion reflection from monocrystalline surfaces for thermal properties of the target atoms has been investigated by computer simulation. In addition the uncertainties in comparing experimental results with calculations, introduced by a not well-known interaction potential, have been examined. The calculations have been carried out for 6 keV Ar⁺ ions reflected from a vibrating Cu〈100〉 chain. To achieve the above presented object we varied the mean square value $\text{u}{}^2$ and the correlation coefficients of the atomic thermal displacements. The used ion-atom interaction potential (a Thomas-Fermi potential in the Molière approximation) has been varied by changing the screening length aF. Under certain conditions the shape of the energy spectra of specularly reflected particles depends pronouncedly on both $\text{u}{}^2$ and aF. The effects are the most pronounced for scattering angles between about 20° and 30°. The angular distribution shows also a distinct and simultaneous sensitivity for the used potential and the target-temperature. A most interesting feature is the occurrence of a QT peak at higher temperatures, resulting from quasi triple collisions from surface “thermal pit” structures. At a given scattering angle the cut-off temperature of this QT peak can be related to the mean square displacements of the involved atoms. This cut-off temperature appears to be (almost) independent from the used potential, allowing an estimation of $\text{u}{}^2$. The intensity of the QS peak and the QD peak depend exclusively on the mean square differences of thermal displacements of neighbouring atoms. Correlated atomic displacements have some influence on the angular distributions and on the QT peak intensity. Possibilities to estimate model quantities are discussed briefly.     

A LEED crystallographic analysis for the Rh(111)-(3 × 3)30°-S surface structure

An intensity analysis with low-energy electron diffraction is reported for the $\text{(}\text{3}\text{×}\text{3}\text{)30°}$ surface structure obtained by the adsorption and presumed dissociation of H₂S on the (111) surface of rhodium. Intensity-versus-energy curves were measured with a video LEED analyser for nine diffracted beams at normal incidence, and comparisons made with the renormalised forward scattering method for four different types of structural models in which the metal atoms remain in their regular bulk positions. The best correspondence between the experimental and calculated intensities occurs with sulphur atoms adsorbed in the “expected” 3-coordinate adsorption sites. The reliability index proposed by Pendry is minimised with S atoms 1.53 Å above the topmost metal layer; this corresponds to nearest-neighbour RhS bond distances equal to 2.18 Å. Comparisons are made with structural data available for related systems, and with the predictions of a model analysis of surface bond lengths given recently by one of the authors.     

Investigation of the local atomic arrangement on surfaces using low-energy ion scattering

Ions with energies around 1 keV are well suited for detecting atoms on solid surfaces and for investigating their relative arrangement. This is due to the large scattering cross-sections which are of the order of 10^–2Ų/sr. The conceptually simple method is limited by the fact that interaction potentials and, more so, charge exchange processes are only approximately known. Progress in low-energy ion scattering has recently been made by applying special scattering geometries and by using alkali ions, in addition to noble gas ions.Among the successful applications there are studies of the arrangement of atomic layers on supported catalysts, ordered adsorption systems on metal surfaces, surface reconstruction, and surface disordering due to defects and thermal motion. Energy spectra of recoil atoms and ions convey additional information. The fundamental physical features of low-energy ion scattering are discussed on the basis of examples of recent results.     

Velocity dependence of azimuthal anisotropies in ion scattering from rhodium {111}

The scattering of He⁺, Ne⁺ and Ar⁺ ions from Rh {111} is measured as a function of the azimuthal angle of the primary ion for an incident polar angle of 70° from the surface normal and an inplane collection angle of 60°. In this case anisotropy is defined as the ratio of the yield of ions scattered having the azimuth of 〈110〉 to the yield of those having the azimuth of 〈211〉. The yield ratio for all particle types correlates with particle velocity. The ratio is ～ 1 at low velocities, decreases to ～ 0.2 at 8 × 10⁶$\text{cm}\text{s}$ and then increases to a value of 1.4 at 25 × 10⁶$\text{cm}\text{s}$. Molecular dynamics calculations have been performed for Ne⁺ ion scattering from Rh{111} in order to understand the changes in anisotropy with particle velocity. Qualitative agreement with the experimental results is obtained without having to account for neutralization. A neutralization probability that depends on the collision time improves the agreement between the calculated and experimental yield ratios. A velocity dependent probability will not affect the ratio of yields in two different azimuthal directions.     

A proposed experiment to measure surface roughness in Si/SiO2 system

The adsorption of oxygen on clean cleaved (111) silicon surfaces has been investigated by high resolution electron spectroscopy (HRES), Auger electron spectroscopy (AES) and ellipsometry. Localized vibrations ($\text{h}\text{?}\text{ω = 94}$, 130 and 175 meV) which are related to the binding state band of oxygen are identified with HRES. AES measures the concentration of adsorbed atoms basically independent of their binding state while ellipsometry refers additionally to the optical properties of the adsorbed layer. The same adsorption kinetics was found with the three methods. Oxygen therefore adsorbs in a single likely molecular state. The sticking coefficient S increases exponentially with the surface step concentration. S is also enhanced by the presence of nude ion gauges. Depending on these parameters sticking coefficients between 2 × 10^?4 and 10^?1 have been obtained. This result might contribute to an explanation of the large differences in earlier works.     

Oxygen adsorption on Cu(110): Determination of atom positions with low energy ion scattering

The position of adsorbed oxygen on Cu($\text{1}$10) surfaces was determined with Low Energy Ion Scattering (LEIS). The experiments were performed by bombarding the copper surface at small angles of incidence with low energy Ne⁺ ions (3–5 keV). Measurements of the Ne⁺ ions scattered by adsorbed oxygen showed regular peaks in the azimuthal distribution of the scattered ions due to a shadowing effect. From the symmetry of the azimuthal distributions it follows that the centre of an adsorbed oxygen atom on the Cu(1̄10) surface lies about 0.6 Å below the midpoint between two neighbouring Cu atoms in a 〈001〉 row. A comparison of the azimuthal distributions of Ne⁺ ions scattered from clean Cu surfaces and oxygen-covered Cu surfaces showed that hardly any surface reconstruction had occurred in the oxygen-covered surfaces. The applied method seems to be an appropriate one for locating adsorbed atoms because it uses only simple qualitative considerations about azimuthal distributions of scattered ions.     

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.     

Electronic structure of the Si (111) reconstructed surface in the vacancy model

The self-consistent pseudopotential method is applied to the Si (111) 7 × 7 reconstructed surface in the vacancy model with a simplified $\text{3}\text{×}\text{3}$ superlattice structure. Numerical results with and without relaxation of surface atoms are presented. It is concluded that the relaxation, if any, is to be much smaller than the atomic distance to explain the photoemission spectrum of the 7 × 7 surface. The importance of the many-body effect is suggested in the photoemission process associated with the dangling bond surface states of Si.     

Application of low energy ion blocking for adsorption site determination of Na Atoms on a Cu(111) surface

Ion blocking in the low keV energy range is demonstrated to be a sensitive method for probing surface adsorption sites by means of the technique of time-of-flight scattering and recoiling spectroscopy (TOF-SARS). Adsorbed atoms can block the nearly isotropic backscattering of primary ions from surface atoms in the outmost layers of a crystal. The relative adsorption site position can be derived unambiguously by simple geometrical constructs between the adsorbed atom site and the surface atom sites. Classical ion trajectory simulations using the scattering and recoiling imaging code (SARIC) and molecular dynamics (MD) simulations provide the detailed ion trajectories. Herein we present a quantitative analysis of the blocking effects produced by sub-monolayer Na adsorbed on a Cu(111) surface at room temperature. The results show that the Na adsorption site preferences are different at different Na coverages. At a coverage θ = 0.25 monolayer, Na atoms preferentially populate the fcc threefold surface sites with a height of 2.7 ± 0.1 Å above the 1st layer Cu atoms. At a lower coverage of θ = 0.10 monolayer, there is no adsorption site preference for the Na atoms on the Cu(111) surface.     

Surface dependence of field ion energy distributions,He on W

Kinetic energy distributions of helium ions produced by field ionization above the (100), (110), and (111) planes of tungsten field emitters are reported. The energy resolution of these measurements if 0.3 eV fwhm. The main peak of these distributions is found to shift by as much as 0.7 eV at $\text{F = 5.3 V/}\text{A}\text{?}$, T = 21 K, when the origin of the ions collected is changed slightly. Several possible explanations for this shift are discussed. The most plausible of these involves electron tunneling through a field adsorbed helium atom. These results have important implications for the use of field ion energy distributions as a probe of the density of electronic states of the emitter.     

The growth and alloy formation of copper on the platinum (111) and stepped (553) crystal surfaces; characterization by Leed,AES, and CO thermal desorption

Epitaxial layers of copper were formed on Pt(111) and Pt(553) single crystal surfaces by condensation of copper atoms from the vapor. Surface alloys were formed by diffusing the copper atoms into the platinum substrate at temperatures above 550 K. The activation energy for this process was found to be ~ 120 $\text{kJ}\text{mol}$. These Pt/Cu surfaces were characterized by LEED, AES, and TDS of CO. The copper grows in islands on the Pt(111) surface and one monolayer is completed before another begins. There is an apparent repulsive interaction between the copper atoms and the step sites of the Pt(553) surface which causes a second layer of copper to begin forming before the first layer is complete. Epitaxial copper atoms block CO adsorption sites on the platinum surface without affecting the CO desorption energy. When the copper is alloyed with the platinum however, the energy of desorption of CO from the platinum was reduced by as much as 20 $\text{kJ}\text{mol}$. This reduction in the desorption energy suggests an electronic modification that weakens the Pt-CO bond.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1̄0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Azimuthal dependence of impact scattering in electron energy loss spectroscopy

The azimuthal dependence of electron energy loss spectroscopy (EELS) dipole and impact scattering intensity has been measured. Spectra for a saturation coverage of H adsorbed on W(110) exhibit loss peaks due to impact scattering from adsorbate vibrational modes. The intensity of the 160 meV loss peak has been measured as a function of the azimuthal angle between the scattering plane and a mirror plane of the $\text{s}$urface. The angular pattern has strong maxima oriented perpendicular to the <111 > rows of atoms on the surface, and has the C_2v symmetry of the W(110) surface. This azimuthal dependence is strikingly different from the nearly isotropic angular dependence of dipole scattering from Cl adsorbed on W(110). Selection rules for impact scattering account for the general features of the angular pattern based on asymmetric stretch modes associated with bridge site H atoms.We have shown that the azimuthal dependence of the 36 meV Cl/W(110) dipole scattering loss peak is isotropic and that the 160 meV H/W(110) impact scattering loss peak exhibits a striking azimuthal pattern with C_2v symmetry. The symmetry and deep minima suggest that selection rules play a central role in determining the azimuthal pattern. Application of these rules to two orthogonal directions (as in ref. 6) may be misleading, as is clear from Fig. 2, because essential features of the pattern will not be observed. Our analysis of the full pattern has suggested two bridge sites may be occupied at saturation coverage, but has still not resolved certain questions about the H/W(110) system.

1. Impact scattering selection rules for potential adsorbate sites. The listed directions are the intersections of the planes with the (110) surface for the mirror planes and the scattering planes, and the displacement directions for the adsorbate vibrational modes. Modes are assumed to be strictly parallel to the surface. The long bridge site is between two W atoms along the <001 > direction, the short bridge site is between two W atoms along the <$\text{111}\text{-}$ > direction, and the distorted bridge site is displaced from the long bridge site along the <$\text{110}\text{-}$ > direction (ref. 6) The asterisks (*) denote that the scattering amplitude is zero for all directions in the scattering plane, otherwise it is zero only in the specular direction. The <$\text{110}\text{-}$ > mode of the distorted bridge is not covered by the selection rules of ref. 2.

     

16.

Superlattices formed by interaction of hydrogen bromide and hydrogen chloride with Pt(111) and Pt(100) studied by LEED,Auger and thermal desorption mass spectroscopy     

Gerald A. Garwood  Arthur T. Hubbard 《Surface science》1981,112(3):281-305

HBr and HCl react with Pt(111) and Pt(100) surfaces to form adsorbed layers consisting of specific mixtures of halogen atoms and hydrogen halide molecules. Exposure of Pt(111) to HBr yielded a (3×3) LEED pattern beginning at $\text{Θ}{}_{\mathrm{Br}}\text{=}\text{2}\text{9}$ and persisting at the maximum coverage which consisted of $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{=}\text{1}\text{3}$ plus $\text{Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{9}$. The most probable structure at maximum coverage, Pt(111)[c(3 × 3)]-(3 Br + HBr), nas a rhombic unit cell encompassing nine surface Pt atoms, and containing three Br atoms and one HBr molecule. On Pt(100) the structure at maximum coverage appears to be Pt(100)[c(2√2 × √2)]R45°-(Br + HBr), $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{= Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{4}$; the rectangular unit cell involves four Pt atoms, one Br atom and one HBr molecule. Each of these structures consists of an hexagonal array of adsorbed atoms or molecules, excepting slight distortion for best fit with the substrate in the case of Pt(100). Treatment of Pt(100) with HCl produced a diffuse Pt(100)(2 × 2)-(Cl + HCl) structure at the maximum coverage of Θ_Cl = 0.13, Θ_HCl = 0.11. Exposure of Pt(111) to HCl produced a disordered overlayer. Thermal desorption, Auger spectroscopy and mass spectroscopy provided coverage data. Thermal desorption data reveal prominent rate maxima associated with the structural transitions observed by LEED. Br and HBr, Cl and HCl were the predominant thermal desorption products.     

17.

An electron diffraction study of the structure of silicon (100)     

Thomas D. Poppendieck  Tran C. Ngoc  Maurice B. Webb 《Surface science》1978,75(2):287-315

The (100) surface of silicon has been studied by low-energy electron diffraction and a preferred model of its atomic arrangement is proposed. A clean, well ordered, and reproducible surface was prepared by repeated heating to 1250°C, annealing at 900°C, and cooling at less than $\text{l°}\text{C}\text{sec}$. This surface is reconstructed from the bulk structure and has a c(4 × 2) unit mesh. The diffracted intensity consists of the Bragg maxima expected from the bulk and secondary peaks whose properties suggest they are due to the diffraction from the reconstructed selvedge. The data are averaged at constant momentum transfer to extract the quasikinematic intensity. The magnitude and width of the secondary structure suggest the selvedge extends three to five atomic layers into the material. No previously proposed model of the surface is consistent with the data. A model which gives a reasonable fit to the data has alternate rows of atoms in each of the outermost two planes removed, which exposes small facets of (111) surface. Then, atoms in the third layer are displaced — half moving to the plane of their back-bonding neighbors and the other half making orthogonal back bonds. Additionally, there are outward displacements of atoms in the first two layers. The proposed model can be rationalized by arguments due to W.A. Harrison in which rehybridization is the major contribution to the energy of the reconstruction.     

18.

On the interaction of cesium with cleaved GaAs(110) and Ge(111) surfaces: Work function measurements and adsorption site model     

H.J. Clemens  J. Von Wienskowski  W. Mönch 《Surface science》1978,78(3):648-666

The work function of UHV cleaved p-Ge(111) and n-GaAs(110) surfaces has been measured in dependence of the Cs coverage. At very low coverages θ < 0.001 the decrease of the contact potential difference is extremely steep. For GaAs the initial slope of the CPD versus coverage curve amounts to ?740 eV for Ge to ?130 eV per monolayer. Up to the saturation coverage the curves exhibit straight line segments with breaks at distinct coverages. Breaks are found for GaAs at approximately $\text{1}\text{12}$, $\text{1}\text{6}$, and $\text{1}\text{3}$ of a monolayer, for Ge at about $\text{1}\text{12}$, $\text{1}\text{4}$, $\text{1}\text{2}$, and $\text{3}\text{4}$. A new model is developed to explain this behaviour. It is based on the assumption of specific adsorption sites for the Cs atoms at the surfaces. With this model the experimental results, including the breaks, may be described in the whole coverage range from θ = 0.03 up to the saturation. Furthermore the dipole moments derived from the straight line segments are in excellent agreement with those values calculated for different surface molecules between the adsorbed cesium and substrate atoms at the specific adsorption sites.     

19.

Substrate steps/adsorbed layer interaction; selective effect on the possible epitaxial relationships: Case of the S/Cu ∼ (111) system     

J.M. Moison  J.L. Domange 《Surface science》1977,69(1):336-348

The interaction between monoatomic steps on a vicinal ～ (111) copper surface and the adsorbed sulphur monolayer has been investigated by a method involving the existence of two possible structures of the monolayer. The parts of the surface occupied by each structure, which are equal on a perfect (111) crystal plane, and differ on a vicinal surface (selective effect of the steps), have been deduced from LEED intensity measurements. Experiments on a wide range of surfaces have revealed a very strong selective effect of the steps, even for misorientations as small as 4° off the (111) plane. On surfaces with $\text{〈3}\text{2}\text{1}\text{〉}$ steps, only one monolayer structure exists, which appears to fit perfectly the steps. The behaviour of the other surfaces having the same misorientation angle allowed us to devise a model of the step/monolayer interaction, which includes a partial decomposition of a step into the two nearest $\text{〈3}\text{2}\text{1}\text{〉}$ steps (2D “faceting”). A general framework for exploiting experiments of the kind described above, on other adsorption systems, is also outlined.     

20.

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment     

A.G.J. De Wit  R.P.N. Bronckers  Th.M. Hupkens  J.M. Fluit 《Surface science》1979,90(2):676-687

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

SITE	LONG BRIDGE	SHORT BRIDGE	DISTORTED BRIDGE
MIRROR PLANES	[001], [1$\text{1}\text{-}$0]	NONE	[1$\text{1}\text{-}$0]
2-FOLD ABOUT Z	YES	YES	NO
PARALLEL MODES	[001]	[1$\text{1}\text{-}$0]	[1$\text{1}\text{-}$1]	[$\text{1}\text{-}$12]	[001]	[1$\text{1}\text{-}$0]
DIRECTIONS OF ZERO SCATTERING	[001] * [1$\text{1}\text{-}$0]	[001] * [1$\text{1}\text{-}$0]	[1$\text{1}\text{-}$1] [$\text{1}\text{-}$12]	[1$\text{1}\text{-}$1] [$\text{1}\text{-}$12]	[001] * [1$\text{1}\text{-}$0]	NA

Superlattices formed by interaction of hydrogen bromide and hydrogen chloride with Pt(111) and Pt(100) studied by LEED,Auger and thermal desorption mass spectroscopy

HBr and HCl react with Pt(111) and Pt(100) surfaces to form adsorbed layers consisting of specific mixtures of halogen atoms and hydrogen halide molecules. Exposure of Pt(111) to HBr yielded a (3×3) LEED pattern beginning at $\text{Θ}{}_{\mathrm{Br}}\text{=}\text{2}\text{9}$ and persisting at the maximum coverage which consisted of $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{=}\text{1}\text{3}$ plus $\text{Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{9}$. The most probable structure at maximum coverage, Pt(111)[c(3 × 3)]-(3 Br + HBr), nas a rhombic unit cell encompassing nine surface Pt atoms, and containing three Br atoms and one HBr molecule. On Pt(100) the structure at maximum coverage appears to be Pt(100)[c(2√2 × √2)]R45°-(Br + HBr), $\text{Θ}{}_{\mathrm{<mtext>Br</mtext>}}\text{= Θ}{}_{\mathrm{<mtext>HBr</mtext>}}\text{=}\text{1}\text{4}$; the rectangular unit cell involves four Pt atoms, one Br atom and one HBr molecule. Each of these structures consists of an hexagonal array of adsorbed atoms or molecules, excepting slight distortion for best fit with the substrate in the case of Pt(100). Treatment of Pt(100) with HCl produced a diffuse Pt(100)(2 × 2)-(Cl + HCl) structure at the maximum coverage of Θ_Cl = 0.13, Θ_HCl = 0.11. Exposure of Pt(111) to HCl produced a disordered overlayer. Thermal desorption, Auger spectroscopy and mass spectroscopy provided coverage data. Thermal desorption data reveal prominent rate maxima associated with the structural transitions observed by LEED. Br and HBr, Cl and HCl were the predominant thermal desorption products.     

An electron diffraction study of the structure of silicon (100)

The (100) surface of silicon has been studied by low-energy electron diffraction and a preferred model of its atomic arrangement is proposed. A clean, well ordered, and reproducible surface was prepared by repeated heating to 1250°C, annealing at 900°C, and cooling at less than $\text{l°}\text{C}\text{sec}$. This surface is reconstructed from the bulk structure and has a c(4 × 2) unit mesh. The diffracted intensity consists of the Bragg maxima expected from the bulk and secondary peaks whose properties suggest they are due to the diffraction from the reconstructed selvedge. The data are averaged at constant momentum transfer to extract the quasikinematic intensity. The magnitude and width of the secondary structure suggest the selvedge extends three to five atomic layers into the material. No previously proposed model of the surface is consistent with the data. A model which gives a reasonable fit to the data has alternate rows of atoms in each of the outermost two planes removed, which exposes small facets of (111) surface. Then, atoms in the third layer are displaced — half moving to the plane of their back-bonding neighbors and the other half making orthogonal back bonds. Additionally, there are outward displacements of atoms in the first two layers. The proposed model can be rationalized by arguments due to W.A. Harrison in which rehybridization is the major contribution to the energy of the reconstruction.     

On the interaction of cesium with cleaved GaAs(110) and Ge(111) surfaces: Work function measurements and adsorption site model

The work function of UHV cleaved p-Ge(111) and n-GaAs(110) surfaces has been measured in dependence of the Cs coverage. At very low coverages θ < 0.001 the decrease of the contact potential difference is extremely steep. For GaAs the initial slope of the CPD versus coverage curve amounts to ?740 eV for Ge to ?130 eV per monolayer. Up to the saturation coverage the curves exhibit straight line segments with breaks at distinct coverages. Breaks are found for GaAs at approximately $\text{1}\text{12}$, $\text{1}\text{6}$, and $\text{1}\text{3}$ of a monolayer, for Ge at about $\text{1}\text{12}$, $\text{1}\text{4}$, $\text{1}\text{2}$, and $\text{3}\text{4}$. A new model is developed to explain this behaviour. It is based on the assumption of specific adsorption sites for the Cs atoms at the surfaces. With this model the experimental results, including the breaks, may be described in the whole coverage range from θ = 0.03 up to the saturation. Furthermore the dipole moments derived from the straight line segments are in excellent agreement with those values calculated for different surface molecules between the adsorbed cesium and substrate atoms at the specific adsorption sites.     

Substrate steps/adsorbed layer interaction; selective effect on the possible epitaxial relationships: Case of the S/Cu ∼ (111) system

The interaction between monoatomic steps on a vicinal ～ (111) copper surface and the adsorbed sulphur monolayer has been investigated by a method involving the existence of two possible structures of the monolayer. The parts of the surface occupied by each structure, which are equal on a perfect (111) crystal plane, and differ on a vicinal surface (selective effect of the steps), have been deduced from LEED intensity measurements. Experiments on a wide range of surfaces have revealed a very strong selective effect of the steps, even for misorientations as small as 4° off the (111) plane. On surfaces with $\text{〈3}\text{2}\text{1}\text{〉}$ steps, only one monolayer structure exists, which appears to fit perfectly the steps. The behaviour of the other surfaces having the same misorientation angle allowed us to devise a model of the step/monolayer interaction, which includes a partial decomposition of a step into the two nearest $\text{〈3}\text{2}\text{1}\text{〉}$ steps (2D “faceting”). A general framework for exploiting experiments of the kind described above, on other adsorption systems, is also outlined.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.