LEED Results Research Articles

AB initio cluster calculations of the equilibrium distance and geometry of oxygen on the Fe{001} surface

We present the results of Spin Unrestricted Hartree-Fock cluster calculation of oxygen at the four-fold, bridge and head-on surface sites. The four-fold site is the lowest energy configuration, in agreement with a LEED structure analysis, and the equilibrium distance above the surface, 0.38 Å, is close to the LEED result, 0.48 Å. These results indicate that cluster calculations may be used to corraborate the LEED analysis and in some cases to predict independently the correct adsorption geometry.

Transition metal compound surfaces I: The cubic sodium tungsten bronze (NaxWO3) surface

The two dimensional surface structure and the surface chemical composition of (100) cubic sodium tungsten bronze (NaxWO3) has been investigated using low energy electron diffraction and Auger electron spectroscopy. Single crystals with bulk x values ranging from 0.5 to 0.9 were examined. The (100) surface exhibits the square symmetry of the WO3 lattice, with fractional order features corresponding to ordered sodium row structures which depend on the crystal x value. Auger spectra support covalent bonding models of these materials, in which sodium contributes little to the lattice binding. LEED and Auger results indicate sodium depletion in the surface region. In addition, evidence of a sodium order/disorder transition in the surface region is obtained.

A multimethod investigation of the adsorption of NO on Ru(001) : II. Δφ, thermal desorption and LEED results

A multimethod investigation of the adsorption of NO on Ru(001) : II. Δφ, thermal desorption and LEED results

The influence of correlated atomic vibrations on low energy ion scattering: I. Quasi-triple scattering from a 〈100〉 row of copper atoms

It is shown that the correlations between atomic thermal vibrations, implied by the Debye model, have a significant effect on low energy ion scattering. In the case of copper the estimated surface Debye temperature is approximately 85% of that found if correlations are neglected. The calculated surface Debye temperature of 258 ± 25 K is in reasonable agreement with LEED results and theoretical estimates.

Nitridation of silicon (111): Auger and LEED results

Clean silicon (111) (7×7) surfaces at up to 1050°C have been reacted with nitrogen ions and neutrals produced by a low energy ion gun. The LEED patterns observed are similar to those previously reported for reaction of silicon (111) (7×7) with NH3. The nitrogen KLL peak exhibits no shift or change in shape with nitride growth. At the same time the magnitude of the elemental silicon LVV peak at 92 eV decreases progressively as a new peak at 84 eV increases. The position of both peaks appears to be independent of the degree of nitridation. Since the Auger spectra are free of oxygen and other impurities, these features can be attributed only to silicon, nitrogen, and their reaction products. Characteristic features of the Auger spectra are related to LEED observations and to the growth of microcrystals of Si3N4.

New study of CO adsorption at low temperature (90 K) on Pt (111) by EELS

We have performed a new study of the vibrational modes of CO adsorbed at low temperature (90 K) on Pt(111). As in previous work a sequential occupation of top and bridge sites is observed. At a certain bridge site coverage, however, a new vibrational mode with a frequency of 720 cm−1 appears. The mode is not observed at room temperature since sufficiently high coverages are not achieved there. The appearance of the 720 cm−1 mode is attributed to the C–Pt asymmetric stretching vibration becoming dipole active at higher coverages when, according to LEED results, an out-of-registry CO overlayer is formed.

An ELS vibrational study of carbon monoxide adsorbed on the Cu(110) surface

The adsorbate site location for CO on Cu(110) has been investigated with electron energy loss vibrational spectroscopy. With low coverage at 80 K vibrational energies of 85 and 258 meV are observed. At high coverages and/or near the thermal desorption temperature of 170 K, only a single peak at 260 meV is observed. A simple spring model analysis of the vibrational energies indicates that a bridge position is occupied by CO in the first case and the on-top position in the second case although LEED results indicate that CO cannot be highly localized at any one site in the high coverage case.

Electron energy-loss spectra of co-adsorbed Ni(100) surfaces

Electron energy-loss spectra of CO-adsorbed Ni(100) surfaces have been observed with concomitant measurements of LEED, AES, work function change, and electron impact desorption. Five CO-induced loss peaks are observed for the first time. The loss peaks at 5.8 and 8.6 eV are ascribed to the charge-transfer transitions ( Ni d π → CO 2 π ∗ and Ni d σ → CO 2 π ∗ , respectively), and the weak peaks at 11.8, 12.6 and 16.0 eV to the intra-molecular transitions ( 5σ → 2π ∗, 1π → 2π ∗ and 4σ → 2π ∗ , respectively). The COCO adsorbate interaction is shown to shift the loss energy of the 12.6 eV peak. The adsorbed state of CO consistent with the results of LEED, AES, work-function change and electron impact desorption is discussed.

Geometric factors in f.c.c. and b.c.c. metal-on-metal epitaxy III. The alignments of (111) f.c.c.-(110) b.c.c. epitaxed metal pairs

Abstract Deposits (≤ 100 nm thick) of Ni, Cu, Au and Ag condensed in u.h.v. onto (110) tungsten surfaces in the range 300-1200 K have been examined by transmission electron microscopy (TEM). In general agreement with the results of LEED, FEM and FIM on very thin deposits or individual small crystals, the metals epitax (111) f.c.c.// (110) b.c.c. in one of two types of alignments [110] f.c.c.// [001] b.c.c. (Ni and Cu) or [011] f.c.c.// [111] b.c.c. and [101] f.c.c.// [111] b.c.c. (Au and Ag). All deposits of f.c.c. metals on (110) b.c.c. metal substrates and deposits of b.c.c. metals on (111) f.c.c. metal substrates examined so far epitax as above. The distribution of the alignments as a function of ρ = d0 b.c.c./d0 f.c.c., the ratio of the bulk atomic diameters of metal pairs, is analysed. Scaled nets of discs representing (111) f.c.c. and (110) b.c.c. planes are superimposed in the two types of alignments and the resulting patterns compared. Areas where discs in the two nets coincide are more elongated in the adopted alignment. This differentiation is used to predict the alignment of other f.c.c.-b.c.c. metal pairs in the range 0·82 < ρ < 1·15. When W substrates deviate from (110), Ni and Cu also epitax (001) f.c.c.// (110) W with[1 10] f.c.c.//[111] W or [110] f.c.c.//[111] W; f.c.c. metals are expected to adopt this orientation also on (110) b.c.c. when ρ > 1·2.

SIMS and flash desorption studies of nickel oxygen interaction

O2 exposure of polycrystalline nickel at 300 K results in characteristics changes of secondary ion emission. These can be described by a model which is in good agreement with corresponding LEED, AES, XPS, and ΔΦ results of other authors. According to this model, oxygen can be bonded on Ni in at least five different phases: 1) chemisorption, indicated by a rapid increase of Ni+, Ni 2 + , and Ni2O+ (≦5 L); 2) a rearranged chemisorption layer, characterized by a drastic decrease of Ni+, Ni 2 + , and Ni2O+ (5–15 L); 3) nickel oxide (NiO) responsible for a strong NiO−- and NiO 2 − -emission (≦40 L); 4) oxygen on top of this NiO layer, producing a final increase of Ni+ and NiO+ and a O2-flash signal at 400 K (>40 L); 5) bulk dissolved oxygen in thermal equilibrium with a chemisorption layer (after several exposure/heating cycles).

Theory of reconstruction induced subsurface strain — application to Si(100)

The stress produced by surface layer reconstruction on semiconductors can produce sizable elastic distortions of deeper layers. These are calculated using a standard bulk model of interatomic force constants for the pairing model of 2 × 1 reconstruction on the (100) surface of C, Si and Ge. Kinematic analysis for the predicted geometry with subsurface distortions is shown to give good agreement with LEED intensity data for Si. This reconciles the LEED results, for which the simple pairing model fails completely, with spectroscopic measurements, which are best fit by the pairing model.

Chimisorption du monoxyde de carbone sur la face (110) du molybdène: II. Modèle pour l'adsorption de CO/Mo(110)

In this second paper we consider three possible models for the adsorption of CO/Mo(110). Taking into account the size of molecules we establish simple criteria for the adsorption in each of the two states we have determined experimentally (Part I). On this basis we simulate the adsorption kinetics corresponding to the three models. Comparison with experimental results shows that only one model is adequate. In this model the CO molecules of state No. 1 are bridged ones and their adsorption is a random processus. In the state No. 2 the CO molecules are perpendicular to the Mo(110) plane and only the C atoms are bounded to Mo atoms. The simulated structure of the adsorbed film is completely disordered, which corresponds to the LEED results (Part I).

LEED, auger electron spectroscopy (AES) and photoemission studies of the adsorption of cesium on the epitaxially grown GaAs(110) surface

Epitaxially grown GaAs(110) surfaces which are cleaned by argon-ion bombardment followed by annealing show the same behaviour on Cs adsorption in both the LEED, AES and photoemission results, as the vacuum-cleaved GaAs(110) surface. Cs adsorbs in a c(4 × 4) structure up to a coverage of 5.5 × 10 14 atoms cm −2. The structure is enhanced by heat treatment at 260°C and then has a coverage of 4.7 × 10 14 atoms cm −2. Further heat treatment at 320°C gives a GaAs(110)- c(2 × 6) disordered pattern at a coverage of 2.7 × 10 14 atoms cm −2. The Cs coverage is linear with the dosage and shows a sharp break point at 5.5 × 10 14 atoms cm −2, which coincides with the maximum photoemission. NEA can be reached by Cs adsorption only, when the specimen is heated to just below its decomposition temperature (660°C). The photoemission then reaches a value as high as 320 μA lm −1 comparable to the value for vacuum-cleaved surfaces. In other cases the photoemission maximum is only 30 μA lm −1. Activation of the surface with Cs + O 2 leads to photoemission values as high as 1200 μA 1m −1 if in the initial activation with Cs alone NEA is achieved, otherwise the photoemission value is not higher than 650 μA 1m −1.

Crystal face specificity of xenon adsorption on iridium field emitters

Fundamental problems of the adsorption of noble gas atoms on metal surfaces are discussed on the basis of new data of xenon adsorption on well-defined crystal faces of iridium. These data include surface potentials ( = changes in work function), heats of adsorption and their decrease with increasing coverage; they have been obtained by using a field emitter probe-hole assembly. It is found that the heat of adsorption Q hkl is not simply additive in the number of Ir atoms contacting a Xe atom on a given site; in particular for the close-packed faces, Q 111 and Q 100 are relatively too high. Apparently, strong bonding is favoured by high work function of the adsorbing crystal face. This proves a significant contribution of a charge-transfer no-bond interaction to the adsorption bond. A model of Xe polarization by an electric surface field is rejected, as it predicts the wrong sign for the adsorption dipole. While at low coverage adsorption is confined to sites determined by the atomic topography of the adsorbing surface, several possibilities exist for high coverages. Either a two-dimensional close-packed layer is formed with little or no epitaxial relation to surface topography, or adsorption remains confined to certain sites. The present data favour the former possibility for atomically smooth faces in agreement with recent LEED results. For atomically rough faces however, the smallness of the decrease of Q hkl with coverage seems to favour site adsorption even at high coverage. The latter result is of relevance for surface area determinations by means of “physical” adsorption.

Intensites experimentales et theoriques en diffraction d'électrons lents sur la face (001) de l'aluminium

We present some experimental and theoretical LEED results obtained from the (001) surface of aluminum. The experimental curves were obtained with a new goniometer which allows us to set any incidence conditions, and to measure all the diffracted beams. We show that rotation diagrams are more interesting than energy profiles for studying the dynamical interactions and testing various LEED theories. We use several models to compute such diagrams. As a first step, we compare the results obtained in the S-wave approximation with the three following models: One Dimensional Multiple Scattering Model, Inelastic Collision Model, and the Kambe-McRae Multiple Scattering approach. As a second step, we use more elaborate potentials to calculate LEED intensities according to the Kambe-McRae Multiple Scattering approach, and the Band Structure-Matching formalism; the results are discussed and compared with experiment.

The role of interaction energies in ordered adsorption on metal single crystal surfaces

Adsorption phenomena are determined by the interaction energies between both the adsorbed molecules and the substrate and adjacent adsorbed molecules. If there is no chemical reaction involved in the adsorption process (e.g. oxidation) one can treat the behaviour of the adsorbed phase as a lattice gas by statistical methods. Using importance sampling it is possible to simulate the ordered adsorbed phase with a digital computer. So far, two special problems have been calculated: (1) the splitting of LEED-beams, found experimentally in various metal-gas-systems; (2) the order-disorder process in a c(2 × 2)-structure. The results are in excellent agreement with the experimental LEED-data. The influence of the underlying metal on adsorption has been studied using Ni/Cu alloy single crystals. The surfaces were first investigated using LEED-Auger techniques to determine surface composition and homogeneity in the first five underlying layers. Various gases were then adsorbed, and the LEED results are compared with those obtained on the elementary Ni and Cu surfaces. Ordered adsorbate structures peculiar to alloy surfaces have been found. These, together with flash desorption and work function measurements indicate that alloys exhibit adsorption properties which cannot be described by consideration of the component elements alone. The results are discussed in terms of the d-band model and Grimley's MO-theory of adsorption.

Determination of various Mo(110) and W(110) surface structures from LEED results using a multiple-scattering method

A plane wave multiple scattering method employing simplified form factors has been used to determine the surface structure of annealed (110) faces of tungsten and molybdenum from existing LEED results. Assuming the validity of the method, it is found that these faces, after being annealed under LEED conditions, are probably covered by an overgrown monolayer sited directly above, and at a distance of 2.1 Å from, the terminal layer of the bulk structure. It is shown that ion bombardment removes this overgrown layer and reveals the bulk structure of Mo(110). Adsorbed CO on bombarded Mo(110) is shown to take up the α-sites, i.e. to position itself directly above the metal atoms, and, assuming similar form factors for the atoms in the adsorbate and adsorbent, to cover 60% of the surface.