Theoretical studies of the bonding of oxygen to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for an O atom bonded to Ni clusters (consisting of one to five atoms) designed to model bonding to the (100) surface of Ni. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full O overlay er is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni⁺₅O cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96 Å above the surface (four-fold coordinate site) in good agreement with the value (0.90 ± 0.10 Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an O(2pπ) pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 × 2) structure for the O overlayer.     

Structure of the laser annealed Si(1 1 1)-(1 × 1) surface

LEED analysis of the laser annealed Si(1 1 1)-(1 × 1) surface shows that a model with a graphite-like top double layer of atoms with a spacing of 2.95±0.02 Å from the second double layer describes the LEED data as well as the Zehner model, but involves large displacements of the atoms normal to the surface as required by ion scattering results. It is suggested that this model provides a natural interpretation of the low energy He atom scattering data for the Si(1 1 1)-(7 × 7) surface.     

Multilayer distortion in the reconstructed (110) surface of Au

A new LEED intensity analysis of the reconstructed Au(110)-(1×2) surface results in a modification of the missing row model with considerable distortions which are at least three layers deep. The top layer spacing is contracted by about 20%, the second layer exhibits a lateral pairing displacement of 0.07 Å and the third layer is buckled by 0.24 Å. Distortions in deeper layers seem to be probable but have not been considered in this analysis. The inter-atomic distances in the distorted surface region show both an expansion and a contraction compared to the bulk value and range from 5% contraction to about 4% expansion.     

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d₁₂ = 0.70 ± 0.03 Å and for the inner potential V₀ = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).     

Theoretical studies of the geometries of O and S overlayers on the (100) surface of nickel

Geometries for O and S overlayers on the (100) surface of Ni have been calculated using $\text{a b initio}$ wavefunctions for O and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated distance of the adatom from the surface is 0.96 Å and 1.33 Å for O and S, respectively, in excellent agreement with the results of dynamic LEED intensity calculations, 0.9 ± 0.1 Å and 1.3 ± 0.1 Å, respectively. This indicates that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters.     

Leed measurements of Fe epitaxially grown on Cu(100)

Iron was epitaxially grown on a Cu(100) surface. Low energy electron diffraction (LEED) intensity versus energy curves were recorded for 1 and 10 layers of iron on Cu(100) at room temperature. A full dynamical analysis was performed using the renormalized forward scattering perturbation method. The surface Debye temperatures were determined to be 233 K for 1 ML Fe and 380 K for 10 layers of Fe. The value obtained for fcc iron was 550 K. A multiple relaxation approach was employed in analyzing the experimental data. The estimated interlayer spacings for the first and second layers were 1.78±0.02 Å (first) and 1.81±0.02 Å (second) for 1 ML Fe, and 1.81±0.02 Å (first) and 1.78±0.02 Å (second) for 10 layers of Fe on Cu(100). Auger electron spectroscopy was used to determine the thickness of the Fe films, and the LEED measurements indicate approximately a layer-by-layer growth until about 17 layers at room temperature. At higher temperatures there is evidence of iron diffusion or copper surface segregation.     

Structural models of the reconstructed W{001} surface

A LEED intensity analysis of 5 beams from the low-temperature W{001}c(2×2) structure indicates that the surface reconstruction involves shifts of the surface atoms along 〈110〉 directions within the plane of the surface, as suggested by Debe and King. At temperatures 100–140K the shifts are in the range 0.15–0.3 Å, with the first interlayer spacing 1.48–1.58 Å (bulk value 1.58 Å). Similar analysis of the room-temperature W{001}c(2×2)-H phase indicates: (i) none of the models proposed, which ascribe the c(2×2) structure directly to ordered hydrogen adsorption, can explain the experimental data; (ii) the W{001}c(2×2)-H structure is probably impurity stabilized by H at room temperature in the same W lattice as the low-temperature reconstructed phase.     

The termination of the titanium (100) surface

We find that a large quantity of normal incidence LEED data from the “non-unique” Ti(001) surface indicates that it is probably uniquely terminated in one of the two possible bulk interlayer spacings. The favored structure has a small first interlayer spacing of about 0.80 Å, similar to that of one bulk distance (0.85 Å) and quite distinct from the second possibility (1.70 Å).     

Further dynamical and experimental LEED results for a clean W<“001”-(1 × 1) surface structure determination

Full dynamical layer-doubling calculations have been made for comparison with precision LEED spectra for the clean W “001”-(l × 1) surface at approximately 470 K. Using 45 beams and 10 phase shifts, multi-layer spacing calculated spectra are critically compared with 12 experimental curves involving 5 different beams and 5 incidence angles. Both visual judgements and a semi-quantitative peak deviation/penalty evaluation yield the same result. A surface-bulk layer spacing of 1.51 ± 0.05 Å is concluded, a 4.4% contraction, in contrast to the most recent other determination of 1.40 ± 0.03 Å. This analysis re-emphasizes the need for a reliable and objective criterion for comparing observed and calculated LEED spectra, and corrects a potentially important input to the analysis of more complex systems. For example, at low temperatures (<370 K) the W “001” clean surface rearranges to a c(2 × 2) structure.     

On the bond lengths reported for chemisorption on metal surfaces

The bond lengths and geometrical arrangements reported with multiple-scattering analyses of LEED intensities for chemisorption on metal surfaces are assessed with Pauling's bond length-bond order relation. The approach here is partly empirical and it depends on information on the atomic valencies; the hybridisation model proposed for metals by Altmann, Coulson and Hume-Rothery is also used to guide the allocation of bonding electrons for the purposes of estimating the orders of surface bonds. It is shown that this framework allows estimates of surface bond lengths to within 0.1 Å of the values reported with LEED, and often the correspondence is substantially closer. Other factors that are likely to influence surface bond lengths have been recognised, but refinements to the present analysis should probably await clarifications of uncertainties in the structures determined with LEED including, for some chemisorption systems, consideration of possible displacements of metal atoms.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

Radiation damage in single-crystal ice studied by 100 keV proton channeling

A detailed study has been undertaken of the Ni{100} (2 × 2)C structure formed by cracking ethylene on a clean Ni{100} surface. The LEED pattern shows characteristic missing spots which can be attributed to the presence of glide lines and indicate a space group symmetry of p4g. We show that this can be readily interpreted in terms of a distortion of the top nickel layer both parallel and perpendicular to the surface, which accompanies the carbon adsorption. Detailed comparisons of LEED intensity data with dynamical calculations indicate that the top layer nickel atoms are displaced 0.35 ± 0.05 Å parallel to the surface, 0.20 ± 0.05 Å outwards from the surface, and that the carbon atoms are in 4-fold hollows (now distorted) at a spacing of 0.1 ± 0.1 Å from the surface. These conclusions lead to a nickel-carbon nearest neighbour spacing of 1.803 ± 0.015 Å.     

LEED analysis of the Co{001}c(2 × 2)O structure

A LEED intensity analysis of the c(2 × 2) structure obtained upon adsorption of oxygen gas on Co {00l} is reported. Three structural models have been tested on the basis of a total of 13 LEED spectra for three angles of incidence and one azimuth. The correct model has oxygen atoms chemisorbed in the four-fold symmetrical hollows formed by four adjacent substrate atoms. Atomic arrangement and metal-oxygen distances are analogous to those found in Ni{001}c(2 × 2)O.     

The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

Structure of CO adsorbed on Ni (100)

The c(2 × 2) configuration of CO chemisorbed on Ni(100) has been examined by the dynamical LEED method of surface structure analysis. Experimental LEED intensity spectra of the (00), ($\text{1}\text{2}\text{1}\text{2}$) (10) and (11) LEED beams measured at 175 K are compared with the corresponding calculated spectra for two different CO potential constructions and a number of trial structures. The best agreement was found for a structure where the CO molecules sit directly above the Ni atoms with vertical spacings between the Ni and C and the C and O layers of 1.80 ± 0.10 A and 0.95 ± 0.10 Å respectively. It is proposed that the CO molecule is tipped over at an angle of 34° ± 10° with respect to the surface normal so that the actual carbon-oxygen bond length is close to the figure 1.15 Å found in Ni(CO)₄.     

LEED study of the epitaxial growth of the thin film Au(111)/Ag(111) system

An analysis of LEED data from the Ag(111) surface at room temperature and 5° ? Θ ? 16°, φ = 12° has been carried out in order to test three different model potentials for the exchange and correlation part of the one-electron LEED potential. Clean Au(111) surfaces have been grown on Ag(111) at room temperature at a deposition rate of 0.15 Å s^?1. Similar method of calculation and potentials have been employed for the Au overlay er on Ag(111). After the deposition of ? 2.5 monolayers of Au/Ag(111) the growth of Au can proceed in two different ways. One of them matches satisfactorily with the theoretical calculation for the Au(111) overlayer on Ag(111) following the fcc sequence. The other seems to be concerned with the diffusion of Ag during the Au growth. Similar curves have been obtained during the diffusion of Ag through 350 Å of Au(111).     

Effects of oxygen adsorption on the surface barrier structure of copper (001)

The effects of oxygen adsorption on copper (001) have been studied by surface barrier resonance spectroscopy in LEED. As a result of adsorption the Bragg peaks move to lower energies and the resonance profiles are heavily modified. A theoretical analysis of this data indicates that the surface barrier height rises by about 2 eV and the barrier origin moves about 0.2 Å further out from the copper surface as a result of oxygen adsorption.     

Structural and electronic model of negative electron affinity on the Si/Cs/O surface

Structural and electronic models are proposed which correlate Goldstein's LEED, Auger, photo-emission, plasmon, and desorption data for negative electron affinity (NEA) on Si(100) surfaces. In the structural model, the surface Si atoms group into adjacent rows of surface “pedestals” and surface “caves”. Their density is 3.4 × 10¹⁴ cm^?2 each, as inferred from the LEED 2 × 2 reconstruction pattern and other data. Adsorbed Cs resides in fourfold coordination with Si atop the pedestals. Adsorbed oxygen is completely submerged in the caves of aperture 2.98Å to give a Cs-O dipole length of 2.9Å. Similar structural arguments show why Cs must be adsorbed before O₂, and why Si(111) does not exhibit NEA. In the electronic model, the surface dielectric constant, 5.3. obtained from the surface plasmon energy, 7 eV, is used to compute the dipole length from the final work function, 0.9 eV. It is 2.8Å in excellent agreement with the dipole length computed from the above structural model. Some properties of the “induced” surface states in the presence of Cs and O are also described.     

The role of HH interactions in the formation of ordered structures on Ni and Pd single crystals

The interaction between H adatoms on a surface is calculated within the embedded cluster model of chemisorption. The model is first applied to the case of two H atoms on a free electron surface. The interaction energy is found to be an oscillatory function of the H-H separation R_ab. Application of the free electron model to the problem of chemisorption on transition metal surfaces leads to unphysical results with the prediction of formation of ordered H overlayers which are not observed in LEED experiments. We next include the l = 2 TM muffin tins. Results for H adsorption on the low index faces of Ni and Pd substrates are presented. Graphitic structures are predicted for the (111) faces of both Ni and Pd with the H atoms occupying both types of three-fold hollow sites on the surface. This agrees with the results of LEED experiments for H/Ni(111). Comparison with experiment is not possible in the case of H/Pd(111) owing to the lack of low temperature studies for that system. Zig-zag chains with the H atoms adsorbed in sites of three-fold coordination on alternate sides of the TM(110) rows are predicted for both Ni and Pd. This is in agreement with the results of He diffraction experiments for H/Ni(110). No structure determination has been done for H/Pd(110). Adsorption in the four-fold centre sites for H on the (100) faces of Ni and Pd is found to be unfavourable. The H atoms are expected to adsorb in sites of three-fold symmetry below the (100) surface for H on Pd with formation of a c(2 × 2) structure in agreement with the LEED observations. For H/Ni(100) the H atoms are believed to adsorb above the surface, away from the centre site and to bond to two surface Ni atoms. No short-range ordered structures are predicted in this case.     

The structure of atomic nitrogen adsorbed on Fe(100)

Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 × 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED I–V spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Å above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Å). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe₄N, thus supporting the view of a “surface nitride” and providing a consistent picture of the structural and bonding properties of this surface phase.