Adsorption sites of111In and111mCd on Pd(111) surfaces characterized by the electric field gradient

PAC measurements of the electric field gradient at the nucleus of isolated probe atoms on Pd(111) surfaces lead to the identification of five adsorption sites successively occupied by the parents¹¹¹In and^111mCd during annealing between 80K and 600K. The data are consistent with an estimated activation energy for Pd surface self diffusion of E_a=0.76(8)eV.     

On the interaction of halogen atoms with (111) and (100) surfaces of silicon

The chemisorption of F, Cl, Br, and I atoms on the (111) and (100) surfaces of silicon has been studied by the MNDO method and using clusters of Si atoms to simulate the substrate. In the case of the adducts F-Si₄H₉ and Cl-Si₄H₉ the MNDO results are in close agreement with previous ab-initio ones concerning both the equilibrium distances and the chemisorption energies. For all the cases considered, the binding energy decreases in the order F>Cl>Br>I. The most stabel adduct is always obtained upon chemisorption at the bridge position on the (100) surface. Chemisorption at on-top positions leads to slightly less stabel adducts and is nearly isoenergetic on (100) and (111) surfaces. All the results are essentially insensitive to the dimensions of the clusters used to simulate the substrate.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Influence of impurity atoms on the temperature dependence of the electric field gradient in nontransition metals

The temperature dependence of the quadrupole coupling constants of¹¹¹CdIn,¹¹¹CdSn, and¹²⁰SbSn was measured by means of the perturbed angular correlation (distribution) technique. The results are discussed within a generalized model for the electric field gradient in nontransition metals.This work was supported by the Bundesministerium für Forschung und Technologie.     

Electric field gradient at111In on Cu (100) surfaces and its utilization for indium surface diffusion

The electric field gradient at¹¹¹In probe atoms on Cu (100) surfaces was studied. At clean surfaces all probes are exposed to a well-defined surface field gradient. This is used to investigate indium surface diffusion, where the applied PAC method allows to observe diffusion steps on an atomistic scale. The jump rate for indium on Cu (100) was found to be in the order of 10^–3 Hz at 200 K.     

Simulated reaction dynamics of F atoms on partially fluorinated Si(100) surfaces

We have investigated the influence of translational excitation on the reactivity of atomic fluorine with the Si(100) surface via molecular dynamics simulations using a first-principles-derived interaction potential. Surface reactivity is contrasted for both clean and partially fluorinated surfaces with the results of previous simulations of F₂ molecules impinging on Si(100) surfaces, indicating many similarities between the dynamics of F atoms and F₂ molecules. Mechanisms for the reaction are proposed based on reactivity trends and scattered product energy and angular distributions, including evidence for the existence of a precursor-mediated adsorption pathway for low incident energy F atoms on partially fluorinated surfaces.     

A study of copper adsorption on (100) tungsten by high field microscopy

An increase in the work function of a copper layer approximately three monolayers deep on tungsten (100) can be thermally induced at temperatures above 540 K. Helium ion microscopy reveals a consequent but small increase in structural perfection of the tungsten substrate surface, which is akin to the rearrangement of gold, but the change in work function of the copper layer appears not to depend upon the rearrangement. Detailed investigation of changes in work function at (100) using a probe-hole field emission microscope shows that copper in the first monolayer increases φ₁₀₀ by 0.42 eV at monolayer coverage, probably by formation of an array of dipoles of moment μ₀ = (1.6 ± 0.5) × 10^?30 C m with polarizability α = 3.80 ± 1.8 ų. A large apparent increase in φ₁₀₀ of 1.6 eV takes place when a thicker copper layer is spread so that it surrounds and approaches (100), and this is ascribed to a reduction in the field enhancement factor β produced by the surrounding copper. Comparison of the present findings with those of Bauer et al. reveals substantial agreement on the behaviour of the first monolayer and two major differences in the behaviour of thicker layers: (i) an increase in φ₁₀₀ from 4.25 to 7.1 eV which we observe above 540 K, and (ii) absence of any evidence for breakup of third and higher layers to form crystallites. It is tentatively suggested that (i) results from band-structure changes accompanying structural reorganisation within the adsorbed layer, which take place under experimental conditions not used by Bauer et al., and that (ii) is due to the absence of steps in the probed area of (100) which can act as nucleating centres for crystallite formation on a macroscopic (100) surface.     

Heavy particle scattering by atoms in an electric field

Atom—atom collision processes in an electric field are considered. A formula is obtained, in a two-level approximation, for the probability of transitions in which there is a change in the total angular momentum from J=0 to J=1. It is shown that the transition probability is characterized by an anisotropic distribution and a dependence on the electric field strength.V. V. Kuibyshev State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 91–94, October, 1994.     

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of¹¹¹Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyE _b for¹¹¹In at a Cd lattice site and a Cd vacancy (¹¹¹In_Cd–V_Cd–pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.     

Molecular cluster theory for chemisorption of first row atoms on nickel (100) surfaces

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of first row atoms on Ni(100) surfaces are presented. Energy levels and ground state charge distributions for XNi_S clusters with the adatom X = H, C, N, O located in C_{4 V} symmetry at a fixed height of h = 2.0 au above the surface are given. The variation of properties with h was studied in detail for the case of oxygen. Theoretical results compare rather well with experimental photoelectron and energy loss data. Local-densities-of-states diagrams are used to clarify the interaction between adsorbate levels and metal conduction bands.     

Chemisorption of isolated Br atoms on Si(100)-(2 × 1) studied by PAC

Chemisorption and desorption of isolated bromine adatoms on the Si(100)-(2 × 1) surface were investigated with nuclear methods. Br adsorption sites at low coverages of 10⁻³ monolayers (ML) were characterised by measuring the nuclear quadrupole interaction with perturbed γγ-angular correlation (PAC) of ⁷⁷Br→⁷⁷Se probe atoms. Below room temperature, two distinct adsorption sites for Br are revealed by PAC. One of them disappears after isochronal annealing above 300 K. The more stable probe-atom state is associated with single Br atoms saturating a dangling bond of the surface, while the less stable state is attributed to adsorption of Br at a bridge site. The potential barrier between the two adsorption sites is estimated to be 0.9(1) eV. At temperatures above 550 K, the fraction of atoms on distinct sites decreases, presumably due to surface diffusion. By measuring the γ-activity of the sample, complete desorption of the ⁷⁷Br atoms was observed above 620 K.     

Imaging metal surfaces by pulsed laser stimulated field desorption of field adsorbed atoms

Atomic images of Pt and Mo surfaces have been obtained using pulsed laser stimulated field desorption of field adsorbed atoms. The resolution of the desorption images is slightly inferior to low temperature field ion images. The direct correspondence of the images produced by thermally desorbed field adsorbed atoms with field ion images supports previous calculations suggesting apex site field adsorption of image gas atoms on field emitter surfaces.     

Aggregates of chemisorbed copper on the (110) and (100) surfaces of tungsten: electronic structure

In a certain range of temperature and coverage, copper atoms chemisorbed on low-index facets of a thermally annealed tungsten field emitter aggregate to form stable islands surrounded by regions of extremely low adsorbate density, yielding a spatially inhomogeneous overlayer. Spatially resolved field emission and photofield emission measurements of the surface density of electronic states have been carried out for copper aggregates on the (110) and (100) facets. Features detected in the surface density of states near the centre of an aggregate yield clues to its structure. On the (110) facet, the measurements reveal striking differences between the surface density of states near the edge of an aggregate and that near the centre. It is argued that the adsorbate configuration in the region between the aggregates might approximate single-atom adsorption.     

Grain size dependence of the electric field gradient

Precise measurements of the electric field gradient (EFG) with the perturbed angular correlation method (PAC) in Cd and Zn show significant differences between single-crystalline and poly-crystalline samples. These differences (up to four percent at low temperatures) decrease with increasing temperature and vanish at about 2 60 K. In all samples the lowest EFG values are obtained for single crystals. In poly-crystalline samples the EFG as well as the width of the EFG distribution increase with decreasing grain size. This effect is also observed in semiconducting ZnO.This work was supported financially by the Bundesministerium für Forschung und Technologie.     

Density functional theory study of the adsorption of oxygen atoms on gold (111), (100) and (211) surfaces

The interaction of an oxygen atom with various gold surfaces was examined computationally using density functional theory. The relative binding energies for a range of possible adatom locations on each surface were obtained. The results demonstrated the relative importance of low-coordinated gold atoms to bind oxygen for a particular surface and a preference for three-fold adatom coordination over the two-fold and single-coordination sites. Pseudo-potential energy curves were obtained by following the calculated energy as a function of surface position. These general results provide a reference for the interaction of oxygen atoms with gold nanoparticles that may project faces similar to the surfaces studied here.