Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast ³He and ⁴He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.     

Efficient calculation of electron diffraction for the structural determination of nanomaterials

A critical advance in the technique of low-energy electron diffraction is presented and shown to enable determining detailed structures of nanomaterials, based on experimental methods that already exist or have been proposed. Our new cluster approach speeds up the computation to scale as n logn, rather than the current n3 or n2, with n the number of atoms, for example. Applications are illustrated for C60 molecules adsorbed on a Cu(111) surface, with and without coadsorbed metal atoms, exhibiting sensitivity to important structural features such as buckyball size and deformation.     

Influence of surface oxidation on the photoelectron diffraction intensities from InP single crystals

The Monte Carlo method was used to calculate the influence of amorphous overlayers on the angle-dependent intensity distribution due to photoelectron diffraction (XPD) in monocrystalline substrates. The influences of the overlayer thickness, the elastic and inelastic mean free paths in the amorphous overlayer, the shape of the initial intensity distribution, as well as the overlayer chemical composition, on XPD is discussed and some trends are formulated. The simulated P 2p intensities of oxidized InP agree with the experimental data. The effect of elastic scattering on the intensity minimum around the detection angle 18° is stronger than that of inelastic scattering. A new method for the thickness determination of thin overlayers on single ecrystals is discussed.     

Calculated beam intensities for low energy positron diffraction

The intensity of the specular beam for low energy positron diffraction (LEPD) from the Cu(111) surface is calculated in the energy range 20–150 eV. Close agreement is found with the first experimental data for LEPD published recently. This result indicates that LEPD may be exploited as a technique for determining surface structures in the near future. It is suggested that LEPD may have advantages in surface structure analysis in comparison with low energy electron diffraction (LEED).     

Interaction potentials for diffraction of a He atom by a NaCl(001) surface

There exist two quite different sets of experimental results for the He-NaCl(001) interaction potential, inferred from bound-state resonance measurements of associated ⁴He-NaCl(001) bound-state energies E_n: Frankl's group has found a lowest bound-state energy E₀ ≈ −4.1 meV, whereas Benedek's group has found evidence of a deeper interaction potential well, with E₀≈ −7.2 meV. Furthermore, there are strong physical arguments for the existence of the deeper well. In the present paper, interaction potentials, based on empirical pairwise He-Na⁺ and He-Cl⁻ interactions, for diffraction of a He atom by a NaCl(001) surface are considered. Comparisons of close-coupling diffraction calculations with experimental data of Frankl's group are made, and values of the empirical parameters are inferred. On the basis of these comparisons, it is concluded that the lowest bound state proposed by Benedek's group is absent, and that the correct bound-state energies are those proposed by Frankl's group.     

Fourier analysis of leed intensities in the determination of surface structures

The energy dependence of low energy electron diffraction intensities is considered in relation to a quasi-Laue condition which allows the use of Fourier transform methods for the direct determination of the surface structures of Si(100)-4. Some remarks are made on the value of the kinematic expressions for the structure factor amplitude.     

Inverted photoelectron diffraction: a new technique for surface structure determination

A reversal of the conventional X-ray photoelectron diffraction (XPD) experiment is described in which an incident electron beam is diffracted to reach different sites within a crystal with differing relative probabilities, which are expected to mirror those for the escape of photoelectrons from the same sites in the XPD experiment. The diffracted-electron flux arriving at each site is monitored by measuring the intensity of the characteristic X-ray emission following core ionisation by the incident electrons. This new approach potentially offers advantages over XPD in respect of minimum sample size and angular resolution in relation to equipment costs. Preliminary results from a GaAs crystal confirm that a clear correlation exists between results from the new technique and from XPD.     

Linear photoelectron diffraction: application of a rapid approximation for surface structural studies

The linear superposition approximation proposed by Wander, Pendry and Van Hove for efficient low-energy electron diffraction calculations (“linear LEED”) is applied to photoelectron diffraction (“linear PD”). As with linear LEED, linear PD works very well for calculating the effect of displaced atoms. However, due to strong forward scattering at higher energies, linear PD requires that atoms do not move into or out of alignment. This limitation can be removed by suitable simple adjustments to the basic approximation, promising to make the method effective for structural searches of complex surfaces.     

Fast atom diffraction during grazing scattering from a MgO(001) surface

Fast neutral atoms and molecules with energies from 0.4 up to 3 keV are scattered under a grazing angle of incidence from a clean and flat MgO(001) surface. For “axial surface channeling” conditions, we observe defined diffraction patterns in the angular intensity distributions for scattered ³He and ⁴He atoms as well as H₂ molecules. The diffraction patterns are analyzed in terms of semiclassical trajectory calculations making use of projectile surface interaction potentials derived from density functional theory and from pair potentials calculated from Hartree–Fock wave functions. From comparison of measured and calculated diffraction patterns we deduced the rumpling of the topmost surface layer of MgO(001), i.e. an inward shift of Mg²⁺ ions with respect to O^2? ions, of (0.03±0.03) Å.     

He atom diffraction from a silicide precipitate on a Pt(100) surface

We have observed the formation of an ordered silicide at the surface of a nominally pure Pt(100) crystal. The silicide appears after thermally activated migration of a trace Si impurity (<10^?4 at%) and subsequent cooling. The silicide is about one monolayer thick and forms a large area unit mesh, $\text{(6}\text{2}\text{×6}\text{2}\text{)R45°}$, which is observed with both He diffraction and LEED. A thermal attenuation analysis of the specular and a diffracted He beam yields a consistent value for the attractive depth of the He-surface potential D=13 meV. From a simple classical analysis of the He diffraction pattern we have obtained an estimate for the corrugation of the He-surface potential, $\text{ξ=0.07}\text{A}\text{?}$. The well depth D is shown to be consistent with the wavelength dependence of the angular distribution of the diffraction pattern.     

Global search algorithms in surface structure determination using photoelectron diffraction

Three different algorithms to effect global searches of the variable-parameter hyperspace are compared for application to the determination of surface structure using the technique of scanned-energy mode photoelectron diffraction (PhD). Specifically, a new method not previously used in any surface science methods, the swarm-intelligence-based particle swarm optimisation (PSO) method, is presented and its results compared with implementations of fast simulated annealing (FSA) and a genetic algorithm (GA). These three techniques have been applied to experimental data from three adsorption structures that had previously been solved by standard trial-and-error methods, namely H₂O on TiO₂(110), SO₂ on Ni(111) and CN on Cu(111). The performance of the three algorithms is compared to the results of a purely random sampling of the structural parameter hyperspace. For all three adsorbate systems, the PSO out-performs the other techniques as a fitting routine, although for two of the three systems studied the advantage relative to the GA and random sampling approaches is modest. The implementation of FSA failed to achieve acceptable fits in these tests.     

The determination of surface debye temperatures from low-energy electron diffraction data

The temperature dependence of low-energy electron diffraction (LEED) intensities has often been interpreted with kinematic theory in terms of an effective Debye temperature θ_D^eff of the surface atoms. The validity of this procedure, often questioned in the literature, is tested with a computer experiment in which LEED spectra are calculated from dynamical theory (layer-KKR method) for a model of Ag{111} with a given value of θ_D^eff and then the usual kinematic formulae are used to re-extract the value of θ_D^eff. The results of the experiment indicate that this procedure yields rough values of the surface Debye temperature for electron energies higher than about 40 eV, which fluctuate substantially and tend to be somewhat smaller than that originally introduced into the model. At energies lower than about 40 eV the kinematically deduced values of θ_D^eff are too large by 10 to 15 %.     

Inversion of acoustic diffraction fields in anisotropic solids

We show that the fast Fourier transform (FFT) technique provides a computationally efficient method of calculating 2D amplitude and phase images of complex wave fields generated and measured in elastically anisotropic solids by phase sensitive acoustic microscopy. Further, we discuss how this technique can be used to treat inverse problems such as source reconstruction, image quality assessment, and the determination of elastic constants.     

Three-dimensional coherent x-ray diffraction imaging of a ceramic nanofoam: determination of structural deformation mechanisms

Ultralow density polymers, metals, and ceramic nanofoams are valued for their high strength-to-weight ratio, high surface area, and insulating properties ascribed to their structural geometry. We obtain the labrynthine internal structure of a tantalum oxide nanofoam by x-ray diffractive imaging. Finite-element analysis from the structure reveals mechanical properties consistent with bulk samples and with a diffusion-limited cluster aggregation model, while excess mass on the nodes discounts the dangling fragments hypothesis of percolation theory.     

Calculations of low-energy electron diffraction intensities from the polar faces of ZnO

If it is assumed that surface compounds obtained by gas adsorption on metal single crystals faces, are formed by adsorption of atoms on the “sites” of an undisturbed substrate, or of a slightly disturbed substrate, one is lead also to assume that the structures of surface compounds have high symmetries. For well-defined coincidence mesh between the surface compound and the substrate, by applying the 2D space groups (with or without a colored character), information can be obtained concerning the distribution of the adsorbed atoms, or the reconstruction of the substrate. In order to be able to go as far as a complete model, it is necessary to know the concentration of the adsorbed atoms accurately. The hypothesis of a high symmetry has been applied to simple surface compounds: the models of the structures which have been obtained are compared with previously proposed models.