Diffraction Of He Atoms Research Articles

Quantum reflection of rare-gas atoms and clusters from a grating

Quantum reflection is a universal property of atoms and molecules when scattered from surfaces in ultracold collisions. Recent experimental work has documented the quantum reflection and diffraction of He atoms, dimers, trimers and Neon atoms when reflected from a grating. Conditions for the observation of emerging beam resonances have been discussed and measured. In this paper, we provide a theoretical simulation of the quantum reflection in these cases from a grating. We confirm, as expected the universal dependence on the incident de Broglie wavelength only of the threshold angles for the observation of emerging beam resonances. However, the angular dependence of the reflection efficiencies, that is the ratio of scattered intensity into specific diffraction channels relative to the total intensity is found to be dependent on the specifics of the incident particle. The dependence of the reflection efficiency on the identity of the particle is intimately related to the fact that the incident energy dependence of quantum reflection depends on the details of the particle surface interaction.

Diffraction of He atoms from Xe monolayer adsorbed on the graphite (0 0 0 1) revisited: the importance of multiple scattering processes

We comment and discuss the findings and conclusions of a recent theoretical study of the diffraction of He atoms from a monolayer of Xe atoms adsorbed on the graphite (0001) surface [Surf. Sci. 496 (2002) L13]. By revisiting the problem we demonstrate that all main conclusions from this reference that pertain to the studied system are at variance with the available experimental and theoretical evidence and the results of multiple scattering calculations presented in this comment.

Inelastic diffraction of He atoms from Xe overlayer adsorbed on the graphite ( [formula omitted

A simple model for the normal vibrations of noble gases adsorbed on graphite in the commensurate phase is presented. The external layer is presented as a lattice of atoms bonded to each other by threads with their tension defined by lateral interaction. The proposed model provides analytical expressions for phonon dispersion curves. It also allows one to construct the one-phonon dynamic structure factor, which is used in expressions for single-phonon inelastic surface-scattering probabilities. A quantitative comparison with experiments at the normal angle of incidence has also been completed.

Power laws in the growth kinetics of metal clusters on oxide surfaces

The possibility of expressing the growth kinetics of metal on weakly interacting substrates (oxides and insulators) by a power law is examined. By a model taking into account for direct growth and growth by capture of diffusing adatoms it is shown that the growth rate can be locally expressed by a power law. The model is restricted to the growth of three-dimensional clusters. In the case of a strong competition between clusters for the capture of adatoms a single exponent, between 0.33 and 0.35 is obtained before the coalescence stage. In the limit of isolated clusters the exponent increases continually from 1 3 –1 during the growth. The model is compared with growth kinetics of Pd clusters on MgO(100) obtained in situ and continuously by diffraction of He atoms. The agreement with the calculations is very good if we take into account for the finite nucleation rate. The present calculations have been also applied satisfactorily to the published data for other systems.

Semiclassical rainbow analysis in atom-surface scattering

A specular reflection by a terrace is currently associated to a classical surface rainbow. In this Letter, we present a semiclassical analysis of such a dynamical process to differentiate it from a real rainbow scattering by considering the diffraction of He atoms by the vicinal metal surfaces Cu(115) and Cu(117). A new interpretation of this specular reflection is given in terms of semiclassical arguments.

Investigation by thermal energy atom scattering of the (2×1) clean Si(100) surface and of the C(8×8) and (2×8) reconstructions induced by copper

For many years it has been firmly established that the primary reconstruction of the Si(100) surface is the (2×1) induced by the formation of dimers. But numerous studies have recently shown that other reconstructions attributed to various arrangements of dimers and vacancies can be obtained: C(4×2), C(2×2), C(4×4), C(8×8), (2×8), (2×n)... The possible influence of Ni contamination at very low concentration has been also pointed out in the formation of the (2×n) phases but the C(8×8) is attributed to the clean surface. In our study by diffraction of He atoms the C(8×8) reconstruction is obtained (after Ar+ bombardment, annealing and a slow cooling 10°/mn) and the first order phase transition (T=875 K) associated to the emergence of this phase is observed. A second first order phase transition (T=1175 K) is also pointed out and is associated to the appearance of a (2×8) reconstruction after quenching. A characterization of the surface by SIMS and x‐ray fluorescence reveals a maximum copper contamination of 0.08 at. unit cell. Then the clean surface is studied and these experimental results are discussed.

Scattering of He from a Cu(110) surface

Using the close-coupling method we have investigated the diffraction of He atoms from a Cu ( 110 ) surface. Interaction between He and the copper surface is represented by a corrugated Mie-type potential. The softness parameter β is “optimized” for a given incident energy ( E i) and incident angle (θ i) so as to reproduce the experimental results. An empirical expression for the softness parameter β( E i, θ i) is obtained. Our results are in good agreement with previous calculations and experimental observations.

The MgO(001) surface studied by helium diffraction

The diffraction of He atoms at thermal energy (~64 meV) from the (001) cleavage plane of MgO was studied. A hard-wall intensity analysis of in-plane spectra obtained at near normal incident angles and in the main azimuthal planes yields the corrugation function, which is simply sinusoidal in both unit-cell directions with a total corrugation amplitude of 0.47 ± 0.02 Å. This value is larger than the one previously reported by K.H. Rieder. A more detailed study of the surface is in progress.

Diffraction of He atoms from a Xe overlayer adsorbed on graphite (0001)

He beam scattering from a (√3 × √3)R30° lattice of xenon adsorbed on the graphite (0001) surface is measured. The experimental data at normal incidence, fitted by the hard-wall model and the eikonal approximation, yield a peak-to-peak corrugation amplitude of 0.88 ± 0.02 Å. The agreement is only qualitative and points to the need for a more accurate analysis, with calculations suitable for large corrugation amplitudes.

Diffraction of He atoms from A Xe overlayer adsorbed on graphite (0001)

He beam scattering from a (√3 × √3)R30° lattice of xenon adsorbed on the graphite (0001) surface is measured. The experimental data at normal incidence, fitted by the hard-wall model and the eikonal approximation, yield a peak-to-peak corrugation amplitude of 0.88±0.02 . The agreement is only qualitative and points to the need for a more accurate analysis, with calculations suitable for large corrugation amplitudes.

Diffraction of He atoms from A Xe overlayer adsorbed on graphite (0001)

He beam scattering from a (√3 × √3)R30° lattice of xenon adsorbed on the graphite (0001) surface is measured. The experimental data at normal incidence, fitted by the hard-wall model and the eikonal approximation, yield a peak-to-peak corrugation amplitude of 0.88 ± 0.02 Å. The agreement is only qualitative and points to the need for a more accurate analysis, with calculations suitable for large corrugation amplitudes.

Helium Diffraction as a Probe of Surface Structure

AbstractThe diffraction of He atoms from solid surfaces has been recently used to determine the electron distribution of electrons at surfaces. From this, the position of underlying atoms can usually be inferred. Recent developments which test the validity of the hard corrugated wall method to calculate diffraction intensities are discussed. Structural studies which have been carried out to date, on various surfaces with He beams, are tabulated.

The diffraction of He atoms at the GaAs(110) surface

We present an experimental study of the diffraction of He beams at the GaAs(110) surface. Diffraction angular scans and specular intensity scans are obtained for a wide range of incident angles, azimuths, and at two energies ( E = 0.063 eV and E = 0.021 eV ). The data have been qualitatively analyzed from a classical scattering viewpoint and the hard wall eikonal scattered wave approximation. A hardwall corrugation function of the form ζ(x, y) = 1 2 d x cos( 2πx L x) + 1 2 d y cos( 2πy L y) yields a qualitative fit to the data with d x ~ 1.1 Å and d y ~ 0.3 Å, w deep corrugation corresponds to going across the surface troughs. This value of d x is approximately half the corrugation of the nuclear positions. Specular intensity scans are analyzed in terms of interference in the normal momentum transfer, Δk ⊥, which yields structural information about vertical displacements. We discuss what is required in order to appropriately describe scattering from strongly corrugated surfaces.

The diffraction of He atoms at the GaAs(110) surface

The diffraction of He atoms at the GaAs(110) surface

Diffraction of He Atoms at a Si(111) 7 × 7 Surface

We report the diffraction of He atoms at the Si(111) 7\ifmmode\times\else\texttimes\fi{}7 reconstructed surface. The atom diffraction patterns consist of numerous sharp peaks filling out the seventh-order net observed by low-energy electron diffraction (LEED). In contrast to low-energy electron diffraction, the diffraction intensities in this seventh-order net are determined by only the outermost exposed layer of silicon atoms. These results should be sufficient to allow a semiclassical scattering calculation to test critically the various structural models which have been proposed for this reconstruction.

Diffraction of He Atoms at a Si(100) Surface

Diffraction of helium atoms at a clear Si(100) surface has been observed. Integral-order, half-order, and quarter-order peaks consistent with the two-domain $c(4\ifmmode\times\else\texttimes\fi{}2)$ surface reconstruction have been recorded. Additional peaks at the ($\frac{\overline{1}}{2}\frac{\overline{1}}{2}$) and ($\frac{\overline{3}}{2}\frac{\overline{3}}{2}$) positions are also seen. The diffraction intensities amount to \ensuremath{\sim}${10}^{\ensuremath{-}3}$ of the incident beam. The major portion of the scattering is spread over a large range of solid angle with broad maxima characteristic of rainbow scattering.