Scattering Of Atomic Beams Research Articles

He Atomic Beam Scattering Study of Pt(111) Super-Structure Surface Induced by High Energy Molecular Beam of H2O

The super-structure, Pt(111)-(2×2), is investigated both by He atomic beam scattering (HAS) and low energy electron diffraction. The structure is produced by irradiation of a high energy H2O molecular beam, which is accelerated up to about 300 meV by dilution in He gas molecules. The HAS result shows that the structure is stable at 200 K and becomes unstable as the sample temperature is raised to 300 K. The observed structure is believed to be due to reconstruction of the Pt(111) surface, which is caused by H atoms produced during the collision process of H2O molecules with sufficiently high energy for dissociation.

A Scaling Calculation of the Scattering of 4He Atomic Beams

We describe a simulation of the scattering in beams of helium atoms. The number of atoms N in the beams is reduced by a large scaling factor λ while the collision cross-section is increased by λ. This leaves the rate of scattering for each particle unchanged. As an example, we predict the outcome of a low temperature atomic beam experiment to measure the4He-4He atomic scattering cross-section σ at low energies. Because of the existence of a very weakly bound dimer, the low energy cross-section is expected to be unusually large, ∼1.83 × 105A2. In the simulation N/λ is small enough for the trajectories of all the scaled atoms to be calculated numerically. The simulation shows that the experiment is quite practicable. The proposed apparatus is just over 20 cm long, and a few centimeters wide, small enough to fit in a dilution refrigerator. The heaters and bolometers are assumed to be similar to those used in previous low temperature scattering experiments. We show that, using low intensity beams, the cross-section can be measured as a function of the relative velocity vrbetween ∼2 and ∼8 m/sec, corresponding to relative energies between ∼1 and ∼16 mK. By fitting σ(vr) one can determine the scattering length and effective range of the interaction. We predict that, at high intensity where multiple scattering is very important, the two beams coalesce into one.

Helium Atom Beam Scattering as a Probe for the Kinetics on Singular and Vicinal Surfaces : Fundamental Aspects of Ordering, Growth and Ablation

Helium Atom Beam Scattering (HAS) allows for a detailed structural characterization of growth and ablation processes in situ, on an angstrom scale, and in real time. Its application to the growth, ablation and ordering of thin Cu films on singular and vicinal Cu templates is briefly reviewed. The investigation of such homoepitaxial systems allows to address the kinetic aspect of growth and ablation in its pure form. It appears this way that in both processes an asymmetry in the horizontal and vertical mobility of atoms, associated with the presence of an excess energy barrier to diffusion at step edges, plays a key role in the formation of the observed selforganized patterned structures on the nanometerscale.

Adsorption and bonding of molecular icosahedra on Cu(100)

Closo-l,2-dicarbadodecaborane (C 2B 10H 12, otherwise known as orthocarborane) adsorption and bonding on Cu(100) were investigated over the substrate temperature range 150 to 400 K using atomic beam scattering (ABS), photoemission and Auger electron spectroscopy. At 150–180 K, orthocarborane adsorbed molecularly with one or two carbon atoms forming bonds with the copper surface in the monolayer coverage range. The molecular orientation within the film becomes more disoriented as coverage is increased above 1 monolayer. For adsorption at 250 K or higher, there is clear evidence that orthocarborane dissociatively adsorbs on the Cu(100) surface. The photoemission features attributable to the orthocarborane molecular orbitals have been compared with gas phase measurements and theory.

Metal adatom induced corrugation of Cu(001)

We report the discovery of a metal adatom induced corrugation of Cu(001) as probed by atom beam scattering (ABS). At Pb or Bi coverages of 0.05 (fraction of Cu layer), and while a (1×1) LEED pattern is still observed, the ABS diffraction pattern from uncovered Cu(001) areas has changed considerably from that of clean Cu(001). We show that these uncovered areas have become up to ten times more corrugated than the clean Cu(001). We performed several carefully designed experiments to test whether our data could be interpreted as a result of scattering from adsorbate islands or from a lattice of randomly occupied sites. Instead, we ascribe the nature of this metal adatom induced corrugation to the redistribution of the Cu(001) surface charge density due to the adsorption of Pb or Bi atoms. No effect is seen following adsorption of weakly chemisorbed adatoms, such as Hg. A model consistent with all our observations is presented.

Corrugation enhancement of Cu(001) induced by low-coverage Pb and Bi adsorption.

We report the discovery of a metal-adatom-induced corrugation of Cu(001) studied by surface-sensitive atom beam scattering (ABS). At a Pb or Bi coverage of 0.05 (fraction of Cu layer), and while a (1×1) low-energy electron diffraction pattern is still observed, the ABS diffraction pattern of Cu(001) has changed considerably from that of clean Cu(001). We show that the surface has become up to 10 times more corrugated than the clean Cu(001). The nature of this metal-adatom-induced corrugation is ascribed to the redistribution of the Cu(001) surface charge density

Ordering of metal overlayers on metal substrates studied using atom beam scattering

We will illustrate how atom beam scattering (ABS) can be used to study the growth and ordering of adsorption of Hg on Cu(001). At low coverage (less than 2% of a monolayer), we obtain that the scattering cross section of He with (adsorbed) Hg is about 50 Å2. We find that mercury forms two stable phases on Cu(001) in the temperature range between 200 and 330 K, a c(2×2) and a high density c(4×4) phase. From ABS diffraction data, we can deduce the corrugation of the surface electron charge density of the mercury layer in the two ordered phases. From the analysis of our data, we obtain a Debye temperature for the Hg overlayer of about 115 K. We will report preliminary calculations on the adsorption energy of Hg on Cu(001) and on the interaction of He with a Hg preplated Cu(001) surface.

Structural and Electronic Changes in the Growth of Mercury Overlayers on Cu(001): A Helium Beam Scattering, Leed and Arpes Study

AbstractWe used atom beam scattering (ABS), LEED and angle-resolved photo-electron spectroscopy (ARPES) to study bilayer films of Hg on Cu(001). In the surface temperature range of 180 to 330 K, the first Hg layer forms two ordered phases, a c(2×2) (with coverage=0.5 of Cu(001)) and a high density (partially commensurate) c(4×4) (coverage=0.62 ). ARPES data show that there is little or no dispersion of the 5d band of Hg. ABS data show that this layer is not flat, with in-registry Hg atoms lying about 0.15 below the not-in-registry Hg atoms. From ABS we find that the second layer forms a completely registered c(4times;4) phase. From ARPES we obtain that the second layer has an electronic structure, particularly the 5d levels, characteristic of bulk mercury.Preliminary results of calculations of the structure of the bilayer are given.

Structure of physisorbed overlayers of dipolar molecules: A combined study by atomic-beam scattering and molecular dynamics.

Structure of physisorbed overlayers of dipolar molecules: A combined study by atomic-beam scattering and molecular dynamics.

Atomic Beam Scattering

Atomic Beam Scattering

Scattering of atomic beams off a single surface step: An exact solution

The scattering of atomic beams off a single rectangular step on a planar hard-wall surface is calculated exactly using the Wiener-Hopf technique. The diffuse scattering consists of a sharp peak around the specular direction which is due to interference effects between the two surface levels and which oscillates in magnitude with incident angle. A second part then describes a smooth background scattering which strongly depends on the actual shape of the step whereas the former does not. Since the step is an infinitely extended defect a finite part of the incident atoms is scattered into the peak around specular, in contrast to the case of a compact scattering object, where only an infinitesimal part is removed from the specular beam.

Scattering of atomic beams off a single surface step: An exact solution: G. Blatter. Theoretische Physik, ETH Hönggerberg, 8093 Zürich, Switzerland

Scattering of atomic beams off a single surface step: An exact solution: G. Blatter. Theoretische Physik, ETH Hönggerberg, 8093 Zürich, Switzerland

Scattering of atomic beams off rough stepped surfaces

Scattering of atomic beams off rough stepped surfaces

Scattering of atomic beams off rough stepped surfaces

The differential scattering power for diffraction of atomic beams by a rough surface is calculated in eikonal approximation using a hard-wall potential for the atom-surface interaction. The surface is described by the harmonic rotator Hamiltonian with an external field. The phase correlation, which is related to the surface height correlation, is calculated using perturbation theory and renormalization group methods. For the rough surface state the phase correlation function decays only algebraically leading to narrow peaks in the differential scattering power at low temperatures. For an instrument with an angular resolution better than 0.25° the oscillations of the shape of the specular peak with varying incident angle should be detectable. This allows for distinction between the smooth and rough surface state and determination of the temperature dependent exponent for the phase correlation and the transition temperature.

Semiclassical Approach to the Scattering of Atomic Beams by a Corrugated Surface Potential: An Approximate Analytical Solution

An approximate analytic solution to the scattering problem of atoms by a weakly corrugated Morse potential is presented. In the classical limit the resulting scattering amplitude coincides with that given by the Kirchhoff approximation for the corrugated hard-wall model with a corrugation function identical to that of the Morse potential. Quantum mechanical effects appear as deviations from the Kirchhoff formula. For a general potential it is shown that it is the potential corrugation near vanishing potential which dominates the scattering process and not that at the classical turning point.

Scattering of atomic beams off stepped surfaces

Surface steps, because they are extended defects, give rise to strong diffuse scattering due to interference between the scattering off the different surface levels. We have calculated this diffuse scattering for a number of different configurations of random arrays of steps, using eikonal approximations which compare well with exact results in the case of a single step. This strong diffuse scattering is concentrated near the specular direction and its width is inversely proportional to the average separation of steps. Its shape gives information on the distribution and the degree of short-range order among the steps. The variation of the pattern with incident angle depends on the step height and also on the details of the step distribution.

Scattering of atomic and molecular beams at metal surfaces

The principles, variable parameters and potential information underlying experiments which involve the scattering of neutral particles with thermal kinetic energies at metal surfaces are briefly outlined. A few examples for the application of ground-state particles are presented: Diffractive scattering, studies on the kinetics of adsorption and desorption as well as reactive scattering. Scattering of electronically excited particles may give rise to electron emission as is demonstrated for metastable He-atoms impinging on clean and adsorbate covered Pd surfaces. Analysis of the electron density distributions yields information on the valence electron states of the outmost layers.

Scattering of atomic beams by surface and bulk phonons

The general expressions obtained in a previous work are further developed to get general formulae for one-phonon scattering of atomic beams in closed form. The scattering by bulk and surface phonons are discussed separately and the respective scattered energy spectra and behaviour near the Bragg angles are carefully treated. Interesting differences are found, which may have experimental importance in separating events of scattering from different modes. The expressions for the inelastic scattering differential cross-sections obtained are applied to the experiment by Williams et al. on the He/NaF(001) system. Despite the use of estimated values for the unknown parameters, a good quantitative agreement with the experiment is obtained, and the structure observed by Williams near the specular peak is clearly explained. The contribution of transverse bulk modes is found to be about 30% of that of transverse surface modes. The contribution of longitudinal modes in smaller than the contribution of the respective transverse modes by a factor 0.01.

Scattering of potassium atomic beams from various cleaved surfaces

Scattering of potassium atomic beams from cleaved surfaces of LiF, KCl and mica were studied. The observed spatial distributions follow the cosine distribution in most cases. At large incident angles the distributions from LiF and KCl deviate from the cosine and show apparent backscatterings. A simple model is proposed to interpret the backscattering in terms of macroscopic roughness of cleaved surfaces. The sticking coefficient S ( t) is defined and the mass balance between the incident and outgoing atoms is examined. Mass balance holds within the limits of errors on the surfaces of LiF and KCl at temperatures above 400 K. On a mica surface deposition of potassium atoms is observed even at temperatures higher than room temperature. It is shown from the change of the sticking coefficient that the deposition does not proceed linearly in the initial period.

Variable Energy Experiments on Atomic Beam Scattering from a NaF (001) Surface

Velocity selected hydrogen and deuterium atoms with kinetic energies from 20 meV to 140 meV have been scattered and diffracted from a NaF (001) cleavage plane. Highly resolved minima of selective adsorption yield binding energies and the gas-atom-surface interaction potential perpendicular to the surface. Specular intensity versus incident energy curves and specular intensity versus angle of incidence curves for different surface temperatures are compared to available theories. From these measurements the Debye-Waller-factor is extracted, which depends on the angle of atomic incidence and is characterised by a surface Debye temperature of θD⊥=370 K and the depth of the attractive potential D0=17.9 meV. The strength of the (first order) periodic part of the potential is found to be D1=0.025 D0.