He atom diffraction

This is attributed to the different nature of the bonding of sulphur to silver as compared to gold and the slightly different packing density. The coherence length detennined with He atom diffraction was found to be 12 mn [162]. 

The de Broglie wavelength of thermal He atoms is comparable with the interatomic distances of surfaces and adsorbed layers. Thus, from measurements of the angular positions of the diffraction peaks the size and orientation of the 2D unit cell, i.e. the structure of the outermost layer, can be straightforwardly determined. Analysis of the peak intensities yields the potential corrugation, which usually reflects the geometrical arrangement of the atoms within the 2D unit celP. 

Besides the inelastic component, always a certain number of He atoms are elastically scattered in directions lying between the coherent diffraction peaks. We will refer to this scattering as diffuse elastic scattering. This diffuse intensity is attributed to scattering from defects and impurities. Accordingly, it provides information on the degree and nature of surface disorder. It can be used for example to study the growth of thin films or to deduce information on the size, nature and orientation of surface defects Very recently from the analysis of the diffuse elastic peak width, information on the diffusive motion of surface atoms has been obtained. ... 

The detailed data from He-scattering experiments provide information about the electron density distribution on crystalline solid surfaces. Especially, it provides direct information on the corrugation amplitude of the surface charge density at the classical turning point of the incident He atom, as shown in Fig. 4.13. As a classical particle, an incident He atom can reach a point at the solid surface where its vertical kinetic energy equals the repulsive energy at that point. The corrugation amplitude of the surface electron density on that plane determines the intensity of the diffracted atomic beam. 

Fig. 4.6. Angular dependence of the diffraction of He atoms from a LiF(lOO) surface...

An important simplification in the analysis of He scattering is the large mass mismatch between He and most other atoms that are the constituents of solid surfaces. Therefore, energy transfer between the He atom and the surface is very much limited and elastic He diffraction or inelastic He scattering can be modelled rather easily and quantum mechanically. The situation is very different when heavier atoms or molecules are scattered from a surface. In this case energy exchange between projectile and surface will be facile, and in most cases only classical mechanics can be used to model the interaction. Most of the... 

First we want to know the interaction with the bare substrate does the molecule become physisorbed, chemisorbed, dissociate or react with the surface atoms, what is the interaction time To answer these questions we have to prepare a clean MgO surface, which is obtained by cleavage under UHV (the cleanliness and the perfect surface order are checked by diffraction of He atoms and AES [23]). Then, a pulsed beam of NO is directed to the MgO clean surface and the (possible) desorbed products (NO, N2, N20, N02, 02) are simultaneously detected by the mass spectrometer. 

The Ge(100)p(2xl) surface has been the subject of fewer studies that the analogous Si surface. In particular, the symmetric vs asymmetric dimer question remains unresolved for this surface. Photoemission (Kevan, 1985), He-atom scattering (Lambert et al., 1987), STM (Kubby et al., 1987) and an earlier XRD study (Eisenberger and Marra, 1981) indicate that the asymmetric is the majority species at the surface. However a recent, more detailed, X-ray diffraction study favors symmetric, or almost symmetric, dimer formation (Grey et al., 1988).The structural parameters for two relatively complete XRD determinations of the Ge(100)(2xl), reconstructed surfaces are given in table 15. 

We shall be concerned principally with electron diffraction, but recently some important results have been obtained using neutral atom (He) beam diffraction, to which we make brief reference in Sect. 3. The applications of interest are to the determination of the crystallography of clean surfaces and also of surface—adsorbate systems. 

Such waves can be used to probe the structure of crystal surfaces, through low-energy electron diffraction (LEED) or atomic beam diffraction. The latter is usually confined to He atoms, but even Ar atom diffraction can be discerned... 

The phonon excitation can be probed by scattering with neutrons or He-atoms which have De Broglie wavelengths comparable to those of the phonons. For the probing of the softer surface phonons, scattering with low-energy He atom beams is ideal. The elastic scattering probabilities, i.e., the diffraction peaks, contain two factors. One depends on the structure of the solid while the other involves the temperature-dependent effects. Thus the fact that the experiments are... 

G. Boato, P. Cantini, R. Tatarek, and R. Felcher, Diffraction of He atoms by the basal (0001) face of graphite A study of bound state resonances. Surface Sci. 80 518 (1979). 

The main advantage of diffraction studies is the direct relationship between the scattered intensity and the surface periodicity. Because of the extreme sensitivity of He scattering to the surface region, He-beam diffraction experiments have been able to unequivocally identify surface structures that have not been seen by other approaches. Unfortunately further interpretation of diffraction data in order to obtain atom-surface potential information requires the use of a fairly detailed theoretical model of the scattering. 

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