He-atom scattering HAS

Fig. 9. Left He-atoms with thermal energies (10-60 meV) from a molecular beam source are scattered off the very top surface charge density (left), whereas electrons penetrate several layers. As a result, He-atom scattering, HAS, offers a unique surface sensitivity.

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. 

The ultimate resolution available today with a state-of-the-art He-atom scattering apparatus is better than 100 peV and allows to study diffusion of adsorbates on surfaces by analyzing the energetic width of the quasi-elastic scattering peak [88FRE]. The method has been applied to quite different systems, e.g. H/Pt(lll) [99GRA] andXe/Pt(lll)[99EEE]. 

Since the probe particles used in He-atom scattering carry no charge, the method can be applied to all kinds of substrates. Eimitations apply to the maximum energy which can be detected with this technique. So far the detection of vibrations above 30 meV has been limited to only very few cases. 

The energy barriers of surface diffusion can further be inferred from quasi-elastic hehum atom scattering (HAS) [39, 40]. The kinetic energy of the He atoms that are scattered from diffusing atoms reveals a small additional broadening due to momentum transfer. In analogy to quasi-elastic neutron scattering, which has been used to study diffusion in solid and liquid bulk samples, this can be used to measure diffusion rates. Other than STM, this technique can be applied up to T [41]. 

The peculiar behavior of H might be relevant to understand the hydrogen bond, which deforms the electronic cloud of the proton. On the other hand, it is surprising to discover an anomalous behavior for a closed-shell atom like He. However, it has been demonstrated in helium-atom-scattering that interactions between He atoms... 

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. ... 

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