Adsorbed Monolayers on Solid Surfaces by G.A. Somorjai, M.A. van Hove

By G.A. Somorjai, M.A. van Hove

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Depending on energy and incidence direction (including channeling and blocking effects) the depth resolution of ISS can vary from a monolayer to about 300 A. Quantitative evaluations of the chemical composition of surfaces may be achieved in special cases, but are generally hampered by uncertainties in the ion-atom scattering potential (especially at the lower energies), the possibility of multiple scatterings and the ever-present question of the depth distribution of individual species. 1 IR (Infrared Spectroscopy) Absorption of infrared radiation by characteristic vibrations of a surface can be used to obtain information about that surface, by comparison with known absorption frequencies in molecules of known structure.

With increasing energy the diffraction spots converge toward the specular reflection spot, here hidden by the crystal sample requires an analysis of the intensities of diffraction. Nevertheless, two-dimensional LEED already can give a very good idea of essential features of the surface geometry, in addition to those mentioned before. Thus one may follow the variation of the diffraction pattern as a function of exposure to foreign atoms: it is often possible to obtain semi-quantitative values for the coverage, for the attractive and/or repulsive interactions between adsorbates s), for some details of island formation 6), etc.

5y ' ' ' ' ' X . 5 I-- N Z tlJ o ttJ rr Ld os c) 03 I O, 30 60 ANGLE FROM SURFACE NORMAL 90 Fig. 7. Normalized scattered intensity vs. angle from surface normal for an azimuthal angle o f 4~= 24 ° with a fixed angle of incidence o f 45 ° on a stepped Pt(553) surface for classical scattering for helium atoms. At grazing angles of incidence, the surface appears less spatially rough and thus allows a more intense elastic scattering contribution. At more normal angles of incidence, the elastic scattering distribution is less intense due to an increase in spatial roughening of the surface as seen by the helium atom.

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