LEED surface crystallography

Some experimental techniques [e.g., low-energy electron diffraction (LEED)-surface crystallography] can detect the structural changes that occur on both sides of the surface chemical bond. However, most currently used techniques are only capable of detecting the structural changes that occur on the adsorbate side (e.g., infrared spectroscopy) or on the substrate side (e.g., electron microscopy). As a result, we often gain only incomplete information about the surface chemical bond, leading to a one-sided molecule-centric or surface-centric view of the adsorbate-surface compound that is produced. 

Low energy electron diffraction (LEED)-surface crystallography studies clearly indicate that the surfaces of most clean metals relax inward. The rougher the surface (the more open the surface structure), the greater is the relaxation. The (Iff) and (211) iron surface structures as determined by LEED-surface crystallography are shown in Fig. 3.84. d90... 

Surface crystallography started in the late 1960s, with the simplest possible structures being solved by LEED [14]. Such structures were the clean Ni (111), Cu(l 11) and Al(l 11) surfaces, which are unreconstructed and essentially unrelaxed, i.e. very close to the ideal temrination of the bulk shown in figure B 1.211 a) typically, only one unknown structural parameter was fitted to experiment, namely the spacing between the two outennost atomic layers. 

Clarke L J 1985 Surface Crystallography, An Introduction to LEED (Chichester Wiley)... 

M. A. Van Hove and S. Y. Tong, Surface Crystallography by LEED, Springer-Verlag, Berlin, 1979. 

The computational effort is larger in ARUPS than in LEED, since more physical processes are involved, so that for the limited purpose of surface crystallography LEED seems more appropriate. Furthermore, ARUPS is best done with synchrotron radiation, which limits its availability. 

Over the past several years the surface structures of several clean monatomic solid surfaces and a variety of adsorbed atoms on solid surfaces have been determined by LEED (7). This field of study is now called surface crystallography and is one of the most rapidly growing fields of surface science. By studying the atomic surface structure of clean surfaces and adsorbed molecules, the nature of the surface chemical bond can be explored in a systematic manner. 

If we limit ourselves to observed LEED patterns, we find that over the years about 2000 ordered structures have been reported./202/ Among these, perhaps 180 have been structurally solved by various techniques of surface crystallography. Intensity analyses of low-energy electron diffraction have contributed about 150 of these. The remaining 30 structures were obtained primarily with ion scattering (MEIS, HEIS), SEXAFS or photoelectron diffraction (NPD, ARXPS). 

L. J. Clarke, Surface Crystallography An Introduction to Low Energy Electron Diffraction , John Wiley Sons, Hoboken, 1985. A starter s book for LEED. 

It seems that no systematic treatment has been given of the problem of domain structure in surface crystallography, yet it is of the greatest importance in the understanding of LEED intensities. This subject has been considered by Lander 71) and by Park 303). Whenever a surface structure is in registry with the substrate, it is implicit that the origin of the surface structure unit mesh can be translated to other equivalent positions. In the general case these translation vectors are neither unit vectors of the sui face structure nor of the substrate, but are combinations of these vectors. And when a surface has steps, as real surfaces do, the top layer is at different levels at different places, and further equiva-... 

In addition to attempting to unravel basic surface crystallography, the other main application of LEED is the identification of surface defect structures [15, 33], including point defects, arrays of atomic steps, domain structures and facets. 

Early LEED studies [97] indicated that the basic structure of a clean 100 surface was 2x1. (For a discussion of the nomenclature of surface crystallography, see ref. 113.) Three different models have been proposed to account for this form of reconstruction the vacancy model [78, 79, 114] assumes that half the surface atoms are missing, while the other two suggestions invoke surface dimerization [80, 114] and the formation of complex conjugated chain structures [115, 116], respectively. We will consider each of these in a little more detail. 

Figure 1 A Model of the surface crystallography of iron oxide (Fe20s) as determined from LEED analysis of a thin fdm grown at Imbar oxygen pressure. The numbers indicate the positional changes of the atomic layers in percent with respect to the position in the bulk structure. B LEED image of a film before and after use as catalyst of dehydrogenation of ethylbenzene (EB) to styrene reaction temperature 873 K, reactant pressure Ibar, composition steam to EB 10 1, LHSV 0.5 h. The unit cell reflections for (001) Fe20s are indicated by circles. C Evolution of the conversion to styrene as function of time on stream under the conditions given in (B)...

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