Surfaces low-energy electron diffraction

Figure 8b. The surface structure of ethylidyne adsorbed on Pt(l 11) as obtained by low-energy electron diffraction surface crystallography.

When a low energy electron diffraction-surface crystallography study is carried out on the same system, a second layer of carbon can be detected under the graphitic overlayer (Figure 11b). In this second layer the carbon atoms are in the troughs and atomic crevices provided by the metallic substrate and the metal surface atoms also occupy positions different from their equilibrium bulk-like positions. 

Over the past twenty years over 250 surface structures have been solved, most of them by low energy electron diffraction-surface crystallography(9). From these studies an entirely different model of the structure of surfaces emerged on the atomic scale. This model indicates a dynamic structure, a structure where the atomic positions of the surface atoms are different from that predicted by the rigid lattice model when the surface is clean and change again when chemisorption occurs. 

Figure 11b. Low energy electron diffraction surface crystallography determination of the structure of a graphite adsorbed layer on the platinum (111) crystal face. The presence of two layers of carbon is clearly visible. (Reproduced with permission from Lawrence Berkeley Laboratory.)...

Ueb] AES (auger electron spectroscopy), XPS (X-ray photo-electron spectroscopy), LEED (Low energy electron diffraction) Surface cosegregation and surface preeipitation on Fe-3V-C 100 single crystals... 

FEED Low Energy Electron Diffraction Surface Mono-energetic electron beam 10-1000 eV Diffracted electrons 0.4-2 nm <6 pm Crystallographic structure ol surtace resolution 0.01 nm 36... 

LEED Low-energy electron diffraction [62, 75, 105] Elastic backscattering of electrons (10-200 eV) Surface structure... 

The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

We will, in the latter part of this discussion, focus only on those few methods that have been the most productive, with low-energy electron diffraction (FEED) receiving the most attention. Indeed, LEED has been the most successfiil surface stmctiiral method in two quite distinct ways. First, LEED has become an almost universal characterization... 

Rous P J 1993 A global approach to the search problem in surface crystallography by low-energy electron diffraction Surf. Sc 296 358-73... 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

G. Ertl and J. Kiippers. Low-Energy Electrons and Surface Chemistry. Verlag Chemie, Weinheim, 1974, Chps. 9 and 10. An introductory treatment of diffraction. 

L. J. Clarke, Surface Crystallography -An Introduction to Low Energy Electron Diffraction, John Wiley and Sons, Chichester, 1985. 2-245 U. ScHEiTHAUER, G. Meyer, M. Henz-LER, Surf Sci. 178 (1986) 441. 

Discovery of Surface Phases by Low Energy Electron Diffraction (LEED) John W. May... 

The most appropriate experimental procedure is to treat the metal in UHV, controlling the state of the surface with spectroscopic techniques (low-energy electron diffraction, LEED atomic emission spectroscopy, AES), followed by rapid and protected transfer into the electrochemical cell. This assemblage is definitely appropriate for comparing UHV and electrochemical experiments. However, the effect of the contact with the solution must always be checked, possibly with a backward transfer. These aspects are discussed in further detail for specific metals later on. 

H. Ohtani, C.-T. Kao, M.A.V. Hove, and G. Somorjai, A tabulation and classification of the stmctures of clean solid surfaces and of adsorbed atomic and molecular monolayes as determined from low energy electron diffraction patterns, Progress in Surface Science 23(2,3), 155-316 (1986) and reference therein. 

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