Low-energy electron diffraction intensities

Atrei A, Bardi U, Rovida G, Torrini M, Zanazzi E, Ross PN (1992) Structure of the (001)- and (lll)-oriented surfaces of the ordered fee Pt Sn alloy by low-energy-electron-diffraction intensity analysis. Phys Rev B 46 1649... 

M.A. Van Hove, R.F. Lin, and G.A. Somorjai. Surface Structure Determination of Coadsorbed Benzene and Carbon Monoxide on the Rhodium (111) Single Crystal Surface Analyzed with Low-Energy Electron Diffraction Intensities. J. Am. Chem. Soc. 108 2532 (1986). 

Adsorption of ethylene and acetylene on platinum in the presence of hydrogen was investigated using elastic recoil detection analysis (ERDA). Below 200 K no change was observed for acetylene, while at room temperature C2H3 radicals were formed The geometry of acetylene adsorbed on the Ni (100) face was determined by LEED (low energy electron diffraction) intensity analysis. ... 

LEED-IV (low energy electron diffraction, intensity/voltage analysis). 

E. G. McRae, Multiple-scattering treatment of low energy electron diffraction intensities, J. Chem. Phys. 45, 3258-3276 (1966). 

Determinations of the surface structure by computing the diffraction beam intensities from low energy electron diffraction are concentrated in two frontier areas at present. One is the determination of the surface structures of adsorbed molecules of ever bigger size and the other is the determination of the atomic locations in reconstructed clean solid surfaces. 

Fig. 2.4 Low-energy electron diffraction (LEED). (a) Apparatus, showing how electrons reflected from a surface are detected by a fluorescent screen, (b) LEED pattern obtained from the surface of a tungsten oxide crystal. The bright spots show reflected electron beams. Measurement of their angles and Intensities gives information about the positions of atoms on the surface.

Atoms are not rigidly bound to the lattice, but rather vibrate around their equilibrium positions. If we were able to examine the crystal over a very brief observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this is for example the reason that in low-energy electron diffraction (LEED) the spot intensities of diffracted beams depend on temperature. At high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms is not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in extended X-ray absorption fine structure (EXAFS) and in Mossbauer spectroscopy. 

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). 

The effect of surface atom vibrations is seen clearly in low energy electron diffraction (EEED). Experimentally, an exponential decrease in the intensity of scattered beams and an increase in background intensity are observed with increasing temperature. This arises as a result of the increased vibrational amplitude of the surface atoms that occurs at... 

Experimental and theoretical results support the fact that the CO molecules are adsorbed on the densely packed surfaces of Pt, Pd, Rh, and Ir with the C-O axis normal to the surface and with the carbon atom directed to the surface. Experimental evidence is derived from angular resolved UPS, ESDIAD, ion scattering, low-energy electron diffraction (LEED) intensity analysis, and EELS. 

Fig. 6. Low energy electron diffraction patterns at normal incidence from clean tungsten surfaces, (a) Ball model of W(llO) face. Some of the net lines (hk) are indexed in terms of a centered rectangular unit mesh (outlined), (b) Clean W(llO), 75 V. Diffuse brightness and central bright spot are caused by light from electron gun filament, (c) Clean W(llO), 300 V. (d) Ball model of (112) surface, the third densest of the boo lattice, (e) Clean W(112) at 90 V. Note the asymmetric intensities of the A/c and hA beams. The unit mesh contains only a single mirror plane perpendicular to surface. There is a strong scattering contribution from the exposed second layer which is asymmetrically positioned.

Monoenergetic electrons ( 100 eV) Low energy electron diffraction (LEED) Elastic scattering Diffracted intensity Ordering... 

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