Diffuse LEED

Heinz K 1994 Diffuse LEED and local surface structure Phys. Status. Solid A 146 195-204... 

Usually, the collection of LEED 7-Vcurves requires single crystal surfaces with long-range order in the upper-most layers. Structural information can, however, also be obtained in a similar way for certain disordered surfaces, when the energy dependence of the diffusely scattered intensity is analyzed (diffuse LEED [2.264, 2.265]). 

Heinz K, Starke U, Vanhove MA, Somoijai GA. 1992. The angular-dependence of diffuse LEED intensities and its structural information-content. Siuf Sci 261 57 -63. 

Another system that has received recent intensive study by both diffraction and spectroscopic methods is that of ethene on Pt(lll). In recent papers, Somorjai et al. have investigated the adsorption of ethene by diffuse LEED to give the unexpected result that the di-cr species [which on Pt(lll) gives an extreme type I spectrum] is adsorbed across threefold sites with... 

There are a number of lattice-gas chemisorption systems, where atoms or molecules are adsorbed in well-defined sites on the surface, as determined by vibrational spectroscopy, but where there is no long range order. The adsorption sites and bond lengths for such systems can be determined by diffuse LEED calculations, as has been done for oxygen on the tungsten (100) surface./13/... 

Photoelectron diffraction is most useful for systems where the photoexcited atoms all have the same local geometry, as in a chemisorption problem. If there are source atoms in different local geometries, there will be interference between multiple sets of scattering paths, and the resulting interference spectrum will be harder to interpret. For these cases LEED experiments are probably better, with diffuse LEED used for disordered systems. 

This diffuse LEED theory consists of three parts ... 

Different surface-sensitive techniques respond differently to the various kinds of disorder. The measurement of LEED beams is unique in largely filtering out all defects that are unrelated to the superlattice periodicity that defines the beams. Other techniques (including diffuse LEED) generally include contributions from all defects, for instance from adsorbates located at undesired steps and crystallite boundaries. Then only a reduction of the defect concentration can remove defect contributions from the experimental data. Rotational disorder of adsorbed molecules does not matter for techniques which measure only bond lengths, and not bond orientations. Thus, NEXAFS is more sensitive to such disorder than SEXAFS. 

Diffuse LEED (DLEED) has been applied by Barnes et al. [46] to probe the local geometry of the Cu 100 /Pd system in the Pd coverage range 0.10 to 0.55 ML. Figure 4 illustrates DLEED (figures 4(a) and (b)) and conventional LEED I(V) spectra (figure 4(c)) from the (1/2,1/2) beam position. 

FEED pattern shows directly the size and orientation of the surface unit cell. Flowever, the symmetry of the unit cell is not necessarily identical to the experimentally observed symmetry in LEED including spot intensities. The properties of symmetries such as rotation axes or mirror planes affect the intensities of the spots. The distinction between the structural symmetries of the surface and symmetries observed among spot intensities in LEED can be performed by changing the direction of the incident electron beam. At oblique incidence only mirror planes can be observed when the incident beam is parallel to the symmetry plane, otherwise the diffraction pattern exhibits no symmetry. In cases where the surface is not well ordered one may obtain additional symmetry information from the diffused LEED spots. The characteristic distribution of diffuse intensity in reciprocal space indicates the existence of short-range ordered antiphase domains or twin domains. 

A. M., Buskotte, U., Neumann, M., Starke, U., Heinz, K., Andres, P. L. d., Saldin, D., Pendry, J. B. Adsorption and reaction ofC02 on Ni(llO) X-ray photoemission, near-edge X-ray absorption fine-structure and diffuse LEED studies Surf. Sci. 206 (1988) 1. 

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