Diffuse LEED spots

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. 

Many fonns of disorder in a surface structure can be recognized in the LEED pattern. The main manifestations of disorder are broadening and streaking of diffraction spots and diffuse intensity between spots [1]. 

Atoms are not rigidly bound to the lattice, but vibrate around their equilibrium positions. If we were able to look at the crystal with a very short observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this, for example, is the reason that in 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 are 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 EXAFS and in Mossbauer spectroscopy. 

Ordered LEED patterns were evident on all three surfaces. A Cu(100)(/2X/2)R45°-C1 was observed with IEED, the same pattern present on the surface before immersion, but with a slight increase in spot diffuseness. A 28% decrease in Cl Auger current was observed and was responsible for the degradation in LEED pattern clarity. It... 

Suppose now that only a proportion of these surface layer sites are occupied by adsorbate. The consequent combination of order and randomness in the surface layer will give a LEED pattern which shows the same spots (but at reduced intensity) together with some diffuseness around the centre spot. 

The diffraction pattern observed in LEED is one of the most commonly used fingerprints of a surface structure. With XRD or other non-electron diffraction methods, there is no convenient detector that images in real time the corresponding diffraction pattern. Point-source methods, like PD, do not produce a convenient spot pattern, but a diffuse diffraction pattern that does not simply reflect the long-range ordering. 

If the crystal surface is well-ordered, a diffraction pattern consisting of bright, well-defined spots will be displayed on the screen. The sharpness and overall intensity of the spots are related to the degree of order of the surface. When the surface is less ordered, the diffraction beams broaden and become less intense, while some diffuse intensity appears between the beams. A typical set of diffraction patterns from a well-ordered surface is shown in Fig. 4. The presence of the sharp diffraction spots clearly indicates that the surface is ordered on an atomic scale. Similar LEED patterns have been obtained from solid single-crystal surfaces of many types including metals, semiconductors, alloys, oxides, and intermetallics. 

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