LEED—See Low energy electron diffraction

LEED, see Low energy electron diffraction Lennard-Jones... 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

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. 

As early research on oscillatory reactions in heterogeneous catalysis began, little attention was given to the state of the catalyst surface. These first studies recorded the reaction rate by analysis of the product concentrations (see, e.g.. Refs. 3,81) or by measurement of catalyst temperatures 3,162). Later, however, attempts were also made to monitor the catalyst surface during the oscillations, first by measurement of the work function 81), and later by methods such as infrared (IR) spectroscopy 108) and low-energy electron diffraction (LEED) for HV oscillations 245). Table III lists methods employed to study oscillations. 

Haneman, 1961 and Lander, Gobeli, and Morrison, 1963) this lowering of symmetry is called reconstruction. For the ideal unreconstructed crystal it is not difficult to construct primitive surface translations (see Section 3-C for discussion of primitive translations), which are the smallest translations in the plane of the surface by which each atom is replaced by another atom. Let two of these translations be written Tj and X2. Low-Energy Electron Diffraction (LEED) measurements tell what the actual translational symmetry of a surface is and frequently give a lower symmetry. For example, it may indicate primitive translations of 2ti and tj this is called a 2 x 1 reconstruction. Similarly, an observed translational symmetry of t, and mx2 is called ann x m reconstruction. 

Low energy electron diffraction (LEED) - See Techniques for Materials Characterization, page 12-1. 

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