Physical Chemistry Essentials by Andreas Hofmann

Author:Andreas Hofmann
Language: eng
Format: epub, pdf
Publisher: Springer International Publishing, Cham

7.3.4 LEED

The arrangement of atoms close to the surface can be determined by low energy electron diffraction ( LEED). Here, the wave character of electrons is used. An incident electron beam of relatively low energy (typically in the range 20–200 eV) is diffracted by the atomic lattice formed by atoms on or close to the surface. The diffracted electrons can be observed by a fluorescent screen, and the resulting diffraction pattern is then visible from behind the sample (modern instruments use a position sensitive detector instead of the fluorescent screen, thus enabling digital data acquisition). The diffraction pattern represents the two-dimensional reciprocal lattice of the specimen’s surface.

As introduced in Sect. 7.2.1, hardly any surface is an ideal surface (i.e. like a cut through the bulk material). Instead, there are many terraces, steps and kinks in surfaces of solids which can be identified with LEED. Besides characterisation of the basic lattice of a solid sample, further information can be gained from particular features caused by super lattices, step lattices or domain formation (Fig. 7.16).

Fig. 7.16Left: Schematics of a LEED instrument. The concentric grids are used for filtering inelastically scattered electrons. Instead of optical observation of the LEED pattern, modern instruments have a position-sensitive digital detector (called delay-line detector) instead of the fluorescent screen. Right: LEED pattern of an Ir(100)−1 × 1 surface recorded at an energy of 195 eV. The image was kindly provided by L Hammer (Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany) and is reproduced with permission

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