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(Spot Profile Analysis) Low Energy Electron Diffraction

Fig1_SPA-LEEDFig. 1: SPA-LEED Instrument

Spot Profile Analysis LEED is a special version of the Low Energy Electron Diffraction technique which uses a channeltron detector for counting the electrons which have been diffracted by the sample surface in a specific direction. Since electron gun and detector are in fixed positions, one has to tune an electrostatic octupole field that is located between the gun/detector unit and the sample in order to vary the angle between sample surface and electron beam. In this way reciprocal space can be scanned and a LEED image is recorded sequentially. SPA-LEED images look similar to normal LEED images, but contain quantitative information which allows the analysis of, e.g., peak profiles. However, more important advantages of SPA-LEED compared to “normal” LEED are the much higher k-space resolution (due to an improved collimation of the electron beam) and a strongly reduced beam current (by a factor of approx. 100). The latter aspect is important when organic adsorbate systems are investigated, since these systems are very sensitive to beam damage that is caused by the electron bombardment.

The photo on the right shows the SPA-LEED instrument in our laboratory. It is mounted in a cylindrical UHV chamber that is additionally equipped with a load-lock and all necessary equipment for sample preparation, for deposition of thin films on different substrates and for Thermal Desorption Spectroscopy (TDS) or Temperature Programmed Desorption (TPD). In particular, the geometry of the chamber allows in-situ SPA-LEED measurements during the deposition of molecules on the surface. This enables fast and straight-forward investigations of kinetic processes on surfaces (like growth, phase transitions, desorption, etc.).



As an example, the growth of the first layer of Cu-Phthalocyanine (CuPc) molecules on a Ag(111) surface is shown in the movie above. It consists of approx. 90 SPA-LEED images which were taken for increasing coverages from 0.3 to 1.0 monolayers (ML). It can clearly be seen that the formation of a long-range ordered structure occurs at a coverage of 0.9 ML. Below that threshold a diffuse ring with changing radius indicates a diluted gas-like phase, while above that threshold one observes the signature of well-ordered structures, the unit cells of which are shrinking further upon increasing the coverage. This behavior demonstrates that a repulsive intermolecular interaction is dominant within the adsorbate layer (see New J. Phys. 12, 083038 (2010), Nature Physics 5, 153 (2009) and New J. Phys. 9, 50 (2007)).
The machine shall be extended in the near future by a satellite chamber for photoemission spectroscopy (PES) within the same vacuum system. Hence, the investigation of geometric structure (SPA-LEED) and electronic structure (PES) can be performed on the very same sample preparation. This allows a comprehensive characterization of surfaces and thin adsorbate layers. We plan to install a (monochromatized) ultra-violet light source, an x-ray source and a hemispherical electron analyzer.

Contact: C. Kumpf