X-RAY PHOTOELECTRON DIFFRACTION - A SENSITIVE PROBE OF ADSORBATE AND SURFACE-STRUCTURE

Recent experiments are discussed in which forward-scattering intensity enhancements observed in X-ray photoelectron diffraction (or in Auger electron diffraction) yield structural information about surfaces. Two types of surface structural problems are highlighted which are particularly suitable for studies using this technique: (1) adsorbate and coadsorbate layers on surfaces, and (2) thin epitaxial metal films in the early stages of epitaxy. In the former one can determine molecular orientations to accuracies approaching ± 1° and also gain information on the molecular vibrations ('wagging' modes arising from frustrated molecular motion) in the adsorbate layer. The transition observed on Ni(110) from perpendicular CO is tilted CO with increasing coverage, induced by adsorbate 'crowding', provides a good example of the technique's capabilities in this regard. In epitaxial films the growth mode in the early stages (up to several layers) can be determined, e.g. layer-by-layer growth can be distinguished from clustering. The crystal structure and degree of elastic strain in such films can also be studied. Examples of such studies include Co and Cu films on Ni(001). © 1990 Pergamon Press plc.