Introductory lecture. Structural investigations of adsorbed layers using synchrotron radiation

Abstract

A brief review is given of experiments aimed at elucidating the structure of adsorbed layers on single-crystal metal surfaces using synchrotron radiation. The techniques based on the phenomena of photoabsorption or photoemission, which require no long-range order in the adlayer and may prove to be of wider applicability than electron or X-ray diffraction, particularly for absorbed molecules. To illustrate the potential of the various methods the adsorption systems Cu{110}(2 × 1)/O and Cu{110}/H.CO₂ are considered in more detail. In the case of the ordered oxygen overlayer, recent low-energy electron diffraction structural analyses are available for comparison. The application of a range of techniques is of particular advantage in this system because of the relative complexity of the structure: the adsorbate induces a so-called missing-row reconstruction. The energetics of such surface restructuring processes are also briefly discussed. In the case of the surface formate species the molecular orientation and adsorption site have been determined by X-ray absorption spectroscopy and photoelectron diffraction, respectively. This adsorption system shows no long-range order, a feature common to most adsorbed molecular fragments formed via simple heterogeneous reactions. Finally, some consideration is given to the requirement of high spectral brilliance for surface structural experiments, particularly in view of the construction of third-generation synchrotron radiation sources.