SURFACE CORE LEVEL SPECTROSCOPY OF TRANSITION METALS: A NEW TOOL FOR THE DETERMINATION OF THEIR SURFACE STRUCTURE

It has been recognized since 1964 that shifts in the binding energies of core electrons detected by high-resolution X-ray photoelectron spectroscopy (XPS), are a very sensitive probe of the chemical environment of the atom which undergoes the X-ray transition. Recently this technique has been widely applied to surface studies. Actually the surface atoms have their structural environment modified and therefore their core levels are shifted from their bulk positions. Such an effect has been first observed on polycrystalline Au in 1978 and on W(110) in 1979. Since, a very large number of experiments have been performed on the 4f core level positions of elements of the 5d series for various orientations of the surface plane. Systematic trends have been put forward and explained by theoretical models. When adsorbed atoms are present on the surface, a given core level shift should correspond to a given chemical environment and therefore this technique can be used to check the validity of structural models. We review the experimental and theoretical work done in this field, critically compare the various theoretical models and discuss their limit of validity.