QUANTITATIVE ASPECTS OF ULTRAVIOLET PHOTOEMISSION OF ADSORBED XENON - A REVIEW

Ultraviolet photoelectron spectroscopy (UPS) is usually not considered to be a quantitative technique for the analysis of solid surfaces. However, careful examination of the He(I)-induced 5p spectra of adsorbed xenon provides the basis for a quantitative description of non-uniform solid surfaces and/or adsorbed layers of xenon themselves. The shape, intensity and electron binding energy of the Xe 5p emission can be associated with the distribution, concentration and local surface potential of particular adsorption sites, respectively. Structural defects, such as steps, edges of evaporated metal islands and pores, can be characterized in this way. A particularly important application of the photoemission of adsorbed xenon (PAX) is the determination of the coverage and average island size of metal adsorbates. The early stages of film growth and the thermally activated healing of surface roughness of such deposits can be followed quantitatively by PAX. Interesting quantitative information can also be obtained on the structure of the adsorbed xenon layer itself, such as the critical coverage for the two-dimensional 2D Xe gas <----> 2D Xe solid phase transition, the nucleation of three-dimensional Xe clusters at defects and the presence and equilibrium topography of adsorbed Xe multilayers. These analytical possibilities of the PAX method are illustrated with applications to the Ru(001) surface, the epitaxially grown Ag film, the bimetallic Ag/Ru(001) system and the trimetallic AgAu/Ru(001) system.