THEORY OF PHOTO-EXCITATION OF ADSORBATES - ANALYSIS OF ATOMIC, ADLAYER, AND SUBSTRATE EFFECTS

Photoexcitation of electrons from electronic states at crystal surfaces is studied for angle-resolved UPS and surface EXAFS within the one-electron approximation using the Green function formalism. The two-dimensional KKR method is used for deriving explicit forms of the Green function and local density of states at surface layers both for the initial and final states. A systematic analysis is done by separating the Green function into the atomic, adlayer, and substrate parts. Model calculations show that the overall characteristics of the angular distribution of photoemission intensities are reproduced already in the single scattering approach, in which only the atomic part of the Green function is taken into account. Relatively simple analytical expressions are derived in the single scattering approach and tabulated to serve in a first analysis of experimental results. The initial states are analysed using local density of states, the main interest being the effect of the substrate on the two-dimensional adsorbate band structure which can be observed in angle-resolved UPS. A close relation of the present type of analysis to the Anderson model of adsorption is demonstrated. © 1979.