Theory of photoemission from localized adsorbate levels

A detailed analysis of a recently developed theory of the angle- and energy-resolved photoemission from localized adsorbate levels is presented. The non-plane-wave nature of the final state originating in the repeated scattering of the outgoing wave by the surrounding atoms constitutes the essential feature of this theory. It is shown that the resulting interferences lead to experimentally observable structure in photoemission spectra which is sensitive to the position of the adsorbate relative to the substrate. Emphasis is placed on the emission from simple, atomiclike orbitals since they permit an unambiguous identification of final-state interactions. Sample calculations are performed to illustrate these effects in the case of emission from the adsorbate $2s$ level for a sulfur monolayer on a nickel substrate.