Surface electronic excitations and dynamic spectral properties of adsorbates

Abstract

Many-body aspects of screening and relaxation mechanisms encountered in spectroscopic studies of the electronic structure of adsorbates have recently attracted considerable attention from both experimental and theoretical physicists. Interest in these phenomena has also been augmented by the rapid improvement of experimental techniques which have enabled better resolution and analysis of various subtile components of the adsorbate spectra. Many of these spectral features have until recently been ascribed to purely chemical and initial state effects. One of the first major advances of the theoretical development in this field was to seek and attribute the origin of these structures to the many-body properties of adsorption systems and, secondly, to predict how the many-body effects would manifest themselves in surface spectroscopies. We start with a rather detailed description of the formalism of the surface electronic response and discuss the properties of the surface excitation spectrum of idealized and real metals. This formalism is then successively applied to set up a model of screening firstly in the nonbonding levels of mainly physisorbed adsorbates and later in the core and valence levels of chemisorbed species. Various modifications of the model enable a dynamic description of the final state relaxation and shake-up effects typical of spectroscopic measurements. To treat the particularly complicated problem of dynamic relaxation in the adsorbate valence levels a special perturbational approach based on Mayer's cluster expansion is developed in §5. The characteristics, and some limitations of this approach, which may also prove useful in other physical problems, are described in detail and discussed within the context of the interaction of localized adsorbate charge fluctuations with bosonic surface excitations. Experimental support for the presented theoretical framework and its applications has been very important. A qualitative comparison with the experimental findings has been given for the majority of effects predicted and some still existing controversies and interpretational problems have been pointed out.