Chemisorption orbital description

In this section we give a simple and qualitative description of chemisorption in terms of molecular orbital theory. It should provide a feeling for why some atoms such as potassium or chlorine acquire positive or negative charge upon adsorption, while other atoms remain more or less neutral. We explain qualitatively why a molecule adsorbs associatively or dissociatively, and we discuss the role of the work function in dissociation. The text is meant to provide some elementary background for the chapters on photoemission, thermal desorption and vibrational spectroscopy. We avoid theoretical formulae and refer for thorough treatments of chemisorption to the literature [2,6-8],... 

Obviously, chemisorption on d-metals needs a different description than chemisorption on a jellium metal. With the d-metals we must think in terms of a surface molecule with new molecular orbitals made up from d-levels of the metal and the orbitals of the adsorbate. These new levels interact with the s-band of the metal, similarly to the resonant level model. We start with the adsorption of an atom, in which only one atomic orbital is involved in chemisorption. Once the principle is clear, it is not difficult to invoke more orbitals. 

In order to find the local density of states at the surface, one requires a formalism not in terms of delocalized orbitals as we have used so far, but in terms of local quantities. Such a formalism is provided by the Green s function method, which Is also a very convenient tool for the study of chemisorption. Here we will introduce its use by applying it to the open and the closed chain problems discussed before. Within the LCAO basis set description and mean field electron approximation (see section 2.7.3.4), the Green s function is a matrix element that satisfies the following set of equations. 

Chemisorption of H on simple metals and on transition metals has been studied theoretically using approaches such as molecular orbit theory, valence bond theory, density functional theory, cluster calculations (a detailed description up to 1980 can he found in Smith, 1980), and effective medium theory (N rskov, 1984). An extension of the effective medium theory, the so-called embedding atom method - originally developed to study the embrittlement problem - was shown to yield very valuable results for H on metals, particularly for the surface relaxation and the H adsorption sites including subsurface sites (see Pd) (Daw and Baskes, 1984). 

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