Elucidation of surface structure and bonding by photoelectron spectroscopy?

Information obtained on the adsorption of small and medium-sized molecules (CO, O₂, CO₂, NO, C₂H₄, C₆H₆) at three metal surfaces (Mo, W, Ni) by X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) is discussed in an attempt to establish what has been achieved, and what might be achieved in the elucidation of adsorption bonding at metal surfaces. Some of the individual results have been published by the present author with co-workers, and by other authors, but some results represent new work. Emphasis is placed on the detection of different states of adsorption, and then on interpretations of structure and bonding. The latter is divided into two areas, empirical interpretation by analogy between different adsorption systems, and in more absolute terms by consideration of the differences in electronic energy levels between the absorbate molecule in its gaseous and adsorbed state as well as differences between the metal levels in the clean and surface bonded state. Note is made of the problems of suitable reference levels and the phenomena of relaxation energies. Mo and W are taken as representative examples of adsorption confined usually to the monolayer regime; Ni is taken as an example of the situation where further reaction may occur. Most of the work described refers to polycrystalline films since little work has yet been done by XPS on adsorption at single crystal surfaces. A short general review of the quantitative aspects of XPS and UPS for analysis is given.