NEXAFS investigations on ordered adsorbate layers of large aromatic molecules

Abstract

It is a scientific challenge to understand and control the adsorption of large organic molecules on single crystal substrates on the same level as that of small molecules or atoms. This is discussed on the basis of one of the most suitable techniques for this purpose, near edge X-ray absorption fine structure (NEXAFS). As examples we present data for highly symmetric and flat aromates without (perylene) or with reactive groups (PTCDA, PTCDI, NDCA) on various single crystal surfaces such as Ag(111), Ni(111) and Si(111). Preparation parameters such as coverage, temperature and preadsorbate have been varied to study the bonding and molecular orientation on the surface together with the layer growth behaviour. In most cases the experimental comparison between different molecules and simple calculations help for the interpretation of the complicated data but ad hoc Xα-SW calculations which have been carried out for the largest molecule presently possible (NDCA) are required for a more detailed understanding of the rich spectral features.