Orientation and bonding of thiophene and 2,2′-bithiophene on Ag(111): a combined near edge extended X-ray absorption fine structure and Xα scattered-wave study

The adsorption of thiophene and 2,2′-bithiophene on Ag(111) was studied using temperature programmed desorption and near-edge X-ray-absorption fine-structure (NEXAFS). The assignment of the NEXAFS resonances is supported by Xα scattered-wave calculations. For thiophene we identify a compressed and a relaxed monolayer species. Whereas the molecules in the latter state are adsorbed parallel to the surface, they are adsorbed in an inclined geometry in the compressed monolayer. For 2,2′-bithiophene, in contrast, we find a mono- and a bi-layer species, which are both adsorbed in a flat-lying geometry on the Ag(111) surface. All adsorption species are weakly bound to the surface and comprise of undissociated molecules. In addition we demonstrate an X-ray-induced partial polymerization of thiophene multilayers.