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

The adsorption of thiophene and 2,2'-bithiophene on Ag(111) was studied usi ng temperature programmed desorption and near-edge X-ray-absorption fine-st ructure (NEXAFS). The assignment of the NEXAFS resonances is supported by X alpha 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 geom etry in the compressed monolayer. For 2,2'-bithiophene. in contrast, we fin d a mono- and a bi-layer species, which are both adsorbed in a flat-lying g eometry on the Ag(111) surface. All adsorption species are weakly bound to the surface and comprise of undissociated molecules. In addition we demonst rate an X-ray-induced partial polymerization of thiophene multilayers. (C) 2000 Elsevier Science B.V. All rights reserved.