Adsorption site determination of a large π-conjugated molecule by normal incidence x-ray standing waves:: End-capped quaterthiophene on Ag(111) -: art. no. 075412

Using normal incidence x-ray standing waves, we determined the geometric structure of the adsorption site of a large pi-conjugated molecule with six rings, end-capped quaterthiophene (EC4T), on the Ag(111) surface in the commensurate monolayer. The S 1s absorption profiles were measured for the (111) and ((1) over bar 11) Bragg reflections. The vertical coherent position D-co((111)), i.e., the average distance between molecules and Ag surface, was determined to 3.15+/-0.05 Angstrom. This is less than expected for van der Waals bonding and more than known for a local S-Ag bond, thus revealing that the surface bonding involves the entire conjugated pi system, in agreement with earlier photoemission results. The lateral position of the EC4T molecule was obtained from a simulation of the experimental coherent position in [(1) over bar 11] direction (1.02+/-0.1 Angstrom). This results in the first complete determination of the adsorption geometry of a large flat-lying organic molecule on an inorganic surface.