Adsorption site determination of a large π-conjugated molecule by normal incidence x-ray standing waves: End-capped quaterthiophene on Ag(111)

Using normal incidence x-ray standing waves, we determined the geometric structure of the adsorption site of a large π-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 $\left(1\mathrm{}¯11\right)$ Bragg reflections. The vertical coherent position $D_{\mathrm{co}}^{\mathrm{}\left(111\right)\mathrm{}},$ i.e., the average distance between molecules and Ag surface, was determined to $3.15\pm 0.05\AA .$ 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 π 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 $\left[1¯11\right]$ direction $(1.02\mathrm{}\pm 0.1\AA ).\mathrm{}$ This results in the first complete determination of the adsorption geometry of a large flat-lying organic molecule on an inorganic surface.