The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a
function of 8T coverage has been studied with low-temperature scanning
tunneling microscopy, high resolution electron energy loss spectroscopy as
well as with angle-resolved two-photon photoemission and ultraviolet
photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying
adsorption geometry. Upon reaching the monolayer coverage the orientation of
8T molecules changes towards a tilted configuration, with the long molecular
axis parallel to the surface plane, facilitating attractive intermolecular
π–π-interactions. The photoemission intensity from the highest occupied
molecular orbitals (HOMO and HOMO − 1) possesses a strong dependence on the
adsorption geometry due to the direction of the involved transition dipole
moment for the respective photoemission process. The change in molecular
orientation as a function of coverage in the first molecular layer mirrors the
delicate balance between intermolecular and molecule/substrate interactions.
Fine tuning of these interactions opens up the possibility to control the
molecular structure and accordingly the desirable functionality
