The internal geometric structure of a large pi -conjugated organic molecule
adsorbed on a metallic surface (end-capped quaterthiophene on Ag(111)) was
determined in detail by using surface X-ray diffraction. In contrast to th
e common belief that such molecules remain undistorted on a weakly interact
ing substrate, we find significant distortion of the thiophene rings and el
ongation of the bond distances. These results open new perspectives for the
understanding of large organic adsorbates and have important implications
for a deeper insight into organic/inorganic interfaces in general.