Substrate-dependent bonding distances of PTCDA: A comparative x-ray standing-wave study on Cu(111) and Ag(111)

We study the adsorption geometry of 3,4,9,10-perylene tetracarboxylic dianhydride on Ag(111) and Cu(111) using x-ray standing waves. The element-specific analysis shows that the carbon core of the molecule adsorbs in a planar configuration, whereas the oxygen atoms experience a nontrivial and substrate dependent distortion. On copper (silver) the carbon ring resides 2.66 angstrom (2.86 angstrom) above the substrate. In contrast to the conformation on Ag(111), where the carboxylic oxygen atoms are bent towards the surface, we find that on Cu(111) all oxygen atoms are above the carbon plane at 2.73 angstrom and 2.89 angstrom, respectively.