Vertical bonding distances of PTCDA on Au(111) and Ag(111): Relation to the bonding type

The vertical bonding distance of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) above the Au(111) surface has been measured by the normal incidence X-ray standing wave (NIXSW) technique. The carbon skeleton of PTCDA has a vertical distance of D = (3.27 +/- 0.02) angstrom to the Au(111) substrate. This distance corresponds very nearly to the sum of the van der Waals radii of carbon and gold, suggesting the adsorption to be a physisorptive one. In contrast, the PTCDA/Ag(111) interface which according to spectroscopic data follows the standard model of chemisorption very closely, shows a considerably smaller bonding distance of D = (2.86 +/- 0.01) angstrom [A. Hauschild, K. Karki, B.C.C. Cowie, M. Rolilting, F.S. Tautz, M. Sokolowski, Phys. Rev. Lett. 94 (2005) 036106, comment: Rurali et al., Phys. Lett. 95 (2005) 209205, reply: Phys. Rev. Lett. 95 (2005) 209206]. The different vertical adsorption heights of PTCDA on gold and silver are discussed in relation to the different bonding mechanisms on both noble metal surfaces. (c) 2007 Elsevier B.V. All rights reserved.