Dresden 2009 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 44: Metal substrates: Adsorption of organic / bio molecules VI
O 44.5: Vortrag
Donnerstag, 26. März 2009, 11:30–11:45, SCH A118
PTCDA chemisorbed on Ag(110): Dispersion interactions and charge equilibration — •Reinhard Scholz1 and Afshin Abbasi2 — 1Walter Schottky Institut, TU München, Germany — 2Institut für Physik, TU Chemnitz, Germany
The chemisorption of PTCDA on Ag(110) is analysed with 2nd order Møller-Plesset perturbation theory (MP2), accounting therefore for the main part of the dispersion interactions at an ab initio level. Irrespective of the size of the rigid silver cluster used as a substrate model, the optimized geometry consists of a nearly flat perylene core, surrounded by carboxylic oxygens closer to the substrate, but with anhydride atoms at a larger height. The charge equilibration between adsorbate and substrate involves charge injection from the oxygen atoms into the substrate, and back transfer of two electrons into the former LUMO of the free molecule. Surprisingly, the resulting negative charge of the adsorbate has a very small contribution in the core region where the main part of the LUMO is localized, but instead accumulates on the end groups. It can be shown in detail that a rehybridization of molecular σ and π states results in a decoupling of several π states on the oxygen atoms from the aromatic core, as opposed to the free molecule, where the π states delocalize over the entire molecule. These decoupled π states on the end groups are forming the binding Ag-O orbitals injecting electronic charge from the molecule into the substrate. Compared to the dominating mechanisms discussed above, the hybridization between orbitals in the core region and the underlying substrate plays only a minor role for the charge balance.