Regensburg 2004 – scientific programme

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O: Oberflächenphysik

O 14: Postersitzung (Adsorption an Oberflächen, Epitaxie und Wachstum, Organische Dünnschichten, Oxide und Isolatoren, Phasenübergänge, Rastersondentechniken, Struktur und Dynamik reiner Oberflächen)

O 14.2: Poster

Monday, March 8, 2004, 18:00–21:00, Bereich C

Non-dipolar contributions in XPS: X-ray standing wave experiments on ultrathin organic films — •A. Gerlach¹, S. Sellner^2,3, F. Schreiber¹, H. Dosch^2,3, I.A. Vartanyants⁴, J. Zegenhagen⁵, T. L. Lee⁵, and B.C.C. Cowie⁵ — ¹Physical Chemistry Laboratory, Oxford University, UK — ²MPI für Metallforschung, Stuttgart, Germany — ³Institut für Theoretische und Angewandte Physik, Universität Stuttgart, Germany — ⁴Department of Physics, University of Illinois, Urbana, USA — ⁵ESRF, Grenoble, France

We study the adsorption behaviour of organic molecules by means of the X-ray Standing Wave (XSW) technique. By measuring XPS and Auger intensities this method allows the precise and direct determination of absorbate/substrate geometries. Our XSW experiments with the aromatic molecules PTCDA (perylenetetracarboxylic dianhydride) and F16CuPc (perfluorinated copper-phthalocyanine) show that higher multipole excitations can be non-negligible. In fact, the breakdown of the dipole approximation, which can qualitatively change the signal, is a fundamental issue in XPS in general and a challenge for theory. Depending on the atomic number, orbital momentum, photon and initial state energy these non-dipole contributions can significantly influence the resulting effective coherent position and fraction. We discuss approaches to the analysis of these fundamental effects that can be relevant for many XSW applications.