Regensburg 2004 – wissenschaftliches Programm

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SYOH: Organic and Hybrid Systems for Future Electronics

SYOH 5: Poster

SYOH 5.1: Poster

Donnerstag, 11. März 2004, 18:00–21:00, B

Normal Incidence X-ray Standing Waves on Light Elements: The Limits of the Dipole Approximation — •Christian Kumpf, Jörg Stanzel, Christoph Stadler, Wolfgang Weigand, and Eberhard Umbach — Experimentelle Physik II, Univ. Würzburg

The normal incidence x-ray standing wave (NIXSW) technique is a powerful tool for obtaining detailed structural information on surfaces, e.g., exact atomic coordinates which enable quantum chemical calculations. In most XSW experiments either the photoelectron or the Auger-electron yield is used to measure the absorption of a specific atomic species adsorbed on a surface. For the analysis of organic adsorbates, which contain low-Z elements only, this is a difficult issue since the dipole-approximation is no longer sufficient to describe the photoemission process. Using the Auger yield is also problematic since the Auger process is not only stimulated by the x-ray standing wave field but also by (photo-) electrons emerging from the bulk. Therefore, significant effort was put in finding a proper way to correct the photoemission yield for non-dipolar contributions as well as the Auger-yield for its electron-induced portion.

We report on a NIXSW experiment performed on a coherent monolayer of NTCDA on Ag(111) for which this correction was successfully performed and consistent results were obtained from the O1s photo electron yield as well as from the OKLL Auger electron yield. The electron-induced portion of the Auger yield was found to be as large as 50%. Furthermore, a significant covalent character was found for the bonding between the NTCDA molecules and the substrate. The bonding distance is 3.08(3)Å, considerably smaller than a Van der Waals distance.