Regensburg 2016 – scientific programme
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DS: Fachverband Dünne Schichten
DS 4: Organic Thin Films I
DS 4.5: Talk
Monday, March 7, 2016, 10:45–11:00, H11
Origin of F1s NEXAFS Dichrosim in oriented perfluorinated acenes — •Michael Klues1, Paul Jerabek1, Tobias Breuer1, Martin Oehzelt2, Robert Berger1, and Gregor Witte1 — 1Uni. Marburg, Marburg, Germany — 2Helmholtz Zentrum Berlin, Berlin, Germany
NEXAFS is a widely used technique to determine molecular orientations on surfaces. Especially for planar molecules with extended π-systems the analysis of angle-dependent intensity variations (dichroism) is straightforward. Surprisingly the analysis of NEXAFS data of fluorinated aromatic molecules measured at the F1s-edge has led to misleading results in the past. In a recent theoretical study [1] this has been attributed to an unexpected overlap of π and σ resonances in the energy region of interest. To further investigate this phenomenon highly ordered multilayers of Perfluoropentacene (PFP) were grown on HOPG and SiO2. PFP crystallizes flat lying respectively nearly upright standing on these substrates. This structural difference results in diverging dichroisms, enabling a detailed assignment of underlying excitations. To gain detailed insights into the nature of these excitations the NEXAFS-signature was calculated using the StoBe-code. Thereby, relaxation effects turned out to be of central importance. To cover this effects the computational approach was improved to determine excitation energies as well as oscillator strength within a ΔSCF-method. Finally these results were able to reproduce not only the signature but also the angle dependants of the experimental spectra.
[1] Oteya et al. Phys. Rev. B 2012, 86, 075469.