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Abstract:
The analysis of NEXAFS spectra acquired at the absorption edge of heteroatoms like fluorine, nitrogen, or oxygen enables the determination of the molecular orientation of individual compounds even in multinary structures. Such an analysis requires detailed knowledge about the nature of the corresponding electronic excitations, especially because 1s-π* and 1s-σ* excitations frequently feature disparate transition dipole moments in planar molecules. Unlike intuitively assumed, the resulting dichroisms of both excitation types in planar systems are, however, not necessarily inverted. Instead, both the structure of the molecules and their alignment on the substrate determine the actual dichroisms. In this study, the NEXAFS signature and dichroisms of thin films of the organic n-type semiconductor perfluoropentacene (PFP) in different crystalline orientations at the carbon and fluorine absorption K-edge are thoroughly analyzed. For lying molecular orientations, an inverted dichroism at the fluorine K-edge compared to the carbon K-edge is found. In contrast, for upright molecular configurations the dichroisms at both absorption edges are similar. With the help of density functional theory methods, this behavior is explained by the different nature of the excitations. While at the carbon K-edge 1s-π* excitations are most prominent, 1s-σ*-related signals are dominant at the fluorine K-edge. Computations of the NEXAFS signatures and corresponding excitations at different levels of theory are compared with particular focus on electronic relaxation. Energetic positions and oscillator strengths, especially for resonances of σ*-type, depend strongly on the theoretical approach whereas the π*-related signals are barely affected by the methods applied in the present work. Similar effects were also found for the analysis of the smaller perfluoronaphthalene and are explained by the different relaxation effects for 1s-σ*- and 1s-π*-type excitations. As the investigated acenes are representative model systems for π-conjugated molecular semiconductors, the present findings are important for the understanding of the electronic properties and the application of NEXAFS for structural analysis in such materials.
