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We report additional rich fine structures in high-resolution near-edge x-ray-absorption fine structure (NEXAFS) spectra of large organic molecules using NTCDA on Ag(111) as an example. These fine structures are completely interpreted as vibronic coupling to electronic core excitations. The coupling is mode selective; predominantly one vibronic mode couples to each excitation. The fit results suggest the occurrence of a Davydov splitting, first observed for core excitons. Morphological differences substantially influence the electron-vibron coupling, indicating a strong intermolecular interaction. Thus NEXAFS becomes a more subtle probe for organic solids.

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The bonding of 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) on Ag(111) and its molecular orientation were investigated by X-ray photoelectron (XPS) and near edge X-ray absorption fine-structure spectroscopy (NEXAFS). Vapour-deposited NTCDA films show high orientational order in the mono-and multi-layer regime. In the monolayer regime the molecules are strongly bound to the substrate via the conjugated π-system of the naphthalene core which leads to a parallel orientation of the molecular plane to the Ag(111) substrate. For thicker films (e.g., 150 Å) the molecular orientation strongly depends on the substrate temperature during preparation. Film preparation at 285 K results in upright standing molecules, whereas low substrate temperatures yield a uniform molecular orientation with the molecular plane parallel to the substrate. This behaviour is explained by the weak intermolecular interaction and the adsorption kinetics.

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