Resonant Photoemission on Organic Thin Films and Interfaces

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The unique physical properties of organic materials are crucially influenced by the coupling of the single molecule to its local surrounding. In particular, dynamic processes, which determine e.g. the macroscopic transport characteristics or the optical properties, are influenced by complex interaction mechanisms. Ultimately, a microscopic understanding of these mechanisms is the basis for a specific control of the system properties. In contrast to perfectly long-range ordered solids, the respective mechanisms can be less clearly distinguished in condensed organic systems, thus requiring expensive preparation and analysis methods. In this project resonant Photoelectron Spectroscopy (ResPES) will be applied for the systematic investigation of the electronic structure of organic semiconductor films. The method is based on the local excitation of a core electron. The induced many particle processes, which are generally well understood from a theoretical viewpoint, bear characteristic spectral information, which allow conclusions on the dynamic properties of the electronic or molecular system, respectively. These many particle processes occur on a much smaller energy scale than the primary excitation and can contain e.g. a local screening of the core hole, intermolecular charge transfer, or collective vibronic or electronic excitations. In this project we will specifically investigate: - The sensitivity of the ResPES-spectra for external influences on molecules in case of weak binding: ResPES-spectra show characteristic fine structures, which can be attributed to the coupling of the excited intermediate state to molecular vibrations. This vibronic signature shows differences if the molecules are examined in different surrounding, even if the chemical bonding is weak. - The temporal sequence of charge transfer processes via the ‘core-hole clock ‘-method: the charge transfer following the electronic excitation will be investigated in thin molecular films, at molecule/metal-interfaces, and at heterointerfaces (molecule A/molecule B). This project aims on the systematic investigation of the physical coupling of particular organic molecules to the surrounding and its effect on vibronic and electronic excitations. It will improve the microscopic understanding of organic molecular materials.