Über die Dynamik von Elektronen auf Silber-Einkristalloberflächen, an metall-organischen Grenzschichten und in PTCDA-Monolagen

In der vorliegenden Dissertation werden verschiedene Aspekte der Elektronendynamik an Metalloberflächen, in metall-organischen Grenzflächenzuständen und in quasi-isolierten PTCDA-Monolagen mittels zeit- und winkelaufgelöster Zweiphotonen-Photoemission (2PPE) untersucht. Die Rydberg-Serien von Ag(...

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Bibliographic Details
Main Author: Marks, Manuel
Contributors: Höfer, Ulrich (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2012
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Table of Contents: Different aspects of electron dynamics on surfaces, at metal-organic interfaces as well as in quasi-isolated monolayers of a molecular organic semiconductor are investigated using time- and angle-resolved two-photon photoemission spectroscopy (2PPE). The Rydberg series of image-potential states and resonances have been studied by 2PPE in combination with quantum-beat spectroscopy up to quantum numbers of n=7 for the Ag(100) and Ag(111) surfaces, respectively. The lifetimes of the excited electrons follow a scaling law propotional to the third power of the quantum number for both, the image- otential states and the resonances. The decay of the resonances due to the resonant elastic electron transfer is found to be approximately one order of magnitude faster. Furthermore, the experimental results indicate, that for image-potential states as well as resonances the pure dephasing times of the coherently excited electron wave packets are independent from decay due to quasi-elastic scattering events with phonons. In addition, the interband scattering between different image-potential states due to electron-electron-scattering could be quantified for Ag(111). The extracted scattering rates confirm theoretical predictions. In order to obtain information about the influence of the interfacial interaction between organic molecular semiconductors and a metal surface on the binding energy and the wave function overlap of unoccupied Shockley-type metal-organic interface states, different model systems of metal-organic interfaces have been studied using 2PPE. The interfaces between monolayer films of the organic semiconductors 3,4,9,10-perylene-tetracarboxylic-acid-dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) and a Ag(111) surface have been systematically studied and the experimental results are compared to density functional theory calculations. The results reveal that the energy shift of the former Shockley-surface state is mainly influenced by the adsorption height between the molecular monolayer and the metal substrate as well as the density of carbon rings per unit cell area. In contrast, the overlap between the interface state and the electronic states of the molecules is determined to a large extent by the bending of the molecules and the bonding between the carboxylic oxygen atoms and the surface. Electrons that have been excited into the interface states decay back to metallic states predominantly due to electron-hole pair excitation. A reversible temperature dependent structural phase transition is characterized for perfluoropentacene (PFP) using 2PPE, near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). When the sample temperature rises above T=145 K, the long-range ordered commensurate (6x3)-superstructure transforms into a disordered monolayer consisting of mobile molecules. In addition, the influence of the organic adlayer on the properties of image-potential states is studied for PFP/Ag(111) and compared to similar model systems in order to extract general trends. The binding energies of the Rydberg series of image-potentials states can be described without the use of a quantum defect. The local workfunction of the PFP-covered surface areas has been determined with high precision by the convergence of the Rydberg series for high quantum numbers. Apparently, the decay channel via the unoccupied interface state leads to a decrease of the inelastic lifetimes of the excited electrons in image-potential states while the lifetime of electrons in image-potential resonances increases due to the decoupling of the resonances from the substrate by the molecular layer. The investigation of quasi-isolated PTCDA-monolayer films adsorbed KCl/Ag(100) with 2PPE shows that the intramolecular electron dynamics can be studied using photoelectron spectroscopy techniques even when the molecular layer has been decoupled from the conducting substrate by an insulating thin film. The obtained data can be explained consistently with the assumption of an exciton in the PTCDA molecules with a high binding energy (E=-1.7 eV). Highly excited PTCDA-molecules rapidly relax towards the strongest bound exciton with time constants of $tau lesssim 300$~fs. An excitation that is observed in the 2PPE-spectra shows an inelastic lifetime in the range of a few nanoseconds. Together with the UV-excitation induced quenching of the luminescence from the PTCDA monolayer the observation can be explained by the formation of a stabilized charge-transfer exciton.