Zeitaufgelöste Untersuchung der laser-induzierten Diffusion von CO-Molekülen auf gestuften Pt(111)-Oberflächen

In der vorliegenden Arbeit wurde die Dynamik von CO-Molekülen auf einer gestuften Pt(111)-Oberfläche unter Anregung mit einem fs-Laser bei tiefen Temperaturen mit laserspektroskopischen Methoden untersucht. Ausgenutzt wurde dabei die Sensitivität der optischen Methode der Frequenzverdopplung (SH...

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Bibliographic Details
Main Author: Lawrenz, Marco
Contributors: Höfer, Ulrich (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2007
Online Access:PDF Full Text
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Table of Contents: In the present work the dynamics of CO-molecules on a stepped Pt(111)-surface induced by fs-laser pulses at low temperatures was studied by using laser spectroscopy. As approved method the sensitivity of the optical second harmonic generation (SHG) on changes of the step decoration on the surface was used. In the first part of the work, the laser-induced diffusion for the CO/Pt(111)-system could be demonstrated and modelled successfully for step diffusion. The experiments not only provide information about the yield of the reaction (fluence dependence) but from the performance of time-resolved measurements (two-pulse correlation) the energy transfer of the system could be decoded. The good statistics of the experiment offered the possibility for this kind of investigation which provided insight into the energy transfer from the optical excitation to the lateral displacement of the adsorbate. In this manner, the potential of the method could be fully exploited and information on a sub-ps time scale could be gained. At first, the diffusion of CO-molecules from the step sites to the terrace sites on the surface was traced. Using SHG, the system which is a model for many metal-adsorbate systems could be characterized by extracting microscopical information about this special diffusion pathway. It could be demonstrated, that via high temperature dosing cycles CO could be dosed selectively at the step sites of the surface. A step depletion rate as a function of critical laser parameters could be directly extracted via interpreting the step sensitive SHG-signal. With this technique, the dependence on the laser fluence could be observed with an exponent of six where the laser fluence is kept in a regime of a few mJ/cm2. The observed dependence is a typical behaviour in DIMET-processes. The sensible dependency of the hopping rate on the laser fluence offered the possibility for two-pulse correlation measurements which manifest in a narrow distribution typical for electronically induced processes. The experimentally discovered energy transfer time of 500 fs for this process confirms the assumption of an electronically induced process. The transfer time was interpreted as a coupling to the frustrated rotational mode. At this point the molecular character of the adsorbate is distinct. The additional degrees of freedom for the CO-molecule which can be involved for the diffusion were discussed at this point, a feature which expands former studies in our group concerning electronically induced atomic diffusion. In the following it was explained how the experimental results were modelled. A classical model using constant friction is insufficient describing the narrow width of the two-pulse correlation. The fast energy transfer extracted from the measurements could only be described and fully understood in the framework of the electronic, temperature-dependent friction model (3TM). Step diffusion could be described as a clearly electronically induced process with a transfer time of 500 fs which is shorter than any comparable desorption or diffusion process. This fact is surprising, particularly since the fluence dependence with F6 is comparable to typical desorption experiments. A friction coefficient which depends on the electron temperature yields a consistent model, whereas for the understanding of the fluence dependence and time-resolved measurements parallel the same set of parameters was used. Furthermore, the analysis was extended to the CO-terrace diffusion. Small coverages of CO were adsorbed to the terraces and the diffusion was detected as the temporal evolution of the occupation of the step sites acting as traps for the diffusing molecules. For this low energy process, laser fluences of about 1 mJ/cm2 are sufficient to induce the diffusion on the Pt-terraces. A weaker activation scheme was found with a quadratic dependence on the laser fluence. The additional performed two-pulse correlation measurements also indicate an electronically induced process. At the substrate temperature of 40 K the cross-correlation - where an energy transfer time of 1.8 ps was extracted - suggests also an electronically induced energy transfer mechanism. Diffusion experiments were performed for different substrate temperatures. Already at 60 K additional phonon mediated reaction processes appear, for this temperature a much broader feature with a width of 25 ps was observed. Also for this diffusion pathway a special feature can be stressed: Using a description of the data in the framework of the 3TM a consistent model can be found which is based on the parameter set for the step diffusion with a modified terrace diffusion barrier. By means of this simulations in terms of the electronic friction model, the potential of a simple model used for multi-dimensional phenomena can be seen which includes all observed effects in a consistent description.