Zeitaufgelöste Elektronenbeugung : ein Film über ultraschnelle, atomare Prozesse

Der Beitrag stellt mit der zeitaufgelösten Elektronenbeugung eine neue Methode vor, die es erlaubt, die Bewegung von Atomen auf Oberflächen beziehungsweise von Atomen einer Oberfläche selbst sichtbar zu machen. Mit Hilfe dieser neuen Methode sollen fundamentale Fragen zu Prozessen beantwortet werden, die stattfinden, wenn eine Oberfläche mit einem Laser beschossen wird.
The dynamics of atoms following laser excitation is an area of special interest that has not been addressed in great detail in the past. It will yield insight into microscopic processes of the light-matter-interaction. Studies regarding direct atomic movement contribute to the understanding of the total energy transfer chain, starting with the initial laser excitation, which ultimately results in the generation of heat at the surface and in the bulk of a crystal. In recent years, a time-resolved electron diffraction (TRED) experiment has been set up at the University of Duisburg-Essen. In this pump and probe experiment a laser pulse excites a surface. The structural dynamics are probed using short electron pulses generated by photoemission. The heating and cooling of a thin Bismuth film deposited on a Silicon substrate has been investigated in an initial experiment. The heating of the thin film could not be studied in detail because the temporal resolution is not yet sufficient. The cooling, however, occurs on a much longer timescale. It was observed that the cooling rate of the thin film is dramatically reduced compared to a Bismuth single crystal. This is explained by the boundary between Bismuth and Silicon acting as a barrier to heat transport. The observed cooling rate is in good agreement with models describing the thermal boundary conductance.
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