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Ultrafast time-resolved X-ray diffraction using an optimized laser-plasma based X-ray source

Lu, Wei

Femtosecond X-ray pulses are invaluable tools to investigate the structural dynamics triggered by a femtosecond laser pulse. These ultrashort X-ray pulses can be provided by lab-sized laser-produced plasma X-ray sources. This thesis is dedicated to optimizing the X-ray emission from the X-ray source at the University of Duisburg-Essen and using this source to investigate ultrafast structural dynamics in laser excited materials. For these purposes, detailed investigations on how the laser intensities, target thicknesses, angles of incidence and different pre-pulse/pre-plasma conditions affecting the emission of Kα-photons from Cu and Ti targets were performed. The outcomes from these studies are applied to optimize the X-ray production of the existing X-ray source for time resolved X-ray diffraction (TRXD) experiments. In the mean time, in order to improve the measurement sensitivity/accuracy, and automatize and speed up the experimental procedures, several other improvements have been implemented in the experimental setup for TRXD experiments. These improvements of the setup are essential to achieve the results of the three TRXD experiments discussed in this thesis. In the first experiment, Debye-Waller effect in a thin laser-excited Au film was observed. The drop of measured diffraction signal with a decay time constant of 4.3 ± 1 ps was measured for high excitation fluences. This result is in good agreement with previous experimental results as well as the Two-Temperature Model (TTM) calculations at high fluences. The second experiment extends the studies of coherent optical phonons in laser-excited Bi to a higher excitation fluence range that has not been investigated previously. Large amplitude coherent atomic motion and a complete softening of the A1g phonon mode were observed. These observations represents conclusive experimental evidence that the Peierls distortion, which defines the equilibrium structure of Bi, vanishes and the material is transformed into a transient ordered state of higher symmetry. The last study focuses on the experiments performed with X-ray Debye-Scherrer diffraction scheme. A static measurement on a Cu foil (20 μm) and a time-resolved measurement on an Au thin film (200 nm) were carried out. In the time-resolved measurements, the generation of an acoustic wave and its propagation inside the film were observed through the transient shifts of the (311)- and (220)- reflection. The feasibility of performing time-resolved X-ray Debye-Scherrer diffraction experiments with this experimental setup is demonstrated.

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Lu, Wei: Ultrafast time-resolved X-ray diffraction using an optimized laser-plasma based X-ray source. 2013.

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