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Hannover 2010 – wissenschaftliches Programm

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K: Fachverband Kurzzeitphysik

K 4: Laseranwendungen und Lasermaterialbearbeitung I

K 4.7: Vortrag

Donnerstag, 11. März 2010, 12:15–12:30, F 442

Modular setup for femtosecond time-resolved X-ray diffraction — •Wei Lu1, N. Nicoul1,2, U. Shymanovich1, A. Tarasevitch1, P. Zhou1, K. Sokolowski-Tinten1, and D. von der Linde11Institut für Experimentelle Physik, Universität Duisburg-Essen, 47048 Duisburg, Germany — 2I. Physikalisches Institut , University of Cologne, 50937 Köln, Germany

We present here a new setup for time-resolved X-ray diffraction using femtosecond X-ray pulses from a laser-produced plasma. The setup has a modular design and only the X-ray source is placed in vacuum. A pre-pulse scheme is employed to optimize Kα-production [1] resulting in a total Cu Kα-flux (8 keV) of 10e10 photons per pulse. For re-collection of the emitted X-rays, we use a multi-layer X-ray mirror which produces a 5x magnified, monochromatic image of the source [2] leading to a Kα*flux of more 10e5 photons per pulse impinging on the sample. The signal of a small ionization chamber is used in the setup as a reference to normalize the diffraction signals. This significantly improves the accuracy of the experiment and it is possible to observe relative signal changes of less than 1%. With this setup, we have performed time-resolved diffraction experiments on laser-irradiated Bi to directly observe coherently excited optical phonons. The experimental data reveal an extreme softening of the excited A1g-mode and give strong evidence that upon intense laser-excitation the Peierls-distortion, which defines the equilibrium structure of Bismuth, can be transiently reversed. [1] W. Lu, et al., Phys. Rev. E 80, 026404 (2009). [2] U. Shymanovich et al., Appl. Phys. B 92, 493 (2008).

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