For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a long-standing dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time1. Free-electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element-specific and chemical-state-selective information with femtosecond time resolution. The key challenge lies in synchronizing the free-electron lasers with separate optical lasers. We exploit the peak brilliance of the free-electron laser in Hamburg2,3 (FLASH) and establish X-ray- pulse-induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough in the path towards recording a molecular movie and—equally importantly—opens up the field of femtosecond X-ray-induced dynamics, only accessible with high-brilliance X-ray free-electron lasers.
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Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide
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We dedicate this manuscript to our suddenly deceased co-worker Kai Starke, whose enthusiasm and competence was essential to initiate this work. We gratefully acknowledge support by the scientific and technical staff of the FLASH facility, in particular S. Düsterer, H. Redlin and R. Treusch. We thank C. Kumpf (University of Würzburg) for providing the GaAs samples. This work was supported by the German Ministry of Education and Research (BMBF) through grants nos. 05 KS4GU1/8 and 05 KS4GU1/9 and the Helmholtz Joint Research Centre ‘Physics with coherent radiation sources’.
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Gahl, C., Azima, A., Beye, M. et al. A femtosecond X-ray/optical cross-correlator. Nature Photon 2, 165–169 (2008). https://doi.org/10.1038/nphoton.2007.298