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Letter

Nature Photonics 2, 165–169 (1 March 2008) | doi:10.1038/nphoton.2007.298

A femtosecond X-ray|[sol]|optical cross-correlator

Cornelius Gahl , Armin Azima , Martin Beye , Martin Deppe , Kristian D|[ouml]|brich , Urs Hasslinger , Franz Hennies , Alexej Melnikov , Mitsuru Nagasono , Annette Pietzsch , Martin Wolf , Wilfried Wurth & Alexander F|[ouml]|hlisch

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 time. 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 Hamburg (FLASH) and establish X-ray- pulse-induced transient changes of the optical reflectivity in GaAs as a powerful tool for X-ray|[sol]|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.