A femtosecond X-ray/optical cross-correlator

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.