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Electron Diffraction


If you want to get insights into chemical, electronic or structural transitions in molecules or other samples, you will end up with time-resolved x-ray experiments.

These experiments use a pump-probe setup that first “pumps” the sample with a femtosecond laser into an excited state from which it relaxes. The relaxation process is then “probed” with a second laser, e.g. with ultra-short pulses of a free-electron laser (FEL). In such an experiment, all systems must work on a shared timebase given by a master clock. And even though the probing process with an FEL might provide a femtosecond time resolution, in the end, the experiment’s accuracy is often limited by synchronization issues. But how to address these limitations?

The solution is to synchronize the pump and probe laser to the same RF signal, e.g. the facility’s RF master oscillator. And for high-precision optical-to-RF synchronization, a BOMPD is an optimal solution. It detects the phase difference between a laser oscillator and an RF signal in the optical domain. There, it is possible to conduct measurements with higher precision than in the electromagnetic regime using photodetection. Consequently, the pump laser can be optimally synchronized to the facility’s master clock, and timing jitters between the master clock and laser below <20fs can be achieved in the setup.

The latter is precisely what Dr. Tadashi Togashi from the SPring-8 Angstrom Compact free-electron Laser (SACLA) in Harima, Japan reported about in his recent publication. By implementing a BOMPD in his setup, he could improve the timing jitter of his pump-probe structure by almost an order of magnitude and now offers better than 50 fs timing resolution for his ultra-fast electron diffraction setup.

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