Emergence of anomalous dynamics in soft matter probed at the European XFEL

This first µs-XPCS study performed at an FEL source demonstrates the high stability of the European XFEL and reveals non-linear heating of water beyond its boiling point.

The European XFEL is a hard X-ray Free-Electron Laser (FEL) with pulse repetition rates in the megahertz regime. Thus, sample dynamics in the microsecond and submicrosecond regime can be accessed by means of X-ray photon correlation spectroscopy (XPCS). Application of XPCS at FEL sources has been challenged because of the intrinsic fluctuations of FELs and the radiation-induced sample heating and damage. In this work we demonstrate the unprecedented stability of the European XFEL and show the feasibility of microsecond XPCS on a model system. Nonlinear heating on microsecond timescales is observed reaching superheated water states that persist up to 100 μs. Our data indicate a violation of the Stokes–Einstein relation for nanoparticle diffusion at short times.

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PNAS, doi:10.1073/pnas.2003337117 (2020).