Femtosecond snapshots of nano crystalline response to XFEL pulse trains*

The European XFEL enables sequential single-shot diffraction snapshots at megahertz repetition rates. These rapid-fire pulses have been successfully applied for serial femtosecond crystallography [1,2] where a fast flowing target delivers a new sample between shots. However, the use of high-repetition-rate pulses for an individual fixed-in-place target depends critically on understanding the response of the target-- damage to which limits the usable fluence per image. We have investigated the response of a CdSe nanocrystalline target using a novel pulse-on-demand method where sequential single-shot diffraction images were acquired for a variable number of x-ray pulses over a wide range of fluence. Unusual features appear in the diffraction pattern and morphology that can be used to model the target response. [1] M. L. GrŸnbein\textit{ et al}., Nat. Commun.\textbf{ 9}, 3487 (2018 [2] M. O. Wiedhorn\textit{ et al}., Nat. Commun. 9, 4025 (2018)