First Imaging Experiments with Hard and Soft Sub-fs to Few-fs XFEL Pulses

Diffraction-before-destruction imaging with ultrashort X-ray pulses can visualise non-equilibrium processes, such as chemical reactions, with sub-femtosecond precision in the native environment without the need of crystallization. Here, a nanospecimen diffracts a single X-ray flash before the sample disintegrates. The structure of the specimen is reconstructed from the coherent diffraction image (CDI). Such state-of-the-art X-ray snapshots lack high spatial resolution information due to weak diffraction signal i.e. shot noise. Bleaching effects from photo-ionization significantly restrain image brightness scaling and thus, further improvement of the spatial resolution. We find that non-linear transient form factor changes can overcome this barrier if FEL pulses are shorter than those applied in the majority of previous experiments. We compared snapshots from individual ≈ 100 nm Xe nanoparticles as a function of the X-ray pulse duration and incoming X-ray fluence in the vicinity of the Xe M-shell resonance. Surprisingly, images recorded with few to sub-femtosecond pulses are up to 10 times brighter than the semi- classical model predicts. Our Monte-Carlo simulation suggests that transient ion form factors can increase the brightness of X-ray images by several orders of magnitude. This provides a novel avenue towards significant improvement of the spatial resolution in CDI in combination with sub-fs temporal precision at the nanoscale.