Ultrafast self-gating Bragg diffraction of exploding nanocrystals in an X-ray laser.

In structural determination of crystalline proteins using intense femtosecond X-ray lasers, damage processes lead to loss of structural coherence during the exposure. We use a nonthermal description for the damage dynamics to calculate the ultrafast ionization and the subsequent atomic displacement. These effects degrade the Bragg diffraction on femtosecond time scales and gate the ultrafast imaging. This process is intensity and resolution dependent. At high intensities the signal is gated by the ionization affecting low resolution information first. At lower intensities, atomic displacement dominates the loss of coherence affecting high-resolution information. We find that pulse length is not a limiting factor as long as there is a high enough X-ray flux to measure a diffracted signal.