  X-ray Free Electron Lasers (XFEL) are the most advanced pulsed x-ray sources.
Their extraordinary pulse parameters promise unique applications. Indeed,
several new methods have been developed at XFEL-s. However, no methods are
known, which would allow ab initio atomic level structure determination using
only a single XFEL pulse. Here, we present experimental results, demonstrating
the determination of the 3D atomic structure from data obtained during a single
25 fs XFEL pulse. Parallel measurement of hundreds of Bragg reflections was
done by collecting Kossel line patterns of GaAs and GaP. With these
measurements, we reached the ultimate temporal limit of the x-ray structure
solution possible today. These measurements open the way for studying
non-repeatable fast processes and structural transformations in crystals for
example measuring the atomic structure of matter at extremely non-ambient
conditions or transient structures formed in irreversible physical, chemical,
or biological processes. It would also facilitate time resolved pump-probe
structural studies making them significantly shorter than traditional serial
crystallography.
