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Direct comparisons of simulations to experiments are available in SI Appendix, Fig.

The X-ray parameters are chosen to match the experimental. ( Lower) Simulated scattered intensities as a function of scattering vector q, based on MD and NLTE simulations, calculated from the O–O radial distribution function (RDF) and the electronic states of the system. ( Middle) Difference between measured scattering intensity using XFEL pulses and a linear interpolation of the scattering intensity from water at ambient conditions ( 8). All curves are normalized to the maximum of the peak with the minimum subtracted. The SE on normalized intensity for all measurements is smaller than 0.2%. ( Upper) Measured scattering intensity as a function of scattering vector q = 2 ⁡ sin ( θ ) / λ, for experiments using an XFEL pulse of 25 fs and 75 fs duration (both with a fluence of 1.35 ⋅10 6 J/cm 2) and data from water at ambient conditions taken from Hura et al. To explain these observations and unravel the electronic and structural evolution of the system, we have used atomistic simulations linking ionization and MD.įig. The curve for the long (75 fs) exposure shows a behavior similar to that of ambient water, whereas the short pulse (25 fs) exposure shows a narrower peak. At a first glance, the shift in the scattering intensity curves compared with the ambient case seem to contradict what one would expect. The largest shift is seen in the short pulse measurement and the largest broadening in the long pulse measurement. The peak in the XFEL measurements is shifted to lower scattering vector q and is narrower than the ambient water peak. Each measurement condition is found to result in a slightly different scattering curve. The scattering intensity as a function of q for ambient water using a synchrotron beamline ( 8) is also shown for comparison and is used to calculate differences ( Fig. 2, obtained after binning, masking and radial averaging the diffracted signal collected on a Cornell-SLAC hybrid Pixel Array Detector (CS-PAD) detector ( 7) (see Materials and Methods). The main experimental results are the plots of scattering intensity as a function of the magnitude of the scattering vector, q = 2 ⁡ sin ( θ ) / λ (where 2 θ is the scattering angle and λ is the wavelength), shown in Fig. Single 6.86 keV X-ray pulses with 25 (short) and 75 (long) fs duration and an average fluence of 1.35 ⋅ 10 6 J/cm 2 were used. 1) consisted of a water jet produced by a gas dynamic virtual nozzle (GDVN) ( 4, 5), which was intersected by a submicrometer focused X-ray beam of the Coherent X-Ray Imaging (CXI) instrument of the Linac Coherent Light Source (LCLS) ( 6).