  Using X-ray free electron laser (XFEL) radiation to conduct an X-ray pump
X-ray probe experiment, we studied strongly ionized water as part of our
ongoing work on radiation damage. After irradiance with a pump pulse with a
nominal fluence of ~$5 \times 10^5$ J/cm$^2$, we observed for pump-probe delays
of 75 fs and longer an unexpected structural rearrangement, exhibiting a
characteristic length scale of ~9 \r{A}. Simulations suggest that the
experiment probes a superposition of ionized water in two distinct regimes. In
the first, fluences expected at the X-ray focus create nearly completely
ionized water, which as a result becomes effectively transparent to the probe.
In the second regime, out of focus pump radiation produces O$^{1+}$ and
O$^{2+}$ ions, which rearrange due to Coulombic repulsion over 10s of fs.
Importantly, structural changes in the low fluence regime have implications for
the design of two-pulse X-ray experiments that aim to study unperturbed liquid
samples. Our simulations account for two key observations in the experimental
data: the decrease in ambient water signal and an increase in low-angle X-ray
scattering. They cannot, however, account for the experimentally observed 9
\r{A} feature. A satisfactory account of this feature presents a new challenge
for theory.
