1. Distinct transient structural rearrangement of ionized water revealed by XFEL X-ray pump X-ray probe experiment
- Author
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Stransky, Michal, Lane, Thomas J., Gorel, Alexander, Boutet, Sébastien, Schlichting, Ilme, Mancuso, Adrian P., Jurek, Zoltan, and Ziaja, Beata
- Subjects
Physics - Plasma Physics - Abstract
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., Comment: 24 main text pages, 6 supplement pages (30 total), 4 main text figures, 3 supplemental figures, 2 supplemental tables
- Published
- 2024