Ionization by XFEL radiation produces distinct structure in liquid water

communications physics Article https://doi.org/10.1038/s42005-024-01768-6 Ionization by XFEL radiation produces distinct structure in liquid water Check for updates 1234567890():,; 1234567890():,; Michal Stransky Ilme Schlichting 1,2,3 6 , Thomas J. Lane 4,5 , Adrian P. Mancuso , Alexander Gorel6, Sébastien Boutet , Zoltan Jurek4,5 & Beata Ziaja2,4 7 , 1,8,9 In the warm dense matter (WDM) regime, where condensed, gas, and plasma phases coexist, matter frequently exhibits unusual properties that cannot be described by contemporary theory. Experiments reporting phenomena in WDM are therefore of interest to advance our physical understanding of this regime, which is found in dwarf stars, giant planets, and fusion ignition experiments. Using 7.1 keV X-ray free electron laser radiation (nominally 5×105 J/cm2), we produced and probed transient WDM in liquid water. Wide-angle X-ray scattering (WAXS) from the probe reveals a new ~9 Å structure that forms within 75 fs. By 100 fs, the WAXS peak corresponding to this new structure is of comparable magnitude to the ambient water peak, which is attenuated. Simulations suggest that the experiment probes a superposition of two regimes. In the first, fluences expected at the focus severely ionize the water, which becomes effectively transparent to the probe. In the second, out-of-focus pump radiation produces O1+ and O2+ ions, which rearrange due to Coulombic repulsion over 10 s of fs. Our simulations account for a decrease in ambient water signal and an increase in low-angle X-ray scattering but not the experimentally observed 9 Å feature, presenting a new challenge for theory. The theoretical description of matter is challenging at densities and temperatures where condensed, gas, and plasma phases coexist1,2. Often referred to as warm dense matter (WDM), these states are characterized by densities between 10-2 and 104 g/cm3 and temperatures on the order of 103–107 K (0.1–1000 eV), as found in brown dwarf stars, the cores of giant planets such as Jupiter, and in the early stages of fusion ignition. This regime presents a challenge for theory because many disparate energetic contributions are relevant in this state, precluding simplifying approximations. For instance, the thermal energies of electrons and ions are typically comparable to the Coulombic potential energy of interparticle interactions1,2. Therefore, to inspire and validate predictive models of this state of matter, new experimental results reporting unexpected phenomena are extremely valuable. Our understanding of WDM has been greatly advanced by laboratorybased studies. Generating matter under extreme conditions on Earth is possible in large part thanks to laser facilities. High-intensity lasers are capable of reaching the peak powers necessary to produce the requisite temperatures and pressures in samples of interest1,3. Radiation from X-ray free electron lasers (XFELs), for instance, has been used to both create and probe WDM. Specifically, XFELs have been employed to investigate nanoscopic diamonds4,5 created through shock compression, produce highdensity plasmas in silver6, and study high-temperature high-pressure melting of aluminum7, all of which involve transitions through WDM states8. Further, XFELs have been used to create and characterize highly ionized states of water under WDM conditions9. A detailed understanding of the behavior of water is particularly important for two reasons. First, due to its significance on Earth and anomalous properties, the structure of water has been extensively studied, both under ambient and extreme conditions10–13. Any new information about the structure of water can be placed in this context, driving toward a complete description of the water pressure-temperature phase diagram. Second, water is either a direct topic of study or an integral component – e.g. a solvent or carrier medium – of the sample in many experiments, including experiments performed at XFELs. Understanding the perturbative, damaging effects of XFEL radiation on water is necessary to properly design and interpret these experiments. The aforementioned XFEL study on highly ionized water9 was part of an experiment aimed at establishing whether or not specific radiation damage can be observed in protein crystals containing clusters of high-Z atoms14. To this end, data from water and protein microcrystals, 1 European XFEL, Schenefeld, Germany. 2Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland. 3Department of Radiation and Chemical Physics, Institute of Physics, Czech Academy of Sciences, Prague, Czech Republic. 4Center for Free-Electron Laser Science CFEL, Deutsches ElektronenSynchrotron DESY, Hamburg, Germany. 5The Hamburg Centre for Ultrafast Imaging, Hamburg, Germany. 6Max Planck Institute for Medical Research, Heidelberg, Germany. 7SLAC National Accelerator Laboratory, Menlo Park, CA, USA. 8Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, VIC, Australia. 9Present address: Diamond Light Source, Harwell Science and Innovation Campus, Didcot, UK. e-mail: ; ; Communications Physics | (2024)7:281 1 https://doi.org/10.1038/s42005-024-01768-6 respectively, were collected using short (25 fs) and unusually long (75 fs) XFEL pulses, with the latter chosen to maximize radiation damage effects. Damage induced by the XFEL pulse was observed, but the resulting dynamics were integrated over the duration of the pulse, preventing analysis of the temporal evolution of the damage process. Therefore, we performed follow-up X-ray pump X-ray probe studies at the Linac Coherent Light Source (LCLS) XFEL, allowing us to generate highly ionized states in both proteins and water and probe the resulting atomic structures15. Here we describe our measurements on water, revealing that after exposure to a 7.1 keV X-ray pulse with a nominal fluence on the order of 5 × 105 J/cm2, a previously undescribed structure of highly ionized water is formed. It is characterized by a peak in the wide-angle X-ray scattering (WAXS) profile at q = 0.7 Å-1, corresponding to structural order at length scales of approximately 9 Å, i.e., significantly longer than the 2.8 Å and 4.5 Å oxygen-oxygen distances of the first and second solvation shells in liquid water under ambient conditions16. We performed moleculardynamics simulations, which predict that under these conditions the water sample is highly ionized, with two qualitatively distinct regimes. In the first regime, the high intensities found at the center of the X-ray focus are sufficient to strip nearly all electrons from the sample. In the second lowerintensity regime, outside the focal center, the irradiation produces singly and doubly ionized oxygen atoms and the atomic structure rearranges significantly. The simulations, however, cannot account for the 9 Å structure observed, highlighting a gap in our theoretical toolbox or understanding. Results X-ray pump (...truncated)