Ultrafast Structural Reorganization of Supercritical Water Molecules Under XFEL Radiation

Despite their widespread use in chemical synthesis and industrial processes, fundamental questions remain on the nature of supercritical fluids, their microstructure, and thermodynamic response functions. While several research efforts have characterized the macroscopic properties and static behavior of various supercritical fluids, their ultrafast response when subjected to intense X-ray irradiation has largely been unexplored. In this work, we present experimental results conducted with supercritical water at the SPring-8 user facility using the SACLA X-ray Free Electron Laser (XFEL). In contrast to the response of liquid water, which shows a long-lasting, pronounced heating effect for all pressure conditions, we observe that the response of supercritical water strongly depends on the thermodynamic state. In the liquid-like region above the critical point, the dominant effect is concentrated at large length scales with a lifetime of ~5 ps. However, close to the Widom line, the structural changes occur at all length scales but are uniformly separated in time. These observations are consistent with the theory of cluster formation and breakup near the Widom delta. The findings can potentially be used to obtain a more accurate estimate of cluster lifetimes in supercritical media.