Capturing nanometer-picosecond collective dynamics in supercritical water using split-pulse XPCS

Supercritical fluids (SCFs) exhibit strong sensitivities to variations in temperature and pressure. Theoretical studies have suggested that many thermodynamic properties in SCFs are strongly related to collective dynamics at scales up to 100 nm. Therefore, there has been a continued interest in directly measuring the frequency and lifetime of these collective motions in SCFs. While inelastic X-ray scattering (IXS) techniques have enabled measurements of the phonon dispersion and collective-motion lifetime in SCFs on the atomic length scale, the limited energy resolution, along with strong quasi-elastic scattering makes the experimental investigations of collective dynamics at nm-length scales challenging. The split-pulse X-ray Photon Correlation Spectroscopy (XPCS) method combined with X-ray Free Electron Lasers can capture the relevant spatiotemporal scales, allowing for the direct observation of the collective dynamics. We present results from split-pulse XPCS measurements of collective dynamics in normal and heavy water at supercritical conditions. Within 30 nm – 125 nm length scale, we measure the sample dynamics between 0.1 – 10 ps with 1 ps temporal resolution. The measured intermediate scattering function shows oscillatory decay due to the presence of collective dynamics in the system.