Examining molecular cluster dynamics in supercritical fluids using split-pulse XPCS

Supercritical fluids (SCFs) are a distinctive state of matter with strong variations of thermodynamic and transport properties. Near the liquid-gas critical point, supercritical fluids display high degree of density fluctuation due to the formation of molecular clusters that evolve on picosecond time scales. To examine the dynamics of the structural heterogeneities, we use split-pulse X-ray Photon Correlation Spectroscopy (XPCS), which provides nanometer-picosecond spatiotemporal resolution. These measurements show that observed dynamics is significantly faster than simple diffusion and lies between ballistic and Brownian motion of molecules. By utilizing large-scale molecular dynamics simulations, we confirm that the observed dynamics occurs due to the interaction of the clusters with unbound molecules.