Capturing ultrafast collective dynamics in supercritical fluids using X-ray Photon Correlation Spectroscopy

Mesoscopic density fluctuations in supercritical fluids lead to structural heterogeneities, thermodynamic anomalies and highly adjustable transport properties. These structural heterogeneities manifest as the formation of molecular clusters, which are especially pronounced near the liquid-gas critical point. While the equilibrium structure of these molecular clusters has been studied, their dynamic properties remain poorly understood. We present results from split-pulse X-ray photon correlation spectroscopy measurements at the Linac Coherent Light Source of normal and heavy water at supercritical conditions to probe this structural-dynamic behavior. The intermediate scattering function (ISF) is measured with time delays of up to 10 ps over a length scale range of 3 – 12 nm. The damped oscillations of the ISF signify the dispersion and dissipation of the collective dynamics. Theoretical analysis shows that the observed ultrafast nanoscale collective dynamics in supercritical fluids originates from the vibrational motion of clusters.