Examining molecular cluster dynamics of supercritical CO₂ using ultrafast XPCS

Due to their wide range of applications, supercritical fluids (SCFs) have been a topic of active research. A unique property of SCFs is the formation of molecular clusters near the critical point, leading to a complex microstructure. These clusters have a significant influence on the thermophysical properties. The challenge in studying the dynamics of such cluster is their short length scale and lifetime. Split pulse x-ray photon correlation spectroscopy (XPCS), with its capability to capture molecular motion at picosecond time resolution, is an attractive choice to study the dynamics of these supercritical clusters. This work presents the first measurements of the cluster dynamics in SCFs using ultrafast XPCS. The experiments are performed on supercritical carbon dioxide. The measured intermediate scattering function, near the critical point, captures the density fluctuations due to cluster dynamics. A theoretical framework, coupled with molecular simulations, is developed to explain the experimental observations.