Direct observation of ultrafast cluster dynamics in supercritical carbon dioxide using X-ray Photon Correlation Spectroscopy
Arijit Majumdar, Haoyuan Li, Priyanka Muhunthan, Alexander Späh, Sanghoon Song, Yanwen Sun, Matthieu Chollet, Dimosthenis Sokaras, Diling Zhu, Matthias Ihme
Abstract
Supercritical fluids exhibit distinct thermodynamic and transport properties, making them of particular interest for a wide range of scientific and engineering applications. These anomalous properties emerge from structural heterogeneities due to the formation of molecular clusters at conditions above the critical point. While the static behavior of these clusters and their effects on the thermodynamic response functions have been recognized, the relation between the ultrafast cluster dynamics and transport properties remains elusive. By measuring the intermediate scattering function in carbon dioxide at conditions near the critical point with X-ray photon correlation spectroscopy, we directly capture the cross-over dynamics between 4 and 13 picoseconds, revealing the transition between ballistic and diffusive motion. Complementary analysis using large-scale molecular dynamics simulations reveals that this behavior arises from collisions between unbound molecules and clusters. This study provides direct evidence of the ultrafast momentum exchange between clusters, which has significant impact on transport properties, solvation processes, and reaction kinetics in supercritical fluids. A transition from ballistic to diffusive motion within 10 ps is observed in supercritical carbon dioxide with X-ray photon correlation spectroscopy. Collisions of unbound molecules with clusters are responsible for the ultrafast momentum exchange.