The Local Electronic Structure of Supercritical CO₂ from X-ray Raman Spectroscopy and Atomistic-Scale Modeling

Supercritical CO2 is encountered in severaltechnicaland natural systems related to biology, geophysics, and engineering.While the structure of gaseous CO2 has been studied extensively,the properties of supercritical CO2, particularly closeto the critical point, are not well-known. In this work, we combineX-ray Raman spectroscopy, molecular dynamics simulations, and first-principlesdensity functional theory (DFT) calculations to characterize the localelectronic structure of supercritical CO2 at conditionsaround the critical point. The X-ray Raman oxygen K-edge spectra manifestsystematic trends associated with the phase change of CO2 and the intermolecular distance. Extensive first-principles DFTcalculations rationalize these observations on the basis of the 4s sigma Rydberg state hybridization. X-ray Raman spectroscopy is found tobe a sensitive tool for characterizing electronic properties of CO2 under challenging experimental conditions and is demonstratedto be a unique probe for studying the electronic structure of supercriticalfluids.