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

Supercritical CO2 is encountered in several technical and natural systems related to biology, geophysics, and engineering. While the structure of gaseous CO2 has been studied extensively, the properties of supercritical CO2, particularly close to the critical point, are not well-known. In this work, we combine X-ray Raman spectroscopy, molecular dynamics simulations, and first-principles density functional theory (DFT) calculations to characterize the local electronic structure of supercritical CO2 at conditions around the critical point. The X-ray Raman oxygen K-edge spectra manifest systematic trends associated with the phase change of CO2 and the intermolecular distance. Extensive first-principles DFT calculations rationalize these observations on the basis of the 4sσ Rydberg state hybridization. X-ray Raman spectroscopy is found to be a sensitive tool for characterizing electronic properties of CO2 under challenging experimental conditions and is demonstrated to be a unique probe for studying the electronic structure of supercritical fluids.

超临界二氧化碳（Supercritical CO₂）广泛存在于与生物学、地球物理学及工程学相关的多种技术与自然体系中。尽管气态二氧化碳的结构已被广泛研究，但超临界二氧化碳的物性，尤其是临界点附近的物性，尚未得到充分认知。本研究结合X射线拉曼光谱（X-ray Raman spectroscopy）、分子动力学模拟以及第一性原理密度泛函理论（DFT）计算，对临界点附近条件下超临界二氧化碳的局域电子结构进行表征。X射线拉曼氧K边光谱呈现出与二氧化碳相变及分子间距离相关的系统性变化趋势。大量第一性原理DFT计算基于4sσ里德伯态杂化机制，对上述实验现象给出了合理解释。研究表明，X射线拉曼光谱是在挑战性实验条件下表征二氧化碳电子物性的灵敏工具，同时也被证实是研究超临界流体电子结构的独特探针。