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.

中文翻译：

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从 X 射线拉曼光谱和原子尺度建模的超临界 CO2 的局域电子结构

超临界 CO ₂在与生物学、地球物理学和工程学相关的几个技术和自然系统中遇到。_{虽然气态 CO 2}的结构已被广泛研究，但超临界 CO ₂的性质，尤其是接近临界点的性质，并不为人所知。在这项工作中，我们结合了 X 射线拉曼光谱、分子动力学模拟和第一性原理密度泛函理论 (DFT) 计算来表征临界点附近条件下超临界 CO 2 的局域电子结构_{。X 射线拉曼氧 K 边光谱显示了与 CO 2}相变相关的系统趋势和分子间距离。广泛的第一性原理 DFT 计算在 4sσ 里德堡态杂交的基础上使这些观察结果合理化。X 射线拉曼光谱被发现是在具有挑战性的实验条件下表征 CO ₂电子特性的灵敏工具，并被证明是研究超临界流体电子结构的独特探针。