液态水X射线吸收谱中的多体效应
Many-Body Effects in the X-ray Absorption Spectra of Liquid Water
摘要：
X射线吸收光谱（XAS）是一种强有力的实验技术，用于探测具有芯电子激发的材料中的局域有序性。实验解释需要支持理论计算。对于水，这些计算是非常苛刻的，并且迄今为止，只能通过限制计算光谱的准确性的主要近似来完成。这引发了一场激烈的辩论，即是否有必要对四面体键合水的标准图像进行实质性修改，以提高理论和实验的一致性。在这里，我们报告了一个新的第一性原理计算的XAS的水，避免了以前的工作由于电子激发理论的最新进展的近似。计算出的XAS光谱，以及它们随温度和/或同位素取代的变化，与实验结果非常吻合。该方法需要精确的准粒子波函数超越密度泛函理论近似，占准粒子的动力学，包括动态屏蔽以及重整化效应，由于连续的价态激发。在实验光谱中观察到的三个特征明确地归因于激子效应。的前边缘功能相关联的绑定分子内激子，主边缘功能相关联的激子本地化的激发分子的配位壳层内，而后边缘离域超过更远的邻居，如预期的共振态。这三个特征探测了相对于激发分子在短、中和长范围内的局部有序。计算出的光谱与水的标准四面体图像完全一致。
英文摘要：
X-ray absorption spectroscopy (XAS) is a powerful experimental technique to probe the local order in materials with core electron excitations. Experimental interpretation requires supporting theoretical calculations. For water, these calculations are very demanding and, to date, could only be done with major approximations that limited the accuracy of the calculated spectra. This prompted an intense debate on whether a substantial revision of the standard picture of tetrahedrally bonded water was necessary to improve the agreement of theory and experiment. Here, we report a new first-principles calculation of the XAS of water that avoids the approximations of prior work thanks to recent advances in electron excitation theory. The calculated XAS spectra, and their variation with changes of temperature and/or with isotope substitution, are in excellent quantitative agreement with experiments. The approach requires accurate quasi-particle wavefunctions beyond density functional theory approximations, accounts for the dynamics of quasi-particles and includes dynamic screening as well as renormalization effects due to the continuum of valence-level excitations. The three features observed in the experimental spectra are unambiguously attributed to excitonic effects. The pre-edge feature is associated to a bound intramolecular exciton, the main-edge feature is associated to an exciton localized within the coordination shell of the excited molecule, while the post-edge one is delocalized over more distant neighbors, as expected for a resonant state. The three features probe the local order at short, intermediate, and longer range relative to the excited molecule. The calculated spectra are fully consistent with a standard tetrahedral picture of water.
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