Institute for Methods and Instrumentation for Synchrotron Radiation Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , 12489 Berlin , Germany.
Philos Trans A Math Phys Eng Sci. 2019 May 20;377(2145):20170464. doi: 10.1098/rsta.2017.0464.
X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for 'probe-before-destroy' spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable 'bringing back together' different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.
X 射线自由电子激光具有高强度、可调谐和短脉冲的 X 射线辐射，是研究过渡金属配合物和金属蛋白的变革性工具。当将 X 射线光谱学的实验可观测结果与电子结构和 X 射线光谱的现代理论计算工具相结合时，这一点尤其明显。这种结合为化学反应中电荷和自旋密度的变化以及它们如何决定反应性提供了新的见解。这在利用金属 K 边吸收光谱研究结构动力学、利用金属 3p-3d 交换相互作用研究激发态动力学中的自旋态、利用金属特异性 X 射线光谱研究解离和取代反应中的前沿轨道相互作用以及利用飞秒脉冲研究金属氧化态的“破坏前探测”光谱中得到了证明。本文探讨了 X 射线自由电子激光的作用，思考了它们如何使同一问题的不同方面“重新结合”，这超出了传统的综述论文，在传统综述论文中，这些方面以斜体字的形式在引言、插曲和续集中阐述。本文是“用 X 射线测量超快电子和结构动力学”主题特刊的一部分。
