Recently, coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of Kα x-ray emission spectroscopy (XES). Understanding the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics is important for further development of femtosecond time-resolved XES methods and, we believe, generally necessary for interpretation of XES signals from highly non-equilibrium structures that are ubiquitous in photophysics and photochemistry. Here, we use multiconfigurational wavefunction calculations combined with atomic theory to analyze the emission process in detail. The sensitivity of core-to-core transitions to structural dynamics is due to a shift of the minimum energy metal–ligand bond distance between 1s and 2p core-hole states. A key effect is the additional contraction of the non-bonding 3s and 3p orbitals in 1s core-hole states, which decreases electron–electron repulsion and increases overlap in the metal–ligand bonds. The effect is believed to be general and especially pronounced for systems with strong bonds. The important role of 3s and 3p orbitals is consistent with the analysis of radial charge and spin densities and can be connected to the negative chemical shift observed for many transition metal complexes. The XES sensitivity to structural dynamics can be optimized by tuning the emission energy spectrometer, with oscillations up to ±4% of the maximum intensity for the current system. The theoretical predictions can be used to design experiments that separate electronic and nuclear degrees of freedom in ultrafast excited state dynamics.

中文翻译：

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核心对核心X射线发射光谱的起源对结构动力学的敏感性

最近，在铁光敏剂的激发态的相干结构动力学通过振荡观察到K的强度α的X射线发射光谱（XES）。对于进一步发展飞秒时间分辨的XES方法，了解核对核转变对结构动力学的意外敏感性的起源非常重要，我们认为，这对于解释普遍存在的高度非平衡结构的XES信号通常是必需的在光物理和光化学领域。在这里，我们结合原子理论使用多配置波函数计算来详细分析发射过程。核心到核心过渡对结构动力学的敏感性是由于最小能量金属-配体键合距离在1 s之间发生变化和2 p的核孔态。一个关键作用是在1 s芯孔态中非键3 s和3 p轨道的额外收缩，这会降低电子-电子排斥力并增加金属-配体键的重叠。据信这种作用是普遍的，对于具有强键的体系尤为明显。3 s和3 p的重要作用轨道与径向电荷和自旋密度的分析一致，并且可以与许多过渡金属配合物观察到的负化学位移相关。XES对结构动力学的灵敏度可以通过调整发射能谱仪来优化，其振荡高达当前系统最大强度的±4％。理论预测可用于设计在超快激发态动力学中分离电子和核自由度的实验。