Pump-probe measurements aim to capture the motion of electrons and nuclei on their natural timescales (femtoseconds to attoseconds) as chemical and physical transformations take place, effectively making “molecular movies” with short light pulses. However, the quantum dynamics of interest are filtered by the coordinate-dependent matrix elements of the chosen experimental observable. Thus, it is only through a combination of experimental measurements and theoretical calculations that one can gain insight into the internal dynamics. Here, we report on a combination of structural (relativistic ultrafast electron diffraction, or UED) and spectroscopic (time-resolved photoelectron spectroscopy, or TRPES) measurements to follow the coupled electronic and nuclear dynamics involved in the internal conversion and photodissociation of the polyatomic molecule, diiodomethane (${\mathrm{CH}}_2{\mathrm{I}}_2$). While UED directly probes the 3D nuclear dynamics, TRPES only serves as an indirect probe of nuclear dynamics via Franck-Condon factors, but it is sensitive to electronic energies and configurations, via Koopmans’ correlations and photoelectron angular distributions. These two measurements are interpreted with trajectory surface hopping calculations, which are capable of simulating the observables for both measurements from the same dynamics calculations. The measurements highlight the nonlocal dynamics captured by different groups of trajectories in the calculations. For the first time, both UED and TRPES are combined with theory capable of calculating the observables in both cases, yielding a direct view of the structural and nonadiabatic dynamics involved.

中文翻译：

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具有时间分辨光电子能谱和超快电子衍射的激发态分子动力学的光谱和结构探测

泵浦探针测量的目的是在发生化学和物理转换时捕获电子和原子核在其自然时标（飞秒到阿秒）上的运动，从而有效地制作出具有短光脉冲的“分子电影”。但是，感兴趣的量子动力学被所选实验可观察到的依赖于坐标的矩阵元素过滤。因此，只有通过实验测量和理论计算的结合，人们才能洞悉内部动力学。在这里，我们报告了结构（相对论超快电子衍射或UED）和光谱学（时间分辨光电子能谱，${\mathrm{CH}}_2{\mathrm{一世}}_2$）。UED直接探测3D核动力学，而TRPES仅通过Franck-Condon因子充当核动力学的间接探测，但它对电子能量和构型敏感，通过Koopmans关联和光电子角分布。这两个测量值是通过轨迹表面跳变计算来解释的，它们能够从相同的动力学计算中模拟两个测量值的观测值。测量结果突出了计算中不同轨迹组捕获的非局部动力学。UED和TRPES首次与能够在两种情况下计算可观测值的理论相结合，从而直接了解所涉及的结构和非绝热动力学。