Graz University of Technology, Institute of Experimental Physics, Petersgasse 16, 8010 Graz, Austria.
J Chem Phys. 2020 Jan 7;152(1):014307. doi: 10.1063/1.5130145.
Helium nanodroplets can serve as reaction containers for photoinduced time-resolved studies of cold, isolated molecular systems that are otherwise inaccessible. Recently, three different dynamical processes, triggered by photoexcitation of a single atom inside a droplet, were observed in their natural time scale: Expansion of the He solvation shell (He bubble) within 600 fs initiates a collective bubble oscillation with a ∼30 ps oscillation period, followed by dopant ejection after ∼60 ps. Here, we present a systematic investigation of these processes by combining time-resolved photoelectron and photoion spectroscopy with time-dependent He density functional theory simulations. By variation of the photoexcitation energy, we find that the full excess excitation energy, represented by the blue-shifted in-droplet excitation band, is completely transferred to the He environment during the bubble expansion. Surprisingly, we find that variation of the droplet size has only a minor influence on the ejection time, providing insight into the spatial distribution of the ground-state atoms before photoexcitation. Simulated particle trajectories after photoexcitation are in agreement with experimental observations and suggest that the majority of ground-state atoms are located at around 16 Å below the droplet surface. Bubble expansion and oscillation are purely local effects, depending only on the ultimate dopant environment. These solvation-induced dynamics will be superimposed on intramolecular dynamics of molecular systems, and a mechanistic description is fundamental for the interpretation of future experiments.
氦纳米液滴可用作反应容器，用于对冷的、孤立的分子体系进行光致时间分辨研究，这些分子体系在其他条件下是无法接近的。最近，在液滴内单个原子的光激发作用下，观察到了三种不同的动力学过程，它们在其自然时间尺度上被触发：氦溶剂化壳（氦泡）在 600fs 内的膨胀引发了一个具有约 30ps 周期的集体泡振荡，随后在约 60ps 后掺杂剂被逐出。在这里，我们通过将时间分辨光电子和光离子谱与时间相关的氦密度泛函理论模拟相结合，对这些过程进行了系统的研究。通过改变光激发能量，我们发现，由液滴内的蓝移激发带表示的全部过剩激发能，在泡膨胀过程中完全转移到氦环境中。令人惊讶的是，我们发现，液滴尺寸的变化对逐出时间只有很小的影响，这为光激发前基态原子的空间分布提供了深入了解。光激发后模拟的粒子轨迹与实验观察结果一致，并表明大部分基态原子位于液滴表面以下约 16Å 处。泡的膨胀和振荡是纯粹的局部效应，仅取决于最终的掺杂剂环境。这些溶剂化诱导的动力学将叠加在分子体系的分子内动力学上，而机制描述对于解释未来的实验是至关重要的。
