光激发超流He纳米液滴的超快弛豫
Ultrafast relaxation of photoexcited superfluid He nanodroplets
摘要：
光激发纳米系统的弛豫过程是光与物质相互作用的基本过程。取决于内部自由度的耦合，弛豫可以是超快的，在几fs内转换电子能量，或者是慢的，如果能量被捕获在从其环境中释放的亚稳态中。在这里，氦纳米液滴被来自种子自由电子激光的飞秒极紫外（XUV）脉冲共振激发。尽管它们的超流体性质，我们发现，氦纳米液滴在最低的电子激发态进行超快弛豫。通过比较实验光电子能谱与含时密度泛函理论模拟，我们解开了完整的弛豫路径：超快带间跃迁后，空纳米尺寸的气泡周围的本地化激发（He*）在1 ps内形成。随后，气泡破裂并在液滴表面释放亚稳态He*。这项研究突出了高层次的细节，可实现探测光动力学的纳米系统使用可调XUV脉冲。
英文摘要：
The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, helium nanodroplets are resonantly excited by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He*) within 1 ps. Subsequently, the bubble collapses and releases metastable He* at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses.
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