Ultrafast relaxation of photoexcited superfluid He nanodroplets
Ultrafast relaxation of photoexcited superfluid He nanodroplets
复制标题
DOI：
10.1038/s41467-019-13681-6
复制
发表时间：
2020-01-08
影响因子：
16.6
通讯作者：
Stienkemeier, F.
中科院分区：
文献类型：
--
作者：
Mudrich, M.;LaForge, A. C.;Stienkemeier, F.
光激发纳米系统的弛豫是光-物质相互作用的一个基本过程。根据内部自由度的耦合，弛豫可以是超快的，在几秒钟内转换电子能量，或者是缓慢的，如果能量被困在亚稳态中，与环境解耦。在这里，我们研究了由种子自由电子激光器的飞秒极紫外（XUV）脉冲共振激发的氦纳米液滴。尽管具有超流体性质，但我们发现氦纳米液滴在最低电子激发态下经历超快弛豫。通过比较实验光电子能谱与时间相关密度泛函数理论模拟，我们揭示了完整的弛豫途径：经过超快带间跃迁，在1 ps内，在局域激发（He*）周围形成一个纳米大小的空洞气泡。随后，气泡崩溃并在液滴表面释放亚稳态He*。这项研究强调了使用可调谐XUV脉冲探测纳米系统光动力学的高水平细节。
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, we study helium nanodroplets excited resonantly 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.