Abstract
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.
Highlights
The relaxation of photoexcited nanosystems is a fundamental process of light–matter interaction
From the comparison of the experimental and theoretical results, we can map out the full picture of the relaxation dynamics of excited He nanodroplets: Initiated by the excitation of the 1s2p 1P
Nanodroplet state, which is likely delocalized over several He atoms[24], ultrafast interband relaxation to the 1s2s 1S droplet state occurs within
Summary
The relaxation of photoexcited nanosystems is a fundamental process of light–matter interaction. We study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded freeelectron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. The complexity of heterogeneous solid or liquid systems, as well as difficulties in preparing well-controlled samples and performing reproducible measurements, make it difficult to unravel the elementary steps in the relaxation process. Molecular Subsequently, a center HeÃn, void cavity or bubble excited forms around HeÃn due to Pauli repulsion electron and the surrounding ground state between the He atoms[21], which expands up to a radius of 6.4 Å25 within about 350 fs[26]
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