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Ultrafast relaxation of photoexcited superfluid He nanodroplets

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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 pho- toelectron 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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Nature Communications, 2020, vol. 11, num. 112

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Articles publicats en revistes (Física Quàntica i Astrofísica)

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Ultrafast relaxation of photoexcited superfluid He nanodroplets