Ultrafast relaxation of photoexcited superfluid He nanodroplets

Mudrich, M.;LaForge,A.C.; Ciavardini, A.; O’Keeffe,P.;Callegari, C.;Coreno,M.;Demidovich,A.;Devetta,M.; Di Fraia,M.; Drabbels,M.; Finetti,P.;Gessner,O.;Grazioli,C.;Hernando, A.;Neumark,D.M.;Ovcharenko,Y.; Piseri,P.;Plekan,O.;Prince,K.C.;Richter,R.;Ziemkiewicz,M.P.;Möller,T; Eloran, J.
Nature Communications 11, 112 (2020)

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.

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