Mudrich, M.LaForge, A. C.Ciavardini, A.O’Keeffe, P.Callegari, C.Coreno, M.Demidovich, A.Devetta, M.Fraia, M. DiDrabbels, 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.Eloranta, J.Pi, M.Barranco, M.Stienkemeier, F.2020-01-092020-01-092020-01-092020-01-0810.1038/s41467-019-13681-6https://infoscience.epfl.ch/handle/20.500.14299/164480The 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.photodynamicssuperfluidtext::journal::journal article::research article