The relaxation of photoexcited nanosystems is a fundamental process of light-matter interaction. This study investigates helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses. Despite their superfluid nature, ultrafast relaxation is observed in the lowest electronically excited states. Photoelectron spectra, compared with time-dependent density functional theory (TD-DFT) simulations, reveal a three-step relaxation pathway involving an ultrafast interband transition, bubble formation around the excitation, and bubble collapse releasing metastable He* at the droplet surface. The high detail achievable in probing nanosystem photodynamics using tunable XUV pulses is highlighted.

M. Mudrich, A.C. LaForge, A. Ciavardini, P. O’Keeffe, C. Callegari, M. Coreno, A. Demidovich, M. Devetta, M. Di Fraia, M. Drabbels, P. Finetti, O. Gessner, C. Grazioli, A. Hernando, D.M. Neumark, Y. Ovcharenko, P. Piseri, O. Plekan, K.C. Prince, R. Richter, M.P. Ziemkiewicz, T. Möller, J. Eloranta, M. Pi, M. Barranco