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Ultrafast relaxation of photoexcited superfluid He nanodroplets.

  • Author(s): Mudrich, M;
  • LaForge, AC;
  • Ciavardini, A;
  • O'Keeffe, P;
  • Callegari, C;
  • Coreno, M;
  • Demidovich, A;
  • Devetta, M;
  • Fraia, M Di;
  • Drabbels, M;
  • Finetti, P;
  • Gessner, O;
  • Grazioli, C;
  • Hernando, A;
  • Neumark, DM;
  • Ovcharenko, Y;
  • Piseri, P;
  • Plekan, O;
  • Prince, KC;
  • Richter, R;
  • Ziemkiewicz, MP;
  • Möller, T;
  • Eloranta, J;
  • Pi, M;
  • Barranco, M;
  • Stienkemeier, F
  • et al.
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[Formula: see text]) within 1 ps. Subsequently, the bubble collapses and releases metastable He[Formula: see text] 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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