  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, helium nanodroplets are resonantly excited 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.
