Free electron lasers (FELs) offer the unprecedented capability to study reaction dynamics and image the
structure of complex systems. When multiple photons are absorbed in complex systems, a plasma-like state
is formed where many atoms are ionized on a femtosecond timescale. If multiphoton absorption is
resonantly-enhanced, the system becomes electronically-excited prior to plasma formation, with
subsequent decay paths which have been scarcely investigated to date. Here, we show using helium
nanodroplets as an example that these systems can decay by a new type of process, named collective
autoionization. In addition, we show that this process is surprisingly efficient, leading to ion abundances
much greater than that of direct single-photon ionization. This novel collective ionization process is
expected to be important in many other complex systems, e.g. macromolecules and nanoparticles, exposed
to high intensity radiation fields
