  The ionization dynamics of helium droplets in a wide size range from 220 to
10^6 He atoms irradiated with intense femtosecond extreme ultraviolet (XUV)
pulses of 10^9 {\div} 10^{12} W/cm2 power density is investigated in detail by
photoelectron spectroscopy. Helium droplets are resonantly excited in the
photon energy range from ~ 21 eV (corresponding to the atomic 1s2s state) up to
the atomic ionization potential (IP) at ~ 25 eV. A complex evolution of the
electron spectra as a function of droplet size and XUV intensity is observed,
ranging from atomic-like narrow peaks due to binary autoionization, to an
unstructured feature characteristic of electron emission from a nanoplasma. The
experimental results are analyzed and interpreted with the help of numerical
simulations based on rate equations taking into account various processes such
as multi-step ionization, interatomic Coulombic decay (ICD), secondary
inelastic collisions, desorption of electronically excited atoms, collective
autoionization (CAI) and further relaxation processes.
