Autoionization dynamics of He nanodroplets resonantly excited by intense XUV laser pulses

The ionization dynamics of helium droplets irradiated by intense, femtosecond extreme ultraviolet (XUV) pulses is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited to atomic-like 2p states with a photon energy of 21.5 eV and autoionize by interatomic Coulombic decay (ICD). A complex evolution of the electron spectra as a function of droplet size (250 to 10^6 He atoms per droplet) and XUV intensity (10^9 - 10^(12) W/cm2) is observed, ranging from narrow atomic-like peaks that are 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 a numerical simulation based on rate equations taking into account all relevant processes - multi-step ionization, electronic relaxation, ICD, secondary inelastic collisions, desorption of electronically excited atoms, and collective autoionization (CAI).