Crossover in the photoionization processes of neon clusters with increasing EUV free-electron-laser intensity

Electron emission spectra and total electron yields were measured for relatively large (about 5000 atoms) neon clusters that were irradiated by extreme ultraviolet pulses with 20.3 eV, corresponding to the 2p -> 3d transition. The present electron emission spectra are remarkably different from those for Ar clusters undergoing sequential ionization [Bostedt et al., Phys. Rev. Lett. 100, 133401 (2008)] and can be interpreted as a superposition of a low-energy exponential tail, dominant at a low free-electron-laser intensity I, and an intermediate-energy Gaussian-like peak which grows with I. Correspondingly, the total electron yield exhibits interesting I dependence. Especially, a plateau appears in the intermediate I range. These results suggest a crossover in the ionization mechanism from thermal electron to two-photon photoelectron emission. An exciton-Mott transition is proposed for the formation of nanoplasma.