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 $2p\to 3d$ 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.

Figure 1

Two-dimensional velocity map images of emitted electrons from (a) Ne clusters and (b) Ne atoms.

Figure 2

Electron emission spectra [thick solid (blue) lines] for Ne clusters at various FEL intensities. (a) Inset: The FEL intensity distribution is divided into four parts. (b) Inset: Semilogarithmic plot of spectrum (b). Exponential tails are depicted by dotted (red) lines; Gaussian peaks, by dashed (green) lines; and the sum, by thin solid lines. Shaded (blue) areas schematically illustrate uncertainties due to edge effects.

Figure 3

Fitting parameters $A$–$E$ (see text) are plotted as a function of the FEL intensity for the case of four divisions [(red) circles] and eight divisions [(blue) squares].

Figure 4

Total electron yields from Ne clusters are plotted as a function of the FEL intensity for the case of 22 divisions [(red) circles] and 8 divisions [(green) squares]. The solid (blue) line represents Eq. (2). The first and second terms in Eq. (2) are depicted by the dotted line and the dashed line, respectively.