Transient charge dynamics in argon-cluster nanoplasmas created by intense extreme-ultraviolet free-electron-laser irradiation

摘要

We present extreme-ultraviolet (EUV) fluorescence spectra of Ar clusters irradiated by intense EUV free-electron-laser (FEL) pulses focused to intensities of up to 3 x 10(13)W/cm(2) at a wavelength of 51 nm. The spectra reveal fluorescence at wavelengths shorter than that of the incident radiation, which can be assigned to EUV fluorescence lines from excited multiply charged ions Arz+* with z as high as 6. This demonstrates that charge states as high as 7+ are produced by the FEL irradiation. The dependence of the spectra on cluster size shows that the highly charged ions are generated at the cluster surface, indicating inhomogeneous charging. The FEL power dependencies of the spectral features suggest that the inhomogeneous distribution of charge within the clusters reduces ionization thresholds at the cluster surface.