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

Iwayama, Hiroshi*; Harries, J.; Shigemasa, Eiji*

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 310 W/cm 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 Ar with as high as 6. This demonstrates that charge states as high as 7+ are produced by the FEL irradiation. The cluster size and FEL power dependences of the spectral features suggest that an inhomogeneous distribution of charge within the clusters reduces ionization thresholds at the cluster surface.