Charge recombination in soft x-ray laser produced nanoplasmas

The ionization and charge separation processes of nanoplasmas created by resonant excitation of atomic clusters in intense soft x-ray pulses have been investigated. Through irradiation with femtosecond pulses from the FLASH free electron laser (FEL) at lambda = 13.7 nm and power densities exceeding 10(14) W cm(-2) the clusters are highly ionized with transient atomic charge states up to 9+. Variation of the cluster composition from pristine to doped and core-shell systems allows tracking of the spatial origin and charge states of the fragments yielding insight into the nanoplasma dynamics. The data give evidence for efficient charge redistribution processes leading to a Coulomb explosion of the cluster outer part and recombination of the nanoplasma core. The experiments show qualitatively different processes for (soft) x-ray produced nanoplasmas from the optical (IR) strong-field regime where the clusters disintegrate completely in a Coulomb explosion.