Kinetic Boltzmann equations are used to model the ionization and expansion dynamics of xenon clusters irradiated with short intense VUV pulses. This unified model includes predominant interactions that contribute to the cluster dynamics induced by this radiation. The dependence of the evolution dynamics on cluster size, Natoms = 20 − 90000, and pulse fluence, F = 0.05 − 1.5 J/cm 2 , corresponding to intensities in the range, 10 12 − 10 14 W/cm 2 and irradiation times, ≤ 50 fs, is investigated. The predictions obtained with our model are found to be in good agreement with the experimental data. We find that during the exposure the cluster forms a shell structure consisting of a positively charged outer shell and a core of net charge equal to zero. The width of these shells depends on the cluster size. The charged outer shell is large within small clusters (Natoms = 20, 70), and its Coulomb explosion drives the expansion of these clusters. Within the large clusters (Natoms = 2500, 90000) the neutral core is large, and after the Coulomb explosion of the outer shell it expands hydrodynamically. Highly charged ions within the core recombine efficiently with electrons. As a result, we observe a large fraction of neutral atoms created within the core, its magnitude depending on the cluster size. Atomic clusters are excellent objects to test the dynamics within samples irradiated with radiation from short wavelength free-electron-lasers (FELs) [1,2,3]. Their physical properties put them on the border between the solid state and the gas phase. Cluster studies are important for planned experiments with FELs in solid state physics, materials science and for studies of the extreme states of matter [4]. Accurate predictions on the ionization, thermalization and expansion timescales within irradiated samples that can be obtained with cluster experiments are also needed for exploring the limits of experiments on single particle diffraction imaging [5,6,7,8,9,10].During the first cluster experiments performed at the free-electron-laser facility FLASH at DESY with VUV photons of energy, E = 12.7 eV, and power densities up to a few 10 13 W/cm 2 [11] highly charged Xe ions (up to +8) of high kinetic energies were detected. This unexpectedly strong energy absorption could not be explained using the standard approaches [12,13,14]. More specifically, the energy absorbed was almost an order of magnitude larger than that predicted with classical absorption models, and the ion charge states created were much higher than those observed during the irradiation of * Corresponding author. E-mail: ziaja@mail.desy.de isolated atoms in similar conditions. This indicated that at such radiation wavelengths some processes specific to many-body systems were responsible for an enhanced energy absorption.The physics underlying the dynamics within the irradiated clusters is complex. Several interesting theoretical models have been proposed in order to describe the evolution of clusters exposed to intense VUV pulses [15,16,17,18,19,20,21]. The most exp...

Coherent diffractive imaging using x-ray free-electron lasers (XFELs) may provide a unique opportunity for high-resolution structural analysis of single particles sprayed from an aqueous solution into the laser beam. As a result, diffraction images are measured from randomly oriented objects covered by a water layer. We analyze theoretically how the thickness of the covering water layer influences the structural and orientational information contained in the recorded diffraction images. This study has implications for planned experiments on single-particle imaging with XFELs.

We estimate the total cross sections for field-stimulated photoemissions and photoabsorptions by quasi-free electrons within a non-equilibrium plasma evolving from the strong coupling to the weak coupling regime. Such a transition may occur within laser-created plasmas, when the initially created plasma is cold but the heating of the plasma by the laser field is efficient. In particular, such a transition may occur within plasmas created by intense vacuum ultraviolet (VUV) radiation from a free-electron laser (FEL) as indicated by the results of the first experiments performed by Wabnitz at the FLASH facility at DESY. In order to estimate the inverse bremsstrahlung cross sections, we use point-like and effective atomic potentials. For ions modelled as point-like charges, the total cross sections are strongly affected by the changing plasma environment. The maximal change of the cross sections may be of the order of 75 at the change of the plasma parameters: inverse Debye length, κ, in the range κ = 0 − 3Å −1 and the electron density, ρ e , in the range ρ e = 0.01 − 1Å −3 . These ranges correspond to the physical conditions within the plasmas created during the first cluster experiments performed at the FLASH facility at DESY. In contrast, for the effective atomic potentials the total cross sections for photoemission and photoabsorption change only by a factor of seven at most in the same plasma parameter range. Our results show that the inverse bremsstrahlung cross section estimated with the effective atomic potentials is not affected much by the plasma environment. This observation validates the estimations of the enhanced heating effect obtained by Walters, Santra and Greene. This is important as this effect may be responsible for the high-energy absorption within clusters irradiated with VUV radiation.

We investigate the rates for multielectron recombination within a dense plasma environment in local thermodynamic equilibrium (LTE). We find that these multielectron recombination rates can be high within dense plasmas, and they should be treated in the simulations of the plasmas created by intense radiation, in particular for plasmas created by intense VUV radiation from free-electronlaser (FEL) or for modelling the inertial confinement fusion (ICF) plasmas.

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