We investigate the heating of the quasi-free electrons in large rare gas clusters (N exceeding 10 5 atoms) by short laser pulses at moderate intensities (I ≃ 10 15 Wcm −2 ). We identify elastic large-angle backscattering of electrons at ionic cores in the presence of a laser field as an efficient heating mechanism resembling the Fermi shuttle. Its efficiency as well as the effect of collective electron motion, electron-impact ionization and cluster charging, are studied employing a meanfield classical transport simulation. Results for the absolute x-ray yields are in surprisingly good quantitative agreement with recent experimental results.

We study experimentally and theoretically the production of characteristic K␣ x-rays during the interaction of intense infrared laser pulse with large ͑N ϳ 10 4 -10 6 atoms͒ argon clusters. We focus on the influence of laser intensity and pulse length on both the total x-ray yields and the charge-state distributions of the emitting cluster argon ions. An experimental optimization of the x-ray yield based on the setup geometry is presented and the role of the effective focal volume is investigated. Our theoretical model is based on a mean-field Monte-Carlo simulation and allows identifying the effective heating of a subensemble of electrons in strong fields as the origin of the observed x-ray emission. Well-controlled experimental conditions allow a quantitative bench marking of absolute x-ray yields as well as charge state distributions of ions having a K-shell vacancy. The presence of an optimum pulse duration that maximizes the x-ray yield at constant laser energy is found to be the result of the competition between the single cluster dynamics and the number of clusters participating in the emission.

Abstract. We investigate the efficient heating of quasi-free electrons during the interaction of short infrared laser pulses with large rare-gas clusters. In the framework of our mean-field classical transport simulation we are able to explain the emission of characteristic x-rays at moderate laser intensities (I ∼ 10 15 Wcm −2 ) where the ponderomotive energy of the electrons is by far to low to allow for the creation of inner-shell vacancies. We identify large-angle elastic electron-ion scattering as an important heating mechanism at moderate laser intensities.

Abstract. We study the interaction of an intense (I 10 15 Wcm −2 ) femtosecond laser pulse with a large (N > 10000 atoms) rare-gas cluster. The simulations are based on a mean-field classical transport approach. The electronic dynamics during the interaction with the laser are discussed in more detail. In particular we point out the difference in behavior between the fast electrons and the collectively moving slow electrons and try to shed light on the acceleration mechanisms behind the high energy tail of the electron energy distribution. The benchmark for our simulations is experimental X-ray spectroscopy data. We show a comparison with the experimentally found total X-ray yields and charge-state distributions. IntroductionRanging in size from a few atoms to several million atoms agglomerated at solid densities, clusters form a link between the gas and the solid phase. One manifestation is the up-conversion of the incident infra-red laser into keV X-ray photons in rare-gas clusters, which takes place with a relatively high efficiency providing large X-ray yields just like in solids, yet is relatively debris-free, a property shared with gas targets [1]. Similar behavior has been observed for the emission of energetic electrons [2] or highly charged ions [3], thus making the interaction of intense short and ultra-short laser pulses with clusters a topic of considerable interest [4; 5].In a simple picture, the dynamics during the interaction of a strong laser pulse with a cluster can be summarized as follows [6]: the atoms of the cluster are first ionized by the incident laser pulse (inner ionization) and a cold "nano-plasma" of solid density is formed. The quasi-free electrons take part in a collective oscillation driven by the laser field and, moreover, interact with the field of the surrounding particles. Electron-impact ionization of cluster ions produces additional quasi-free electrons and inner-shell vacancies which are at the origin of characteristic X-ray radiation. As a fraction of the electrons leaves the cluster (outer ionization), a net positive charge is left behind and the cluster begins to expand before disintegrating completely in a Coulomb explosion.The size of the system and the abundance of mechanisms at play provide a considerable challenge for the theoretical description of the interaction. One approach to the problem is treating the cluster as a "nanoplasma" submitted to a laser field [7; 8]. The inclusion of the cluster boundaries and the inhomogeneous ionic background during cluster expansion is however problematic. Molecular dynamics simulations on the other hand are limited to clusters of about 1000 atoms [9][10][11], and a scaling of the results to larger cluster sizes is difficult. Clusters of ∼ 10 4 atoms have been simulated using a microscopic particle in cell (MPIC) code [12], but typical

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