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We have studied the ionic outcome from the interaction of intense laser light with large argon and xenon clusters. Ions with initial energies of several 100 keV are charge and energy selected using a magnetic deflection time-of-flight mass spectrometer. For argon clusters, Coulomb repulsion is the key process in the explosion mechanism, whereas for xenon we observe a mixture of Coulomb repulsion and hydrodynamic expansion. Coulomb explosion is the preferred decay channel for smaller clusters and it is also responsible for the production of the most energetic ions. Our results can be understood on the basis of a charged sphere model of cluster-sized plasmas. [S0031-9007 (97)05135-1] PACS numbers: 36.40.Vz, 52.40.NkThe development of relatively compact but powerful short pulse lasers has given strong impetus to the study of matter in intense fields. Solid and gaseous targets have been thoroughly investigated and, generally, highly excited states of matter are reported (see, for example, [1]). Recently, since atomic clusters have an intermediate target size, they have captured the attention as a promising means to produce x rays in the keV range [2-4], high particle velocities [5,6] and highly charged ions [6][7][8]. According to Wülker et al.[9] as well as Ditmire and co-workers [10,11], the very energetic states of matter obtained when irradiating clusters are usually interpreted as hot cluster-sized plasmas, efficiently laser heated by inverse bremsstrahlung. Already for intensities of 10 14 W͞cm 2 we have been able to confirm hydrodynamically expanding nanoplasmas with ion energies of several keV [8]. Much higher particle energies reaching up to 1 MeV, obtained after irradiating xenon clusters with 10 16 W͞cm 2 , are also currently attributed to hydrodynamic expansion [5,6]. It should be mentioned that, after the pioneering work by Ehler [12], ion energies of several MeV͞amu have been reported from intense laser irradiation of solids for more than a decade [13,14]. Such energetic particles have been theoretically interpreted as a departure from ideal isothermal expansion due to overheated coronal electrons [15], rarefaction shocks [16], neutralizing return currents [17], ion acoustic turbulences [18], or self-focusing [19]. Most of the theoretical studies on fast ion generation have assumed that the energy distribution in the coronal plasma is described primarily by two temperatures for thermal and hot electrons. Interestingly, two electron temperatures have also been observed from clusters, although electron energies generally did not exceed 3 keV [20]. A difference in three orders of magnitude between ion and electron energies obtained from similar cluster experiments [5,20] could therefore again indicate a failure of the isothermal expansion mechanism. From previous studies on molecules [21] and small clusters [22], a transition regime can be expected for shrinking particle sizes towards a Coulomb explosion mechanism originat-ing from the charge buildup in the system due to electron losses. In such a case, th...

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8

Argon ions with charge states of at least up to q = 9 are produced with 10 14 W cm −2 , 30 ps laser light pulses at 1064 nm from neutral Ar n clusters, whereas only Ar 3+ can be produced from monomers. Irradiation of cluster targets leads to ions with remarkably high kinetic energies exceeding 4.8 keV, depending on the ion charge state. The experimental results are understood in terms of cluster-sized nanoplasmas with internal electron temperatures of approximately 7 × 10 6 K which are heated by collisional electron-ion absorption. From these thermalized superdense nanoplasmas highly charged ions are ejected isotropically with the corresponding ion sound velocity. A simple Coulomb explosion model cannot account for the observed ion energies.

International audienceWe study the x-ray L-shell production from large krypton clusters submitted to ultrashort and intense laser pulses. The x-ray photon emission pattern appears to be isotropic and the absolute x-ray photon yields per laser pulse are measured as a function of the laser intensity and of the estimated mean cluster size in the supersonic expansion. In particular, up to 4 x 106 x-ray photons per laser shot are detected at intensities approaching 5 x 1017 W cm-2. This allows us to determine precisely a maximum conversion efficiency of 1.7 x 10-8 between the incoming IR photon and the generated x-ray photon energies. Finally, the x-ray photon emission is understood as the result of highly stripped ion production with L-shell electron-impact ionization and excitation in laser-heated cluster-sized nanoplasmas

We have performed studies of keV x-ray production from (Ar) n , (Kr) n and (Xe) n rare gas clusters (with n between 10 4 and 10 6 atoms/cluster) submitted to intense (≤10 18 W/cm 2) infrared (790 nm) laser pulses. We have determined the photon energies and the absolute photon emission yields as a function of several physical parameters governing the interaction: size and atomic number of the clusters, peak intensity of the laser. Up to 10 6 3 keV photons per pulse at a moderate (10 15 /cm 2) atomic density have been observed. High resolution spectroscopy studies in the case of (Ar) n clusters have also been performed, giving unambiguous evidence of highly charged (up to heliumlike) ions with K vacancies production. The results obtained indicate that x-rays are emitted before cluster explosion on a subpicosecond time scale, and shed some light on the mechanisms involved in the first stage of the production of the nanoplasma induced from each cluster.

In this Letter, we demonstrate the instantaneous creation of a hot solid-density plasma generated by focusing an intense femtosecond, high temporal contrast laser on an ultrathin foil (100 nm) in the 10(18) W/cm2 intensity range. The use of high-order harmonics generated in a gas jet, providing a probe beam of sufficiently short wavelengths to penetrate such a medium, enables the study of the dynamics of this plasma on the 100 fs time scale. The comparison of the transmission of two successive harmonics permits us to determine the electronic density and the temperature with accuracies better than 15%, never achieved up to this date in the regime of laser pulses at relativistic intensity.

We have systematically studied emission patterns of fragments arising from the Coulomb explosion of N 2 , Cl 2 , and I 2 molecules when exposed to intense, linearly polarized laser fields. The experiments are performed at 395, 610, and 790 nm, using two different pulse durations ͑130 fs and 2 ps͒ and choosing two different peak intensities ͑10 15 and 2ϫ10 16 W cm Ϫ2 ͒. We show that the anisotropy of the emission patterns increases with the number of absorbed photons necessary to produce the parent molecular ion. The set of experimental results suggest that the alignment process occurs in the leading edge of the laser pulse at moderate intensities (Ͻ10 14 W cm Ϫ2 ) where the multiphoton regime prevails. We explain these observations in terms of the rotational pumping model, as demonstrated by statistical simulations of multiphoton absorption and emission. The present study confirms that, in most cases, the confined emission of fragments along the laser electric field is due to the alignment of the transient molecular ions.