The dynamics of Xe clusters with initial radius between 10 and 100 Å irradiated by an IR subpicosecond laser pulse is investigated. The evolution of the cluster is modeled with a relativistic time-dependent three-dimensional particle simulation model. The focus of this investigation is to understand the energy absorption of clusters and how the absorbed energy is distributed among the various degrees of freedom. The consequence of the initial cluster radius on the absorbed energy, average charge per atom, mean electron and ion energies, ionization, removal of electrons from the cluster, and cluster expansion was studied. The absorbed energy per cluster scales as N 5/3 , and the mean electron and ion energies scale as N 1/3 and N 2/3 , respectively ͑N is the number of atoms per cluster͒. A significant fraction of the absorbed energy ͑ϳ90% ͒ is converted into kinetic energy with comparable contribution to electrons and ions. The energy balance suggests that smaller clusters are more efficient as radiators, while larger clusters are more conducive to particle acceleration. The radiation yield of clusters with initial radius 20-50 Å irradiated by a laser with peak intensity 10 16 W/cm 2 is determined to be 1%-2%.

A relativistic time-dependent three-dimensional particle simulation model has been developed to study the interaction of intense ultrashort KrF (248 nm) laser pulses with small Xe clusters. The trajectories of the electrons and ions are treated classically according to the relativistic equation of motion. The model has been applied to a different regime of ultrahigh intensities extending to 10(21) W/ cm(2). In particular, the behavior of the interaction with the clusters from intensities of approximately 10(15) W/cm(2) to intensities sufficient for a transition to the so-called "collective oscillation model" has been explored. At peak intensities below 10(20) W/cm(2), all electrons are removed from the cluster and form a plasma. It is found that the "collective oscillation model" commences at intensities in excess of 10(20) W/cm(2), the range that can be reached in stable relativistic channels. At these high intensities, the magnetic field has a profound effect on the shape and trajectory of the electron cloud. Specifically, the electrons are accelerated to relativistic velocities with energies exceeding 1 MeV in the direction of laser propagation and the magnetic field distorts the shape of the electron cloud to give the form of a pancake.

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double-vacancy states undergo strong amplification in relativistic self-trapped plasma channels on 3d → 2p transitions in the λ = 2.78–2.81 Å region. The 2P3/2 → 2S1/2 component at λ ≅ 2.786 Å exhibits saturated amplification demonstrated by both (1) the observation of spectral hole-burning in the spontaneous emission profile and (2) the correlated enhancement of 3p → 2s cascade transitions (2S1/2 → 2Pj; j = 1/2, 3/2) at λ = 2.558 Å and λ = 2.600 Å. The condition of saturation places a lower limit of ∼1017 W cm−2 on the intensity of the x-ray beam produced by the amplification in the channel. The anomalous strength of the amplification signalled by the saturation mirrors the equivalently anomalous behaviour observed for all 3d → 2p transitions corresponding to single-vacancy Xeq+ arrays (q = 31, 32, 34, 35, 36) that exhibit gain. The conspicuous absence of amplification involving states with double-vacancy configurations suggests the operation of a selective interaction that enhances the production of states. Overall, the generation of double-vacancy states of this genre demonstrates that an excitation rate approaching ∼1 W/atom for ionic species is achievable in self-trapped plasma channels.