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Abstract:

We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16)  W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.

Abstract:

We report on nonequilibrium molecular dynamics simulations of single crystals of copper experiencing rapid shear strain. A model system, with periodic boundary conditions, which includes a single dislocation dipole is subjected to a total shear strain of close to 10 on time-scales ranging from the instantaneous to 50 ps. When the system is strained on a time-scale short compared with a phonon period, the initial total applied shear is purely elastic, and the eventual temperature rise in the system due to the subsequent plastic work can be determined from the initial elastic strain energy. The rate at which this plastic work occurs, and heat is generated, depends on the dislocation velocity, which itself is a function of shear stress. A determination of the stress-dependence of the dislocation velocity allows us to construct a simple analytic model for the temperature rise in the system as a function of strain rate, and this model is found to be in good agreement with the simulations. For the effective dislocation density within the simulations, 7.8 10 11 cm - 2, we find that applying the total shear strain on time-scales of a few tens of picoseconds greatly reduces the final temperature. We discuss these results in the context of the growing interest in producing high pressure, solid-state matter, by quasi-isentropic (rather than shock) compression. © 2011 American Institute of Physics.

Abstract:

We reply to the comment by Iglesias [HEDP, XXX] regarding our implementation of a solid-state pseudopotential in a model for the calculation of the free--free opacity in warm-dense aluminum [HEDP 5(2009), 124-131]. Some further details are given describing the method used to determine the adjustable parameter in the pseudopotential and several important limitations are discussed. © 2010 Elsevier B.V.

Abstract:

In the past few years, the development of light sources of the 4^th generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of ∼20 fs, micro focusing leading to intensities larger than 10¹⁶ W/cm². Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.