Matter with a high energy density (>10(5) joules per cm(3)) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>10(17) watts per cm(2), previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 10(6) kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray-matter interactions, and illustrate the importance of electron-ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the K-α fluorescence, we can directly measure the position of the K edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential.

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion.

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The propagation of shock waves through polycrystalline iron is explored by large-scale atomistic simulations. For large enough shock strengths the passage of the wave causes the body-centered-cubic phase to transform into a close-packed phase with most structure being isotropic hexagonal-close-packed (hcp) and, depending on shock strength and grain orientation, some fraction of face-centered-cubic (fcc) structure. The simulated shock Hugoniot is compared to experiments. By calculating the extended x-ray absorption fine structure (EXAFS) directly from the atomic configurations, a comparison to experimental EXAFS measurements of nanosecond-laser shocks shows that the experimental data is consistent with such a phase transformation. However, the atomistically simulated EXAFS spectra also show that an experimental distinction between the hcp or fcc phase is not possible based on the spectra alone.

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser-excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes. 78.47.+p 61.10.-i 63.20.-e The absorption of ultrafast laser pulses in opaque materials generates coherent stress when the pulse length is short compared with time for sound to propagate across an optical penetration depth [1]. The resulting strain field consists of both a surface component, static on time scales where thermal diffusion can be ignored, and a bulk component that propagates at the speed of sound (coherent acoustic phonons). This strain is typically probed by optical methods that are sensitive primarily to the phonon component within the penetration depth of the light [1,2]. However, such methods give little information about the surface component of the strain, and, moreover, they are unable to give a quantitative measure of the strain amplitude.Due to their short wavelengths, long penetration depths, and significant interaction with core electrons, x-rays are a sensitive probe of strain. We note that coherent lattice motion adds sidebands to ordinary Bragg reflection peaks due to x-ray Brillouin scattering if the momentum transfer is large compared to the Darwin width, equivalent to phonons of GHz frequency for strong reflections from perfect crystals. This effect was demonstrated many years ago with acoustoelectrically amplified phonons using a conventional x-ray tube [3].With the recent availability of high brightness short-pulse hard x-ray sources, including third generation synchrotron sources and optical laser based sources [4-6], coherent strain generation and propagation can now be probed by x-ray methods in both the frequency and time domains. Recently, time-resolved diffraction patterns of cw ultrasonically excited crystals were obtained with a synchrotron source [7]. Other experiments have employed picosecond time-resolved x-ray diffraction to study transient lattice dynamics in metals [8], organic films [9], and impulsive strain generation and melting in semiconductors [10][11][12][13][14]. In particular Rose-Petruck et al.[10] demonstrated transient ultrafast strain propagation in GaAs by laser-pump x-ray-probe diffraction. In that experiment, x-rays were diffracted far outside the Bragg peak; however, no oscillations in the diffraction efficiency were detected, and the data were consistent with a unipolar strain pulse. In a similar experiment, Lindenberg et al. [13] detected oscillations in the sidebands for an asymmetrically cut InSb crystal using a streak camera. These oscillations were due to lattice compression and were probed for discrete phonon frequencies i...

The angular distribution of bremsstrahlung gamma rays produced by fast electrons accelerated in relativistic laser-solid interaction has been studied by photoneutron activation in copper. We show that the gamma-ray beam moves from the target normal to the direction of the k(laser) vector as the scale length is increased. Similar behavior is found also in 2D particle-in-cell simulations.

In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.