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IntroductionThe properties of materials are driven to extreme conditions under high pressures, the highest of which is generated by shock compression. First described by George Gabriel Stokes in 1851 [1], 'a shock is a travelling wave front across which a discontinuous, adiabatic jump in state variables takes place'. Investigations of materials under shock compression cast light on a broad range of phenomena that are not fully understood in the areas of high-energy-density physics, earth and planetary sciences, aerospace engineering, and materials science. The three main progenitors of shock compression can be considered as: explosion, impact, and plasma [2]. Respectively, the most popular platforms for shock wave generation in the AbstractA high-power, nanosecond pulsed laser impacting the surface of a material can generate an ablation plasma that drives a shock wave into it; while in situ x-ray imaging can provide a timeresolved probe of the shock-induced material behaviour on macroscopic length scales. Here, we report on an investigation into laser-driven shock compression of a polyurethane foam and a graphite rod by means of single-pulse synchrotron x-ray phase-contrast imaging with MHz frame rate. A 6 J, 10 ns pulsed laser was used to generate shock compression. Physical processes governing the laser-induced dynamic response such as elastic compression, compaction, pore collapse, fracture, and fragmentation have been imaged; and the advantage of exploiting the partial spatial coherence of a synchrotron source for studying low-density, carbon-based materials is emphasized. The successful combination of a high-energy laser and ultra high-speed x-ray imaging using synchrotron light demonstrates the potentiality of accessing complementary information from scientific studies of laser-driven shock compression.

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Photofragmentation of mass-selected C+16 to C+36 clusters is investigated at two laser wavelengths: 532 and 355 nm. The depletion dependence on laser fluence leads to the number of absorbed photons, together with the photoabsorption cross section. From the knowledge of the internal energy deposited in the clusters before the fragmentation step, the dissociation energies are deduced. Clusters C+16 to C+29 are found to be less and less stable, whereas above C+30 an increased stability is observed.

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We present an experiment adapted to collisional studies of cluster ions based on a laser vaporization setup coupled to a supersonic expansion. The ions are selected in a first time-of-flight, slowed down to the desired energy, and collided in an octopolar guide. The parent and fragment ions are then reaccelerated in order to be mass analyzed in a reflectron time-of-flight. An original method for the extraction of the ion that uses a double voltage pulse, is proposed. The experiment has been applied to collisions of hydrated cobalt ions. An absolute cross section of 17 A2 for the loss of one water molecule by Co(H2O)2+ in collision with neon at a center-of-mass energy of 10 eV, has been determined, with an accuracy of 10%. The threshold for this reaction has been measured at 1.5 eV and is in good agreement with the existing literature (Dalleska et al. J. Am. Chem. Soc. 1994, 116, 3519). Ions that cannot be formed by conventional ligand exchange methods, can also be studied. As an example, the threshold for dehydration of the Co2(H2O)+ ion has been measured at 1.5 +/- 0.2 eV.

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