The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10(-14) to 10(-12) s. Until recently, only optical techniques were capable of providing adequate time resolution, albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction. However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase, has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance-near the Lindemann limit-leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

The displacive phase transition in SrTiO3 was investigated by means of x-ray diffraction. We used 4.5 keV photons thus probing only a very thin region near the surface. In the low temperature phase the lattice parameters evolve substantially different than in bulk material. We also investigated the phase transition under the influence of an epitaxial coating with YBaCu2O7 and found the nature of the phase transition changed. The near-surface region behaves like an epitaxial thin SrTiO3 film.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

The ablation of solids by high energy femtosecond pulses from an extreme ultraviolet ͑XUV͒ free electron laser has been investigated using picosecond optical imaging. The time-resolved measurements are supplemented by an analysis of the permanent structural surface modifications. Compared with femtosecond optical excitation, distinct differences in the material response are found which are attributed to the increased penetration depth of the XUV radiation and the absence of any absorption nonlinearities. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2405398͔ Ultrashort laser pulses allow to create states of strong electronic excitation and high temperature and pressure in solid materials. Under these conditions phase transitions and ablation occur on very rapid time scales, and often along unusual, nonequilibrium pathways. Up to now mainly femtosecond optical lasers have been used for material excitation, but the interpretation of experimental data is often difficult because of the highly nonlinear nature of the deposition of optical energy in the intensity range of interest ͑Ͼ10 12 W/cm 2 ͒.Unique possibilities for both generating and probing high energy density states of matter are emerging with the recent advent of short-wavelength accelerator-based light sources. 1,2 Among these future light sources the extreme ultraviolet ͑XUV͒ free electron laser ͑FEL͒ FLASH ͑free electron laser in Hamburg͒ 3 at the Deutsches ElektronenSynchrotron ͑DESY͒ in Hamburg, Germany is currently the only self-amplified spontaneous emission FEL 4 operating in the 6 -100 nm wavelength range.The irradiation of solid materials with such shortwavelength femtosecond pulses offers a number of advantages. First of all it permits a high degree of electronic excitation but with a strongly reduced influence of optical nonlinearities ͑i.e., multiphoton absorption and free carrier absorption͒. Moreover, for frequencies higher than the plasma frequency but lower than the frequency of the innershell absorption edge, the absorption depth for some materials can be rather long. Therefore, ultrashort XUV pulses allow the preparation of rather well-defined excitation conditions in relatively large sample volumes as compared with femtosecond optical pulses.In this letter we report on the results of experiments performed at FLASH on the interaction of ultrashort high intensity ͑10 12 -10 14 W/cm 2 ͒ XUV pulses with solid surfaces. In an XUV-pump/optical probe experiment picosecond optical imaging has been used to follow the dynamics of short-pulse XUV-induced phase transitions and ablation. The time-resolved measurements are supplemented by a characterization of the permanent structural modifications of the irradiated surfaces. A comparison with femtosecond optical excitation reveals distinct differences in the material response which we attribute to the absence of absorption nonlinearities and the increased penetration of the XUV light in the materials ͑Si and GaAs͒ discussed in this study.Experiments were performed at beamline BL2 of the FLASH...

By the use of a thin highly oriented pyrolytic graphite crystal (HOPG) bent to a high-performance ellipsoidal shape it was possible to focus monochromatic x-rays of 4.5 keV photon energy with an efficiency of 0.0033, which is 30 times larger than for previously used bent crystals. Isotropic Ti K alpha radiation of a 150 microm source was focused onto a 450 microm spot. The size of the focal spot can be explained by broadening due to the mosaic crystal rocking curve. The rocking curve width (FWHM) of the thin graphite foil was determined to 0.11 degrees. The estimated temporal broadening of an ultrashort K alpha pulse by the crystal is not larger than 300 fs. These properties make the x-ray optic very attractive for ultrafast time-resolved x-ray measurements.