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Featured Application: This article describes the layout of the European XFEL, a soft and hard X-ray free-electron laser user facility starting operation in 2017. Emphasis is put on the photon beam systems, scientific applications and the instrumentation of the scientific instruments of the European XFEL.Abstract: European XFEL is a free-electron laser (FEL) user facility providing soft and hard X-ray FEL radiation to initially six scientific instruments. Starting user operation in fall 2017 European XFEL will provide new research opportunities to users from science domains as diverse as physics, chemistry, geo-and planetary sciences, materials sciences or biology. The unique feature of European XFEL is the provision of high average brilliance in the soft and hard X-ray regime, combined with the pulse properties of FEL radiation of extreme peak intensities, femtosecond pulse duration and high degree of coherence. The high average brilliance is achieved through acceleration of up to 27,000 electron bunches per second by the super-conducting electron accelerator. Enabling the usage of this high average brilliance in user experiments is one of the major instrumentation drivers for European XFEL. The radiation generated by three FEL sources is distributed via long beam transport systems to the experiment hall where the scientific instruments are located side-by-side. The X-ray beam transport systems have been optimized to maintain the unique features of the FEL radiation which will be monitored using build-in photon diagnostics. The six scientific instruments are optimized for specific applications using soft or hard X-ray techniques and include integrated lasers, dedicated sample environment, large area high frame rate detector(s) and computing systems capable of processing large quantities of data.

The transverse acoustic excitation modes were detected by inelastic x-ray scattering in liquid Ga in the Q range above 9 nm(-1) although liquid Ga is mostly described by a hard-sphere liquid. An ab initio molecular dynamics simulation clearly supports this finding. From the detailed analysis for the S(Q,omega) spectra with a good statistic quality, the lifetime of 0.5 ps and the propagating length of 0.4-0.5 nm can be estimated for the transverse acoustic phonon modes, which may correspond to the lifetime and size of cages formed instantaneously in liquid Ga.

We present high resolution inelastic x-ray scattering measurements of the coherent dynamic structure factor S͑Q, v͒ of liquid lithium at momentum transfers 0.36 # Q # 5 Å 21 . The determined S͑Q, v͒ agrees much better with molecular dynamic simulations using the neutral pseudo-atom potential rather than the empty core potential. We observe a positive dispersion in the sound velocity confirming that in liquid lithium the longitudinal dynamics reaches a solid-like response at high frequencies.[S0031-9007 (97)02529-5] PACS numbers: 61.25.Mv, 61.10.Eq, 61.20.Ne Recently, Canales et al. [1] calculated the dynamic structure factor of liquid lithium by means of molecular dynamic (MD) simulations using two different pair potentials: the empty core potential derived by Ashcroft [2]and an ab initio calculation for a pair potential deduced from the neutral pseudoatom method (NPA potential) [3]. Although the shapes of these two potentials are quite different, most of the calculated structural and thermodynamic properties are very similar [1]. Significant differences are observed in the calculations of the coherent part of the dynamic structure factor.Inelastic neutron experiments on the dynamic structure factor of liquid 7 Li were performed by de Jong et al. [4]. However, these measurements yielded no decision which of the two pair potential approaches is the more favorable. The reason is that an essential part of the coherent structure factor-the Brillouin modes-could not be observed at small momentum transfers Q. Because of the momentum-energy relation for a classical particle there exists a maximum energy transfer v at a momentum transfer Q, which is determined by the flight velocity of the incident neutron. In the neutron experiment mentioned above the Brillouin excitations were not detectable for Q , 1.2 Å 21 . Moreover, the high fraction of incoherent scattering at small Q and the uncertainties in the incoherent cross section of 7 Li makes it difficult to extract the coherent part from the experimental data.In contrast, these limitations do not appear in an inelastic x-ray scattering experiment with sufficient high energy resolution. At energy transfers of about a few meV the observed intensity originates dominantly from coherent scattering. The energy-momentum relation of the photon allows an almost unlimited energy transfer at any accessible momentum transfer [5,6].Previous experiments on liquid lithium were performed at the synchrotron laboratory HASYLAB in Hamburg [7,8]. The experiments reported here were carried out during the commissioning phase of the inelastic x-ray scattering beamline (ID16) at the European Synchrotron Facility (ESRF) in Grenoble. The x-ray radiation, coming from an undulator, was monochromized by a combination of a cryogenically cooled heat load monochromator and a silicon backscattering monochromator [Si(7,7,7) with E 13.8 keV]. The analyzer, a two dimensional focusing array of silicon crystals, is positioned at a 2.5 m distance from the sample in backscattering geometry. Details of the back...

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 µm in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.