Studies of the structure of matter with photons from an X-ray free-electron laser

X-ray free-electron lasers offer the potential for fourth-generation ultra-high peak power and coherence X-ray beams in the hard X-ray (8 keV) spectral range. A critical review of three areas of potential application of such sources is presented. Owing to the enormous electric field strength at the focus of such beams, it is shown that matter would very rapidly form a plasma for focal spots of submicrometer dimensions. Thus, hologram formation would only be feasible for crystalline samples with dimensions of a few micrometers or greater. A new version of X-ray holography using interference between a sample and a reference two-dimensional crystal is proposed. Because of the very high peak intensity of the free-electron-laser pulses, a natural application is to time-resolved multibunch 'dynamic X-ray scattering' measurements. It is shown that this could provide information on dynamical processes in condensed matter complementary to that obtained using slow neutron beams. Finally, optical laser-induced pump-probe-type experiments are reviewed. There is the potential for extending the study of the charge distribution of electronic excited states to those with lifetimes in the submillisecond time range.