Mid Sweden University

X-ray Diffraction (XRD) is an outstanding tool for structural analyses at the atomic scale, and both the experimental techniques and the theoretical interpretations are well established. X-rays also have the advantage of being highly penetrating, as compared to electrons for instance, allowing for the study of bulk materials, or to study samples in complicated environments. The high photon fluxes available at third generation synchrotron sources make it possible to collect full diffraction patterns in relatively short times, and thus to follow time varying processes in-situ. In the frst part of this chapter we briefly discuss the advantages and disadvantages of X-rays as compared to other probes like electrons or neutrons. In the second part as an example in-situ surface X-ray diffraction studies of growing films using pulsed laser deposition (PLD) will be presented. The hetero-epitaxial growth process, especially of the first mono-layers can only be understood by in-situ diffraction studies in the PLD chamber under deposition conditions. Also high energy diffraction of buried interfaces will be discussed briefly. The final part of this chapter will present the possibilities for in-situ diffraction studies at the upcoming Free-Electron Laser sources, with fully coherent beams and sufficient intensities to collect full diffraction patterns with single 100 femto-second pulses. The characteristics of the Free-Electron-Lasers and various planned experiments will be presented. © Springer-Verlag Berlin Heidelberg 2014.