Phonon spectroscopy by inelastic x-ray scattering

The present synchrotron sources with brilliant x-ray beams, due to high photon fluxes, small source sizes and high collimation, have revolutionized x-ray physics. Enormous progress has been initiated in all established x-ray methods, with the aim of the development of new types of spectroscopy. This is particularly true for the spectroscopy of the dynamics in condensed matter. Meanwhile, there are two powerful x-ray methods with very high-energy resolution available for the study of low energetic excitations like phonons. This review summarizes the developments of these methods focusing on these instrumental developments of the spectrometers using either crystal optics in close-to-backscattering geometry or nuclear resonant techniques. Applications to measurements of phonon dispersion curves and of phonon density of states in ordered and disordered solids and in liquids are presented. It is shown how x-ray results are stimulating improvements in the theoretical approaches to the dynamics. New insights into the dynamics of liquids are discussed. The sensitivities of the spectroscopies allow the study of vibrational behaviour in very small amounts of material even in nanometre-sized thin films or panicles. We can already analyse the phonon spectrum of a monolayered nuclear resonant isotope. Prospects of the techniques are also demonstrated.