Inelastic X-ray scattering: new possibilities for Raman spectroscopy

Inelastic X-ray scattering with meV resolution has recently become available for studies of dynamical properties and elementary excitations in solids. Contrary to Raman spectroscopy at visible wavelengths, which in first order is limited to the Brillouin-zone center; the wave vectors in hard X-ray Raman scattering are very large, and the crystal-momentum transfer to elementary excitations, whose energies may range from a few meV up to several eV, can be tuned continuously across the whole Brillouin zone. This paper reviews new and unique possibilities offered by X-ray Raman spectroscopy for crystalline solids, such as phonon-dispersion measurements (GaN), the determination of phonon self-energies (isotopically mixed diamond), and resonance effects and studies of electronic excitations (copper oxides).