Source and optics considerations for new generation high-resolution inelastic X-ray spectrometers

The high-resolution inelastic X-ray scattering technique has evolved rapidly at the third-generation synchrotron sources. It is now possible to measure collective excitations with 2 meV resolution. The next level of experiments may require even more stringent conditions in terms of energy resolution, sample size and environment. For example, microscopic single crystals, thin films or multilayers, confined liquids, and samples under high pressure are outside the domain of momentum resolved inelastic X-ray scattering. Inelastic nuclear resonant scattering can measure partial phonon density of states from such samples, provided that the samples contain suitable Mِssbauer isotopes. Based on the experience obtained at the high-resolution X-ray scattering beamline (SRI-CAT 3-ID) of the APS, we present some new perspectives for X-ray sources, monochromators and analyzers to improve the performance of the spectrometer by an order of magnitude.