## 1 Introduction  
With the continuous increase in X-ray flux available from accelerator and lab-based sources, many demanding techniques are now being explored and applied in a variety of research fields. One class of techniques that has recently received significant attention has been the application of dispersive X-ray spectrometer geometries to measure inelastic X-ray scattering signals from a wide-range of samples. Dispersive X-ray spectrometry consists of using a crystal to simultaneously energy-resolve a broad spectral range of the X-rays scattered or emitted from the sample after exposure to an X-ray source. This experimental description covers everything from non-resonant and resonant X-ray emission spectroscopy (XES [1] and RXES [2], respectively) to X-ray Raman scattering (XRS) [3; 4]. In general, these dispersive spectrometers cover a finite range of X-ray bandwidth in a fixed geometry. This allows them to measure a range of the scattered X-ray spectrum in a single measurement, avoiding the necessity of scanning any part of the spectrometer or the incident beam energy. Here, we will describe the characterization and application of such a spectrometer based on the von Hamos geometry using segmented cylindrically bent crystals with 25 cm radius of curvature. This spectrometer is an evolution of our previous design [5], which has been in operation at the SuperXAS beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland) since 2012, as well as being used in temporary installations [6; 7; 8; 9] at the Advanced Photon Source [10] (Argonne National Labs, Lemont, IL, USA), the SACLA XFEL [11] (Spring-8, Hyogo Prefecture, Japan), and the LCLS XFEL [12] (SLAC National Accelerator Laboratory, Menlo Park, CA, USA). Similar spectrometers will also be available at Experimental Station Alvra [13] at the SwissFEL XFEL [14] when it begins user operation in 2018. The inelastic X-ray scattering technique (IXS), also called X-ray Raman scattering (XRS), is based on a photon-in photon-out scattering process of hard X-rays from low Z elements [4; 15; 16; 17; 18]. Through this scattering process, the core electron is excited to an unoccupied electronic state just above the Fermi level, and the energy loss shifts the scattered photon energy to lower values. The energy conservation for the IXS process is expressed by \(E_{1}=E_{initial}+E_{electron}+E_{2}\), where \(E_{1}\) and \(E_{2}\) are the energies of incoming and scattered X-rays, respectively. The sum of \(E_{initial}\) and \(E_{electron}\) represent the total energy loss of the scattered X-ray, where \(E_{initial}\) stands for the binding energy of an electron and \(E_{electron}\) represents its energy above the Fermi level. By changing the energy of either \(E_{1}\) or \(E_{2}\), and monitoring \(E_{1}\) and \(E_{2}\) energies, the unoccupied electronic states probed by the scattering electron may thus be determined. Compared to soft X-ray absorption spectroscopy (XAS) measurements (<1 keV), which are typically employed for electronic state and structure determination in low Z elements, the IXS technique has both advantages and limitations. The primary drawback of applying the IXS technique is the low cross section for the scattering process. As compared to resonant X-ray scattering, the IXS yields are lower by \(10^{-4}\)-\(10^{-6}\). On the other hand, the IXS technique provides capabilities that are complementary when compared to soft-XAS techniques. The IXS technique uses hard X-rays for incoming and emitted X-ray energies; therefore, the penetration depth is much larger, making the IXS technique bulk-sensitive. Thanks to this advantage, the IXS technique may thus be applied to probe the electronic structure of low Z elements in liquids, concentrated heterogeneous materials, or matter under in situ conditions. To date, the IXS technique has been applied in many scientific areas where soft X-ray techniques could either not be applied [19; 20] or could not probe the desired state of matter [21; 22].