Photoelectron spectroscopy, scanning tunneling spectroscopy, and neutron scattering have given us some of the deepest insights in the physics of novel materials, like high temperature superconductors.

Thanks to the development of ultra-bright synchrotron radiation (SR) sources of tunable xrays, a new technique - resonant inelastic x-ray scattering (RIXS) - can now be added to this list.

RIXS exploits the diffusion of x rays, as illustrated in Fig. 1a. An incident photon of energy hV₀ is absorbed and immediately re-emitted - scattered - by a solid sample. Most of the times the process is elastic, i.e. hV_out=hV₀, and the system falls back to its ground state |0> after the interaction. Less frequently, the photon is inelastically scattered : part of its energy is transferred to the solid, which is then left in an excited state |f_j*>. The energy and momentum losses suffered by the scattered photons therefore contain unique information on the sample's excitation spectrum [1]. The reader familiar with optical spectroscopy will recognize here a typical Raman process. But there is an important twist: in RIXS the incident photon energy is precisely tuned to the binding energy of an atomic core level, which participates in the scattering process. These atomic resonances typically occur in the x-ray energy range, e.g. at 930 eV for the Cu 2p level in the cuprates. Working at resonance has several dramatic advantages, namely in terms of sensitivity and chemical selectivity.

Several RIXS experiments are operational or planned at SR laboratories worldwide.

SAXES (Super-Advanced X-ray Emission Spectrometer), the most advanced instrument in the 400-1600

It is the result of a collaboration between the SLS, which provides a focused soft x-ray beam of superior quality, the spectroscopy group of Politecnico di Milano, which designed and built the 5-meter soft x-ray spectrometer [2], and members of MaNEP. The main features of SAXES are its very high resolving power (E/ΔE > 10 000 for energies up to 1100 eV, a fivefold improvement over the present world's best instrument), and the possibility of collecting data at variable momentum transfer. This combination should open the door to a whole new range of accurate measurements of local electronic (Fig. 1b) [3] and spin excitations [4], but also of collective magnetic and orbital excitations, in transition metal compounds, oxides, and other novel materials. After a short commissioning phase, SAXES will be available to the international community as an SLS facility. It will be an ideal instrument to pursue the core mission of MaNEP: understanding the electronic properties of complex systems to design tomorrow's materials.

Captions [ click captions to enlarge ]


Fig. 1a: The RIXS process in a total energy scheme. The ground state of the system is |0>; |i> is an intermediate excited state with a core hole, and |fj*> is an excited final state at energy Ej*. The energy loss is hv0- hvj = Ej .