X-ray spectroscopy and scattering
Spectroscopy gives direct access to the electronic structure that is responsible for the functionality and properties of matter, whereas scattering directly accesses spatial information on the arrangement of the constituents of matter. Using X-rays allows to enhance the information depths of those tools through the specific tuning of the properties of the incident radiation. The specificity of X-rays is twofold:
On one hand the photon energies can be tuned to core resonances. Those resonances involve electronic orbitals that are local to one atom and allow therefore for local probing. Furthermore, the core resonances are energetically well separated for different chemical elements and even for the same elements in different chemical surroundings. This allows for local probing at selected active centers in matter. On the other hand, the tunable wavelengths of X-rays is on the same order like atomic distances in matter or for example typical length scales of ordering phenomena in correlated materials. This allows to select specific spatial features in scattering experiments.
Furthermore, the tunable polarization of X-rays can address transitions of specific symmetry, for example one can select orbitals oriented parallel or perpendicular to the sample surface. With circularly polarized X-rays, it is even possible to access the spin properties of matter.