Resonant Inelastic X-ray scattering and x-ray emission

General sketch of the Resonant Inelastic X-ray scattering
and x-rayemission process. The total energy of initial and
final states isdifferent. The energy difference is reflected in the
decreased photonenergy of the reemitted photon and corresponds
to an elementary lowenergy excitation.

Resonant inelastic x-ray scattering (RIXS) is a method to probe elementary excitations in materials in a very selective manner. Photons that are tuned to the energy of an excitation from a core level to an unoccupied state are scattered by the sample. Besides the elastically scattered photons which scatter without energy transfer also photons are emitted which transfer part of their energy to an elementary excitation in the material. In molecular systems vibrational and rotational excitations occur as well as spin excitations. In solids the corresponding collective excitations are phonons and magnons, orbitons and complex coupled processes of different degrees of freedom, but also local excitations occur. On higher energy scales charge transfer and inter-band excitations are found. Because of the resonance condition for the incoming photon RIXS is very selective with respect to the scattering sites. Only those centers contribute significantly to the RIXS signal, for which the photon energy and polarization allows for an excitation. This selectivity is particularly high in the soft x-ray range, where the core-hole influence leads to very pronounced differences between different ions of the same element that differ in oxidation state, orbital occupation and spin state. On the other hand the intrinsic energy line width of the RIXS process is not limited by the core-hole lifetime broadening but only by the instrumental performance. Recent advances in instrument design pushed the energy resolution so much forward that one now can probe low-energy excitations in a way that is directly complementary to inelastic neutron scattering. Also information about the momentum transfer connected with an excitation can be obtained nowadays such that the dispersion of excitations in solids and the symmetry of excitations in molecular systems become directly accessible.

At HZB in G-I2 RIXS is one of the central tools and the development of improved RIXS instrumentation is a major goal of the institute. A dedicated station for inelastic and coherent x-ray scattering (RICXS) will be commissioned in 2012, a project for momentum resolved RIXS within the newly planned XMETRIX station at BESSY II is presently being pushed as well as a project for time and momentum-resolved RIXS at the free electron laser XFEL. Presently a flexible station for RIXS from solids and liquid samples is available for experiments at BESSY II as well as LCLS. An RIXS station for surface science is in preparation.