In the last two decades the rapid development of instrumentation
for resonant inelastic X-ray scattering (RIXS) spectroscopy has established it as a suitable tool to probe localization of valence electrons in the atom specific picture. RIXS is a second order optical process that involves resonant excitation of a core electron to an unoccupied valence state, and relaxation to symmetry allowed final states
. The schematic of the resonant excitation, the inelastic scattering process, core ionization and X-ray emission are illustrated in the figure below. In the X-ray emission process a core electron is transferred to the continuum and then the system relaxes to a valence-ionized final state via spontaneous radiative decay of the valence electrons to the core-hole. Instead, in the RIXS process these two steps cannot be considered as separate processes but have to be described with a one-step scattering mechanism. RIXS process probes a symmetry specific valence occupied partial density of states (DOS), while XES probes all valence occupied partial DOS of the probed element. The spatial localization of core levels to one specific atom leads to the essential feature of the RIXS technique: probing "local" or "atom specific" unoccupied and occupied valence states as well as their correlation. The unoccupied valence states provide direct information about the energy and symmetry of the empty molecular orbitals, being "fingerprints" for identifying different chemical species and molecular hybridization. Detuning of the resonant excitation across the fingerprints is accompanied with the change of scattering duration time
that can be varied to probe the ultrafast nuclear dynamics
and vibrational progression
or charge transfer
in shorter times than the core-excited state lifetime (few femtoseconds). The final state of the RIXS process is a valence-excited state; therefore RIXS is a direct probe of the occupied-local-valence-states. Accordingly, RIXS profiles would consist of the linearly dispersed features (originating from scattering by localized states) and the constant energy RIXS features
(originating from delocalized charge transfer states). The low energy linearly dispersed features are of particular importance for highly correlated transition metals because they give information about the spin stat
e, crystal field
and the symmetry of orbital excitations
, while the constant energy features reveal electron dynamics and mixed states
Schematic of X-ray absorption process, radiative decay of a valence electron into the core excited state; known as resonant inelastic X-ray scattering process, photoionization of a core electron into the continuum and spontaneous radiative decay of a valence electron into the core hole state; known as spontaneous X-ray emission. Figure taken from the review of Lange et al.
- Lange, Kathrin M., René Könnecke, Samira Ghadimi, Ronny Golnak, Mikhail A. Soldatov, Kai F. Hodeck, Alexander Soldatov, and Emad F. Aziz. “High Resolution X-ray Emission Spectroscopy of Water and Aqueous Ions Using the Micro-jet Technique.” Chemical Physics 377, no. 1–3 (November 25, 2010): 1–5. doi:10.1016/j.chemphys.2010.08.023.
- Lange, Kathrin M, René Könnecke, Mikhail Soldatov, Ronny Golnak, Jan‐Erik Rubensson, Alexander Soldatov, and Emad F Aziz. “On the Origin of the Hydrogen Bond Network Nature of Water: X-Ray Absorption and Emission Spectra of Water–Acetonitrile Mixtures.” Angewandte Chemie 123, no. 45 (September 14, 2011): 10809–10813. doi:10.1002/ange.201104161.
- Lange, Kathrin M., Mikhail Soldatov, Ronny Golnak, Malte Gotz, Nicholas Engel, René Könnecke, Jan-Erik Rubensson, and Emad F. Aziz. “X-ray Emission from Pure and Dilute H2O and D2O in a Liquid Microjet: Hydrogen Bonds and Nuclear Dynamics.” Physical Review B 85, no. 15 (April 4, 2012): 155104. doi:10.1103/PhysRevB.85.155104.