Resonant Inelastic X-ray Scattering (RIXS) Studies in Chemistry: Present and Future

Abstract

This chapter illustrates how resonant inelastic x-ray scattering (RIXS) is used to address questions in chemistry, with special focus on the electronic structure and catalytic activity of first row transition metals. RIXS is a two-photon process that is the x-ray equivalent of resonance Raman spectroscopy. The final states correspond to vibrational, valence electronic or even core excitations. In addition to the advantages of a local element-selective x-ray spectroscopic probe, RIXS gives new information compared to single-photon x-ray absorption and x-ray emission experiments. Metal L-edge RIXS shows intense metalcentered ligand-field transitions, even in cases where they are spin or parity forbidden in optical absorption spectroscopy. By selecting different resonances by appropriately tuning the incident energy, it is possible to isolate different ligand-field and charge-transfer transitions. The observation of a large number of electronic states that can be properly assigned, sometimes with the help of theoretical methods, gives novel opportunities to quantify metal-ligand interactions and their contributions to reactivity. RIXS in the K preedge can be used to obtain L-and M-edge like spectra including insight into charge-transfer excitations all with the advantages of a hard x-ray probe. Finally, it is shown how timeresolved RIXS down to the femtosecond timescale probes the orbitals of transient reaction intermediates. The usefulness of RIXS in chemistry is shown for a diverse set of systems, including coordination complexes, metal enzymes and nanoparticles.

Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers

“…XAS can be used to probe unoccupied orbitals but is insensitive to the occupied part of the valence electronic structure (besides being sensitive in addition to geometric structure changes in the case of K-edge XAS). This is where RIXS, combining the sensitivity to occupied and unoccupied orbitals, comes in [10]. Figure 13 royalsocietypublishing.org/journal/rsta Phil.…”

“…RIXS will be highlighted as a way to access the low-energy excitations from meV to eV thereby giving access to ligand-field and charge-transfer excitations in 3d transition-metal systems. The book chapter on ‘Resonant Inelastic X-ray Scattering (RIXS) Studies in Chemistry: Present and Future’ by Marcus Lundberg and myself further details this aspect of studying 3d transition-metal systems with RIXS [10].…”

“…XAS can be used to probe unoccupied orbitals but is insensitive to the occupied part of the valence electronic structure (besides being sensitive in addition to geometric structure changes in the case of K-edge XAS). This is where RIXS, combining the sensitivity to occupied and unoccupied orbitals, comes in [10]. Figure 13 summarizes some of the important aspects of our own application of time-resolved RIXS at the Fe L-edge with measurements at the LCLS XFEL to probe the frontier-orbital interactions in the dissociation reaction of Fe(CO) 5 in solution [80].…”

“…This establishes the metal-specific probe of the frontier-orbital interactions where the metal specificity is given by the specificity to the Fe L-edge due to the localized nature of the Fe 2p core orbital. In a many-electron total-energy treatment of the process, the RIXS final states can be assigned to ligand-field or charge-transfer valence excited states and numerous examples for steady-state RIXS at the L- and K-edges of 3d transition-metal complexes and metalloproteins are discussed in [10]. In this picture, the transitions discussed here correspond to scattering to ligand-field final states involving exclusively nominally Fe-centred orbitals.…”

Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering

“…VtC RIXS measurements have provided invaluable insights into the electronic conguration of 3d transition metals. 34 In particular, 2p3d RIXS (initial 2p-hole, nal 3d-hole) have been used to provide detailed information on the nature of the 3d frontier orbitals, metal-ligand interactions, energies and nature of charge separated states, including dd transitions, [35][36][37][38] and have also been extended to the time-domain. 39,40 Similarly to the VtC XES, VtC RIXS measurements on 4d elements have been very scarce and only for bulk and concentrated systems.…”