204
M. Lundberg and M. G. Delcey
Proceeding along the incident energy direction, the next resonance is the e g peak,
for which several features along the energy transfer axis can be resolved. After the
elastic peak, there is a broad and intense resonance around 4 eV that corresponds to
t 2g → e g transitions, see Fig. 12. These ligand-field transitions are the most intense
in the RIXS spectrum, in contrast to the weak transitions in UV/Vis absorption
spectroscopy. This is due to the differences in selection rules. The single-photon
g → g transition is parity forbidden and only gain intensity through vibronic coupling, while the two-photon g → u → g transition is parity allowed. In addition, the
strong spin–orbit coupling in the intermediate state breaks the spin selection rules,
and calculations indicate that both transitions to singlet and triplet final states have
appreciable magnitude [53].
Notice that although the σ → t 2g and t 2g → e g transitions have similar finalstate energies, these resonances are clearly separated along the incident energy in
the RIXS map. RIXS thus includes more information than a single-photon absorption, partly due to the enhancement of ligand-field transitions, but also facilitates
the assignment of these resonances to different molecular orbital transitions. With
the help of electronic structure calculations, the RIXS plane can be used to map out
the entire set of valence orbitals.
4.3 Transient Intermediates from Charge-Transfer
Excitations
Full understanding of catalytic reactions requires knowledge of intermediates along
the reaction pathway. The development of intense XFELs with time resolution in
the femtosecond range has opened up new ways to study short-lived intermediates. A prominent example is how the combination of femtosecond RIXS with RAS
modeling has given detailed insight into the spin and ligand-exchange dynamics of
photoexcited Fe(CO) 5 [51, 52, 93]. In general, valence excited states of iron complexes have attracted considerable scientific interest, as charge separation in these
states can be used in light-harvesting applications [57]. Again, iron hexacyanide
serves as a suitable model system to understand how information about electronic,
spin and structural dynamics can be extracted from the combination of modeling and
experiment [39, 66].
In the experiment, ferricyanide absorbs a photon from the UV/Vis probe, which
leads to an LMCT excitation that fills the t 2g shell and at the same time creates a
hole on the ligand. The time evolution of this excited state is then followed using
femtosecond RIXS [39]. Figure 13a shows the difference spectrum of the LMCT state
compared to the ground state of ferricyanide (shown in Fig. 12a). A clear fingerprint
of the LMCT state is the complete loss of the t 2g peak in the RIXS spectrum, because
the hole in that orbital is filled in the valence excitation.
RAS calculations have been used to predict spectra of potential species along the
reaction pathway and offer fingerprints for the dynamics [66]. They can also explain
M. Lundberg and M. G. Delcey
Proceeding along the incident energy direction, the next resonance is the e g peak,
for which several features along the energy transfer axis can be resolved. After the
elastic peak, there is a broad and intense resonance around 4 eV that corresponds to
t 2g → e g transitions, see Fig. 12. These ligand-field transitions are the most intense
in the RIXS spectrum, in contrast to the weak transitions in UV/Vis absorption
spectroscopy. This is due to the differences in selection rules. The single-photon
g → g transition is parity forbidden and only gain intensity through vibronic coupling, while the two-photon g → u → g transition is parity allowed. In addition, the
strong spin–orbit coupling in the intermediate state breaks the spin selection rules,
and calculations indicate that both transitions to singlet and triplet final states have
appreciable magnitude [53].
Notice that although the σ → t 2g and t 2g → e g transitions have similar finalstate energies, these resonances are clearly separated along the incident energy in
the RIXS map. RIXS thus includes more information than a single-photon absorption, partly due to the enhancement of ligand-field transitions, but also facilitates
the assignment of these resonances to different molecular orbital transitions. With
the help of electronic structure calculations, the RIXS plane can be used to map out
the entire set of valence orbitals.
4.3 Transient Intermediates from Charge-Transfer
Excitations
Full understanding of catalytic reactions requires knowledge of intermediates along
the reaction pathway. The development of intense XFELs with time resolution in
the femtosecond range has opened up new ways to study short-lived intermediates. A prominent example is how the combination of femtosecond RIXS with RAS
modeling has given detailed insight into the spin and ligand-exchange dynamics of
photoexcited Fe(CO) 5 [51, 52, 93]. In general, valence excited states of iron complexes have attracted considerable scientific interest, as charge separation in these
states can be used in light-harvesting applications [57]. Again, iron hexacyanide
serves as a suitable model system to understand how information about electronic,
spin and structural dynamics can be extracted from the combination of modeling and
experiment [39, 66].
In the experiment, ferricyanide absorbs a photon from the UV/Vis probe, which
leads to an LMCT excitation that fills the t 2g shell and at the same time creates a
hole on the ligand. The time evolution of this excited state is then followed using
femtosecond RIXS [39]. Figure 13a shows the difference spectrum of the LMCT state
compared to the ground state of ferricyanide (shown in Fig. 12a). A clear fingerprint
of the LMCT state is the complete loss of the t 2g peak in the RIXS spectrum, because
the hole in that orbital is filled in the valence excitation.
RAS calculations have been used to predict spectra of potential species along the
reaction pathway and offer fingerprints for the dynamics [66]. They can also explain