Simulations of valence excited states in coordination complexes reached through hard X-ray scattering.

Erik Källman,Kelly J Gaffney,Marcus Lundberg,Roland Lindh,Drew A Meyer,Mickaël G Delcey,Meiyuan Guo

doi:10.1039/d0cp01003k

Abstract

Hard X-ray spectroscopy selectively probes metal sites in complex environments. Resonant inelastic X-ray scattering (RIXS) makes it is possible to directly study metal-ligand interactions through local valence excitations. Here multiconfigurational wavefunction simulations are used to model valence K pre-edge RIXS for three metal-hexacyanide complexes by coupling the electric dipole-forbidden excitations with dipole-allowed valence-to-core emission. Comparisons between experimental and simulated spectra makes it possible to evaluate the simulation accuracy and establish a best-modeling practice. The calculations give correct descriptions of all LMCT excitations in the spectra, although energies and intensities are sensitive to the description of dynamical electron correlation. The consistent treatment of all complexes shows that simulations can rationalize spectral features. The dispersion in the manganese(iii) spectrum comes from unresolved multiple resonances rather than fluorescence, and the splitting is mainly caused by differences in spatial orientation between holes and electrons. The simulations predict spectral features that cannot be resolved in current experimental data sets and the potential for observing d-d excitations is also explored. The latter can be of relevance for non-centrosymmetric systems with more intense K pre-edges. These ab initio simulations can be used to both design and interpret high-resolution X-ray scattering experiments.

Highlights

Transition metal complexes can facilitate a wide variety of different processes
3.1 Iron(II) hexacyanide RIXS spectrum As mentioned above, the experimental ferrocyanide spectrum has one pre-edge resonance at 7113.0 eV with three energytransfer resonances, see Fig. 2. These resonances can be tentatively assigned to come from emission from the 8t1u, 7t1u, and 6t1u ligand-dominated orbitals, which results in ligand-to-metal charge-transfer (LMCT) final states
In UV-Vis there are strong charge-transfer excitations at 5.7 and 6.2 eV. These do not appear in the experimental LMCT RIXS spectrum where the first resonance is at 7.4 eV

Summary

Introduction

Transition metal complexes can facilitate a wide variety of different processes. Their versatility comes from the potential to tune the energy of close-lying valence levels through metal– ligand interactions. Valence excitations can be studied through the use of resonant inelastic X-ray scattering (RIXS).[1,2] In this two-photon process, absorption of an incident photon is followed by emission of a scattered photon Their energy difference is equal to the energy required to reach the excited state.[3,4,5,6,7,8] For firstrow transition metals, soft X-ray L-edge RIXS (2p - 3d - 2p) directly probes the metal 3d orbitals. The other two complexes have multiple pre-edge resonances, which leads to a large number of different valence-excited states, separated either by incident or emission energy in the RIXS planes These rich data sets can be used to rigorously test the performance of different modeling protocols and to establish the accuracy of the current RAS approach

Computational details

Results and discussion

Exploring method sensitivity

RIXS spectra with metal-centered d–d excitations

Conclusions