Electronic charge transfer and dynamics in manganese and iron coordination complexes studied with resonant inelastic X-ray scattering [electronic resource]
Keywords: Chemistry

X-ray spectroscopy is a powerful tool in the study of electronic structure. I have utilized resonant inelastic x-ray scattering (RIXS) to study electronic charge transfer and electronic dynamics in transition metal complexes. RIXS creates a core-hole by scanning through the x-ray absorption edge while simultaneously measuring the emitted x-ray photons as the system relaxes to a lower energy core-hole state. RIXS is analogous to Resonance Raman spectroscopy but due to the properties of hard x-ray radiation, the tool is an elemental specific probe of electronic transitions. Charge transfer is a vital property of transition metal catalysts and I am able to assign RIXS spectral features to ligand-to-metal and metal-to-ligand charge transfer resonances and clearly characterize the nature of the molecular orbitals that are involved. I have also determined the effectiveness of extracting electronic dynamics from RIXS by carefully analyzing entire 1s3p RIXS data sets. The very short lifetime of the hard x-ray excited core-hole states means the RIXS process is sensitive to ultrafast electronic dynamics. The study of both charge transfer and electronic dynamics has been complimented by theoretical techniques to further the scientific understanding. The combination of RIXS measurements and density functional calculations allows the determination of the strength of the ligand-metal electronic interaction and assignment of the Raman resonances to charge transfer transitions in several manganese and iron cyanide complexes. With x-ray excitation energies resonant with the t2g and eg pre-edge peaks derived predominantly from the Mn 3d orbitals, the observation of Raman resonances in the energy transfer range from 2 to 12 eV which result from the filling of the 1s core-hole from t1u-symmetry occupied orbitals can be assigned as ligand-to-metal charge transfers. Evidence is also presented for the observation of a transition that leaves the state with increased electronic density in a ligand orbital while creating a metal hole, representing a metal-to-ligand charge transfer. The technique is then applied to K3Fe(CN)6, K4Fe(CN)6, and RbMnFe(CN)6. The two iron cyanides show similar results to those obtained with the manganese complex and the peak positions and relative intensities are discussed in relation to the electronic structure of the complexes. The manganese K-shell RIXS for RbMnFe(CN)6 shows significant deviation from the strong field metal-cyanide centers. The demonstration of the power of the technique on well characterized model systems opens the door for RIXS to be applied to more chemically relevant systems which is necessary for RIXS to develop widespread impact. I have also explored the potential for extracting excited state electron dynamics from RIXS spectra. This has involved detailed theoretical analysis of K3Mn(CN)6 and RbMnFe(CN)6 spectra. 'Core-hole clock' resonant soft x-ray studies have been utilized in the past to determine dynamic properties for a number of systems by relying on the lifetime of the excited core-hole. Due to the shortened lifetime of the hard x-ray excited core-holes, the technique is able to probe ultrafast electronic dynamics. The standard Kramer-Heisenberg description of RIXS attributes all dynamical effects to an excitation independent 1s core-hole lifetime and a final-state dependent lifetime broadening. Thorough study of the experimental data demonstrates that the standard implementation of the Kramers-Heisenberg formula cannot fully account for the experimentally observed excited state dynamics. We have proposed an alternative approach to analyzing RIXS spectra based on a density matrix formalism developed by Mukamel. The results demonstrate that while the Kramers-Heisenberg method is able to qualitatively model the spectra, it is unable to account for all aspects of the spectral dynamics within the RIXS spectrum. While the density matrix formalism is able to more accurately describe the spectral features in the RIXS, a more detailed theoretical understanding of the dynamics involved is necessary to robustly extract a dephasing time and better understand the ultrafast electronic response to core-hole creation.