Max Flach

Probing the electronic structure of isolated transition metal model systems using XAS

X-ray absorption spectroscopy on cold gas phase cations provides the opportunity to probe the electronic structure of isolated systems [1], not individually accessible in solid or liquid phase experiments. This project is divided in two parts, which are both focused on investigating model systems of transition metal species, especially iron and nickel in the oxidation state of +2.

Part one:
As excitation energies of transition metal L-edges are widely used to determine oxidation states in a variety of fields and systems. Hence disentangling various effects such as crystal structure, ligand field, spin state and 3d occupation [2] is crucial to further improve the methods for experimental oxidation state determination. Gas phase diatomic systems of iron and nickel halides, enabling the investigation and quantification of the changes in the XAS L-edge excitation energy solely dependent on changes in the 3d occupation on the transition metal. As an example, we were able to quantify a shift in iron L₃-edge excitation energy of -0,42eV solely depend on non-integer changes of the irons 3d occupation of 6.1 electrons in FeF⁺ through 6.6 electrons in FeI⁺[3].

Part two:
Prediction and understanding of chemical reactions in aqueous solutions can be of great importance to develop new catalytic materials [4]. Therefore, providing experimental data on the changes in bonds and electronic structure of transition metal species with a controlled number of water molecules in their solvation shell is of great interest. We investigate these successive changes in the electronic structure using gas phase XAS of the transition metal L-edge and oxygen K-edge of iron aqua complexes and contact ion-pairs.

[1] J. T. Lau et al., Phys. Rev. Lett., 2008, 101, 153401
[2] Frank de Groot, Coordination Chemistry Reviews, 2005, Volume 249, 31-63
[3] Flach et al., Phys. Chem. Chem. Phys., 2022, 24, 19890-19894
[4] Macchioni, Chem. Rev. 2005, 105, 6, 2039–2074