Development of resonant inelastic x-ray scattering spectroscopy

Abstract

This research focuses on the development of Resonant Inelastic X-ray Scattering spectroscopy (RIXS) as a tool in homogeneous catalysis by looking at 3d transition metal compounds and specifically Cr(salen) epoxidation catalysts. Previous studies have demonstrated the sensitivity of transition metal K-edge X-ray absorption pre-edge and edge features to their chemical environment, but the exact origins are unassigned. The aim of this study is to bring together a collection of complementary, and some novel, X-ray techniques to assign these features and obtain more detailed structural and electronic information on the systems under investigation.

Novel high energy resolution data on transition metal complexes have been obtained and the pre-edge region has been simulated with the FEFF9 multiple scattering code. The increase in spectral structure compared to normal XANES allows for a more detailed analysis and as such, provides detailed insights in the electronic properties. L-edge XAS data obtained using soft X-rays enabled the determination of crystal field parameters, which were compared with other X-ray (RIXS) and optical absorption techniques. The novel RIXS experiments provide L and K-edge spectra unaffected by lifetime broadening or background from the main absorption edge. 2D images of X-ray emission as a function of absorption are obtained, revealing the relationship between them and providing direct and detailed information on the presence and position of molecular orbitals. These 1s2p RIXS experiments make use of hard X-rays enabling in-situ experiments, which are important in the field of catalysis, making it a promising tool to monitor the changing electronics of the metal centre as well as ligand coordination during the catalytic process.

Whereas the electronic differences on the Cr metal as a function of salen ligand are not revealed by the current RIXS experiments, which is likely due to resolution issues, new preliminary insights in the different catalytic Cr intermediates of the epoxidation reaction have been obtained.

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