Soft X-ray spectroscopies are ideal probes for local valence electronic charge distributions of photocatalytically active sites in transition metal complexes. Due to their relevance for sustainable energy research, catalysts based on the widely abundant and non-toxic iron (Fe) have in particular been subject to various X-ray spectroscopic studies. By exploiting the elemental and orbital selectivity of soft X-rays, local charge densities in terms of valence orbital occupation and metal-ligand covalency can be determined. Making use of this sensitivity and further extending the method to the time domain for studying electronically excited states, we present femtosecond resolution time-resolved resonant inelastic X-ray scattering (RIXS) data on a charge-transfer excited Fe complex in solution measured at the Linac Coherent Light Source (LCLS). With Fe L-edge RIXS we are able to provide new insight to the transient ligand-to-metal charge-transfer (LMCT) state in aqueous Ferricyanide by locally probing the dynamics of charge densities and valence orbital occupations. In comparison to ground state measurements and quantum chemical simulations, the LMCT state is characterized to exhibit increased electron donation and ligand field splitting as a result of π-to-σ relaxation of the ligand hole orbital. Our investigations provide novel detail to a charge-transfer excited transition metal complex and demonstrate how femtosecond RIXS can make use of established descriptions of chemical bonding to help derive an orbital-based picture of transient charge densities as well as metal-ligand covalency during photochemical reactions.

Dynamcis of local charge densities and metal-ligand covalency in iron complexes from femtosecond resonant inelastic soft X-ray scattering

256th National Meeting (2018)

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