Picosecond X-ray absorption spectroscopy

The photocycle of aqueous ruthenium-(trisbipyridine) [Ru(bpy)3]2+ was studied under high laser excitation intensities and high sample concentrations with picosecond resolved x-ray absorption spectroscopy. In a pump-probe scheme a femtosecond laser pulse promotes a 4d electron from the ruthenium to the ligand orbitals, thus creating a metal-to-ligand-charge-transfer (MLCT) complex. A hard x-ray pulse from a synchrotron source probes the ruthenium L3 and L2 edges, monitoring the electronic and molecular structure of the ruthenium over the photocycle. The measured x-ray absorption spectrum of the MLCT state is in good agreement with the predictions of a theoretical calculation (TT-multiplet software). We extract from the spectrum that the excited-state complex can be described by D3 symmetry and has a 4d5 configuration. The decay kinetics of the MLCT state are found to be strongly dependent on the sample concentration, especially for solutions near the solubility limit of [Ru(bpy)3]Cl2 in water. Besides ground-state quenching and triplet-triplet annihilation a third fast decay component quenches the life-time of the MLCT state, tentatively attributed to a cluster effect. This study is the first application of sub-nanosecond time-resolved x-ray absorption spectroscopy on solvated systems and demonstrates its capability as a new tool for the observation of chemical dynamics in solvated systems.