Using X-rays to probe excited state structural and electronic dynamics: Cobalamins as a case study*

The fate of a photoactive molecule is determined by the electronic and structural rearrangements that follow excitation.  Femtosecond (fs) X-ray free electron lasers (XFELs) have made it possible to use X-ray absorption spectroscopy to probe changes in electronic configuration and atomic structure as a function of time, beginning from the initial excited state.  Hard X-rays permit the investigation at the Co K-edge focused on structural changes, soft X-rays at the Co L-edge and N K-edge permit more detailed investigation of the evolution of the electronic structure at the photoactive Co center. Femtosecond X-ray absorption near edge structure (XANES) and X-ray Emission spectroscopy (XES) at the Co K-edge to characterize the excited state dynamics of cobalamins, B₁₂ coenzymes and analogues. Time-resolved measurements in the pre-edge region and steady state measurements in the soft X-ray region lay the ground work for femtosecond measurements using the soft X-ray liquid jet end stations being developed at both LCLS and the EuXFEL.