Visualizing excited state structural and electronic dynamics

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 and X-ray emission spectroscopy to probe changes in electronic configuration and atomic structure as a function of time, beginning from the initial excited state. Both ‘movies’ of coherent or ballistic motion and ‘snaphots’ of local minima or kinetic intermediates are possible. Polarization anisotropy, long exploited in ultrafast optical measurements, permits decomposition of the X-ray transient difference signal into contributions along the direction parallel to the transition dipole initially pumped, and perpendicular to this transition dipole. This decomposition allows the analysis of asymmetric sequential structural changes of photoexcited molecules in isotropic solution. We have used femtosecond X-ray absorption near edge structure (XANES) at the Co K-edge to characterize the excited state dynamics of cobalamins, B₁₂ coenzymes and analogues. Femtosecond X-ray emission spectroscopy (XES) in the Kβ_1,3 and valence-to-core regions provides additional insight into the electronic evolution.