Light, Molecules, Action: Using Ultrafast Optical and X-ray Spectroscopy to Probe Excited State Dynamics in Photoactive Molecules.*

Light provides a versatile energy source capable of precise manipulation of material systems on size scales ranging from molecular to macroscopic. Photochemistry provides the means for transforming light energy from photon to process via movement of charge, a change in shape, a change in size, or the cleavage of a bond. Put simply, photochemistry produces action. In the work to be presented here ultrafast UV-Visible and X-ray absorption spectroscopies have been applied to the study of cobalamin (vitamin B₁₂) based compounds. Optical measurements provide precise characterization of spectroscopic signatures of the intermediate species on the excited state surface responsible for photochemistry. In contrast time-resolved XANES at the Co K-edge is sensitive to the structural changes that accompany these transformations. Polarized tr-XANES measurements allow identification of concurrent and sequential structural dynamics on the femtosecond and picosecond time scales. The results of recent polarized ultrafast XANES measurements at LCLS on two photoactive B₁₂ compounds will be presented.