Probing transient molecular structures with time-resolved pump/probe XAFS using synchrotron X-ray sources

Abstract

Laser pulse pump, X-ray pulse probe X-ray absorption fine structure (pump-probe XAFS) experiments using synchrotron sources are described from technical considerations and from scientific significance. There are three technical challenges of such experiments: (1) laser photoexcitation, (2) synchronization of laser pulse and X-ray pulse, and (3) detection; each of which is investigated in detail. Based on the results from these investigations, the transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L 2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution has been captured for the first time with the pump-probe XAFS on a 14-ns time scale obtained from the X-ray pulses from a third generation synchrotron source. The experimental results confirm that photoexcitation leads to the rapid removal of both axial ligands to produce a transient square-planar intermediate, NiTTP, with a lifetime of 28 ns. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a non-coordinating solvent before it recombines with two ligands to form the more stable octahedrally coordinated NiTPP-L 2 . No detectable population of a penta-coordinated intermediate was present. This experiment demonstrates the feasibility of determining transient molecular structures in disordered media using the temporal resolution of a synchrotron X-ray source.