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

Laser pulse pump, X-ray pulse probe X-ray absorption fine structure (pump-p robe XAFS) experiments using synchrotron sources are described from technic al considerations and from scientific significance. There are three technic al challenges of such experiments: (1) laser photoexcitation, (2) synchroni zation 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 photo excitation of NiTPP-L-2 (NiTPP, nickeltetraphenylporphyrin; L, piperidine) in solution has been captured for the first time with the pump-probe XAFS o n 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 squ are-planar intermediate, NiTTP, with a lifetime of 28 ns. The transient str ucture of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same st ructure as the ground state in a non-coordinating solvent before it recombi nes with two ligands to form the more stable octahedrally coordinated NiTPP -L-2. No detectable population of a penta-coordinated intermediate was pres ent. This experiment demonstrates the feasibility of determining transient molecular structures in disordered media using the temporal resolution of a synchrotron X-ray source. (C) 2001 Elsevier Science B.V. All rights reserv ed.