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-L2 (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-L2. 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.
Original language | English (US) |
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Pages (from-to) | 161-174 |
Number of pages | 14 |
Journal | Journal of Electron Spectroscopy and Related Phenomena |
Volume | 119 |
Issue number | 2-3 |
DOIs | |
State | Published - Aug 2001 |
Funding
The work is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy under contract W-31-109-Eng-38. The author would like to thank all the collaborators over the past 10 years on the time-resolved pump-probe XAFS and related projects. They are Drs. James R. Norris, Farrel W. Lytle, Marion Thurnauer, Pedro Montano, Michael K. Bowman, Michael R. Wasielewski, Zhiyu Wang, Peter L. Lee, Wighard J.H. Jäger, David J. Gosztola, Guy Jennings, Anneli Munkholm, and Jan Hessler. This work would not be possible without their support and participation. I would also like to express my sincere appreciation to all the staff members at BESSRC-CAT, Advanced Photon Source for their technical assistance.
Keywords
- NiTPP
- Pump-probe
- Structures of intermediates
- Synchrotron
- Time-resolved XAFS
- XAFS
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Radiation
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Spectroscopy
- Physical and Theoretical Chemistry