Ultrafast time-resolved X-ray absorption spectroscopic studies of solvated transition metal complexes.

摘要

A fundamental goal of chemical research has always been to understand the reaction mechanisms leading to specific reaction products. In principle, Ultrafast X-ray Absorption Fine Structure (UXAFS) spectroscopy permits the measurements of bond distances as a function of time after photo-excitation of a molecular system. Furthermore, most chemical processes occur in solution and UXAFS is well suited for condensed-phase studies. A laser-induced plasma x-ray source is the most attractive laboratory-based source suitable for hard x-ray generation due to relatively small, tabletop x-ray source with high source brightness. Thus, ultrafast tabletop laser-driven x-ray absorption spectrometer has been developed.; The structures of solvated Iron Pentacarbonyl (IPC), Fe(CO)5, have been investigated by using XAFS measurements in combination with IR measurements as well as density functional theory (DFT) calculations. The theoretical and experimental data suggest that solvated IPC "pre-assembles" with one solvent molecule before any photo-triggered chemical reaction. This system might therefore undergo ultrafast bi-molecular reaction in solution at room temperature without the need for diffusive reactant encounter. This could permit, for instance, the coherent control of a condensed-phase bi-molecular reaction. Furthermore, the XAFS results demonstrate that a dispersive x-ray absorption setup with the laser plasma x-ray source permits the measurement of spectra with excellent spectral resolution.; A significant first step toward the goal of observing the ultrafast structural motions during chemical reactions was performed: the laser-pump - picosecond x-ray-probe measurements of solvated transition metal complexes were carried out with our tabletop ultrafast laser-driven plasma x-ray source. The XAFS spectra of FeCN4- 6 solvated in water have been measured before and tens of picoseconds after photo-excitation with ultrashort UV laser pulses. The spectroscopic accuracy of the laser-plasma driven XAFS spectrometer has been established by comparing the static x-ray absorption spectra measured at a synchrotron source. Furthermore, these measurements clearly demonstrate the feasibility of UXAFS experiments with the tabletop laser plasma x-ray source.