Capturing excited state molecular structures in disordered media with 100-ps time resolution by laser pump x-ray probe XAFS

The timing structure and the high photon flux of x-ray pulses from the Advanced Photon Source permit pump-probe techniques widely used in ultrafast laser spectroscopy to be extended into the x-ray regime. The intrinsic time resolution of the experiments is determined by the FWHM of the single pulses from the synchrotron at 70-100 ps. The challenges and the solutions in such experiments will be discussed. Using laser pulse pump, x-ray pulse probe XAFS, several excited state molecular structures in solutions were studied. We will mainly describe molecular structures of the photoexcited metal-to-ligand-charge-transfer state of (CuI(dmp)2)+, where dmp is 2,9-dimethyl-1,10-phenanthroline, in toluene and acetonitrile. The experimental results indicated that the copper ion in the thermally equilibrated MLCT state in both solvents had the same oxidation state as the corresponding Cu(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distances 0.04 (angstrom) longer in toluene and 0.04 (angstrom) shorter in acetonitrile than that of the ground state (CuI(dmp)2)+. The results further revealed that what distinguishes the MLCT state structures in non-coordinating and coordinating solvents is not the 'exciplex' formation, but the strength of the interactions between the solvent and the Cu(II)* species at the MLCT state. In addition, future direction of time-resolved XAFS will be discussed.