Picosecond X-ray absorption spectroscopy:application to coordination chemistry compounds in solution

The photocycle of aqueous ruthenium-(trisbipyridine) [Ru(bpy)3]2+ was studied under high laser excitation intensities and high sample concentrations with picosecond resolved x-ray absorption spectroscopy. In a pump-probe scheme a femtosecond laser pulse promotes a 4d electron from the ruthenium to the ligand orbitals, thus creating a metal-to-ligand-charge-transfer (MLCT) complex. A hard x-ray pulse from a synchrotron source probes the ruthenium L3 and L2 edges, monitoring the electronic and molecular structure of the ruthenium over the photocycle. The measured x-ray absorption spectrum of the MLCT state is in good agreement with the predictions of a theoretical calculation (TT-multiplet software). We extract from the spectrum that the excited-state complex can be described by D3 symmetry and has a 4d5 configuration. The decay kinetics of the MLCT state are found to be strongly dependent on the sample concentration, especially for solutions near the solubility limit of [Ru(bpy)3]Cl2 in water. Besides ground-state quenching and triplet-triplet annihilation a third fast decay component quenches the life-time of the MLCT state, tentatively attributed to a cluster effect. This study is the first application of sub-nanosecond time-resolved x-ray absorption spectroscopy on solvated systems and demonstrates its capability as a new tool for the observation of chemical dynamics in solvated systems

Towards structural dynamics in condensed chemical systems exploiting ultrafast time-resolved x-ray absorption spectroscopy

The authors present the case for exploiting time-resolved x-ray absorption to study structural dynamics in the liq. phase. With this aim in mind and considering the large differences between absorption coeffs. in the optical and the x-ray domains as well as the x-ray absorption cross sections due to unexcited species, the authors have estd. the anticipated signal-to-noise ratio (S/N) under realistic conditions with femtosecond laser pump pulses and synchrotron radiation x-ray probe pulses. As a model system, the authors examine I- photodetachment in H2O and detect the appearance of laser-generated neutral I atoms by their x-ray near-edge absorption structure (XANES) and by their extended x-ray absorption fine structure (EXAFS). While the S/N ratio critically depends on the photolysis yield, which itself is governed by the optical absorption cross section, the optimum sample concn. varies in a complex fashion as a function of pump laser intensity and optical absorption cross section. However, concns. yielding near total absorption of the pump laser deliver quite optimum S/N ratios. The calcns. presented here provide guidelines for the implementation of time-resolved x-ray absorption expts. in condensed phase chem. systems. [on SciFinder (R)

Observing Photochemical Transients by Ultrafast X-Ray Absorption Spectroscopy

Accurate detn. of the transient electronic structures, which drive photochem. reactions, is crucial in chem. and biol. The authors report the detection of transient chem. changes on the picosecond time scale by x-ray-absorption near-edge structure of photoexcited aq. [Ru(bpy)3]2+. Upon ultrashort laser pulse excitation a charge transfer excited state having a 300 ns lifetime is formed. The authors detect the change of oxidn. state of the central Ru atom at its L3 and L2 edges, at a temporal resoln. of 100 ps with the zero of time unambiguously detd. [on SciFinder (R)

A setup for ultrafast time-resolved x-ray absorption spectroscopy

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resoln. on liq. samples at the Advanced Light Source at Lawrence Berkeley National Labs. The temporal resoln. is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the expt. and present a single-pulse detection scheme. [on SciFinder (R)