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February 19, 2004

 

 

Ultrafast XANES Technique Reveals Photochemical Transients

Ultrafast, time-resolved x-ray experiments are on the frontier of synchrotron radiation science with several demonstrations of white-light (Laue) x-ray diffraction to track structural changes on the picosecond time scale. Comparable experiments with x-ray absorption are more challenging because data must be recorded at each photon energy over the spectral range of interest. Transient, photochemical intermediate states in solution pose the additional difficulty of a solvent whose effects must be accounted for. A team of researchers from the University of Lausanne, the Swiss Light Source, the University of California, Berkeley, and the ALS have paved the way for experiments of this type by using x-ray absorption near-edge structure spectroscopy (XANES, also known as NEXAFS) to detect the change in oxidation state of the central ruthenium atom in a laser-excited ruthenium complex in solution.

Charge-transfer processes during photochemical reactions occur in a large class of systems from small molecules to large metalloproteins. One thinks of photosynthesis in plants and vision in animals as prime examples in the natural world, but industrial instances abound as well. In principle, XANES is the ideal tool for studying the electrical structure of laser-excited transient states. Similarly, extended x-ray absorption fine structure spectroscopy (EXAFS) would be the choice for elucidating the changes in geometrical structure in these noncrystalline systems. Synchrotron sources are fast and intense enough to probe transients living 100 picoseconds or longer. But there have been no reports of synchrotron-based, time-resolved x-ray absorption spectroscopy of transient, photochemical intermediate states with subnanosecond time resolution.

 

chemical reaction

Photochemical reaction cycle of [RuII(bpy)3]2+.

To record these short-lived states, the Swiss­Berkeley team used an experimental setup sensitive enough to record the weak signals, reduce background to the shot-noise level of the pulsed x-ray source, and minimize interference from the solvent. For their pump-probe measurements, the investigators chose ruthenium (II) tris-2,2'-bipyridine [RuII(bpy)3]2+, a model transition metal complex for studies of ultrafast electron-transfer processes, dissolved in a free-flowing jet of water. As the probe, the team used x rays from ALS Beamline 5.3.1 with the ALS operating in its "camshaft" mode with a single, bright pulse of x rays in the midst of a 100-ns-long dark interval. An amplified, frequency-doubled titanium-sapphire femtosecond laser, synchronized with the ALS, served as the pump.

Faster Than the Blink of an Eye


Without chemical reactions induced by light (photochemistry), plants would not be able to convert sunlight via photosynthesis into energy for growth, thus also depriving the animal world of its food supply. Even if there were food to gather, we wouldn't be able to see it because our vision depends on photochemical reactions initiated when light strikes the eye's retina. But scientists motivated to study these and a host of other natural and industrial photochemical reactions face a problem. The reactions often comprise a sequence of events that follow each other extremely rapidly, less than a trillionth of a second in some steps of the vision process. Ultrafast infrared, visible, and ultraviolet lasers have been widely and successfully applied to the problem, but they are not a universal solution. X-ray spectroscopy can provide essential information not accessible via these lasers, but until now, the combination of high-speed sources that also emit x rays over a continuous range of wavelengths (necessary for spectroscopy) and experimental setups optimized for the low signals generated have not been available. Working at the ALS, Saes et al. have recorded x-ray absorption spectra of short-lived photochemical structures with a time resolution of about 100 trillionths of a second. While this is still much slower than the fastest lasers, their experiments constitute a proof of principle that the era of ultrafast x-ray spectroscopy has now arrived.


experiment setup

A titanium-sapphire femtosecond laser served as the pump for the [RuII(bpy)3]2+ pump-probe measurements.

Laser excitation creates a ruthenium (III) singlet state that decays in 100 fs to a triplet state, 3[RuIII(bpy­)(bpy)2]2+, which has a lifetime of 300 ns and is the transient whose electronic structure was studied by XANES. As a baseline for the ruthenium (II) and ruthenium (III) states, respectively, the researchers measured the static XANES spectrum of the unexcited complex and added from the literature a spectrum for [RuIII(NH3)6Cl2]. They then made the transient measurements in such a way that only the change in absorption (the transient part) was recorded. They determined the temporal resolution of their measurements to be 100 picoseconds, governed by the x-ray pulse width, from the temporal evolution of the absorption at a particular wavelength as the delay between the exciting laser pulse and the probe x-ray pulse increased.

step function

Temporal evolution of the transient absorption is a step function, whose shape is governed by the cross correlation function between the exciting fs laser and the probing ps x-ray pulses. The derivative is Gaussian with a FWHM around 70-80 ps, which corresponds to the electron bunch length of the x-ray pulse (lower blue). With this precise timing measurement, one can then set a fixed time delay between the laser and the x-ray pulses with better than 10 ps accuracy (the arrow indicates a delay of 50 ps).

Two independent approaches to analyzing the transient absorption agreed that the basic features of the transient ruthenium (III) state are a 1.2-eV shift to higher energy of a pre-edge feature at 2841 eV due to a 2p3/2 → 4d3/2(eg) transition and the onset of a new absorption due to a 2p3/2→ 4d5/2(eg) transition. Both of these features, present in the static spectra, were already known, but this is the first time that such details have been extracted from transient XANES. The experimenters believe that the same approach can be applied to EXAFS measurement of local structural changes.

spectra

Top: Transient x-ray absorption spectrum of photoexcited aqueous [RuII(bpy)3]2+ measured 50 ps after laser excitation (red data points) with a fitted curve (solid blue) that accounts for the blue shift of the static absorption after photoexcitation. The peak near T is a new feature that occurs in the photoexcited intermediate. Bottom: Static L3-edge x-ray absorption spectrum of ground state [RuII(bpy)3]2+ (solid black) measured 0.5 ms before the laser strikes the sample, and excited-state absorption spectrum (red dots) generated from the transient data curve T in the upper panel. The blue shift B→B' and the new peak A' are evident.

 

Research conducted by M. Saes (Université de Lausanne and Swiss Light Source, Switzerland); C. Bressler and M. Chergui (Université de Lausanne); R. Abela and D. Grolimund (Swiss Light Source); S.L. Johnson (University of California, Berkeley); and P.A. Heimann (ALS).

Research funding: Swiss National Science Foundation, Swiss Light Source, and U.S. Department of Energy, Office of Basic Energy Science (BES). Operation of the ALS is supported by BES.

Publication about this research: M. Saes, C. Bressler, R. Abela, D. Grolimund, S.L. Johnson, P.A. Heimann, and M. Chergui, "Observing photochemical transients by ultrafast x-ray absorption spectroscopy," Phys. Rev. Lett. 90, 047403 (2003).

 

ALSNews Vol. 227, August 6, 2003

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