X-ray absorption spectroscopy is a powerful probe of molecular structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aqueous iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 femtoseconds. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-femtosecond time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT-->3MLCT-->5T cascade from the initially excited state. The time scale of the 3MLCT-->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

Brown algae of the Laminariales (kelps) are the strongest accumulators of iodine among living organisms. They represent a major pump in the global biogeochemical cycle of iodine and, in particular, the major source of iodocarbons in the coastal atmosphere. Nevertheless, the chemical state and biological significance of accumulated iodine have remained unknown to this date. Using x-ray absorption spectroscopy, we show that the accumulated form is iodide, which readily scavenges a variety of reactive oxygen species (ROS). We propose here that its biological role is that of an inorganic antioxidant, the first to be described in a living system. Upon oxidative stress, iodide is effluxed. On the thallus surface and in the apoplast, iodide detoxifies both aqueous oxidants and ozone, the latter resulting in the release of high levels of molecular iodine and the consequent formation of hygroscopic iodine oxides leading to particles, which are precursors to cloud condensation nuclei. In a complementary set of experiments using a heterologous system, iodide was found to effectively scavenge ROS in human blood cells.algae ͉ Laminaria ͉ x-ray absorption spectroscopy ͉ cathodic stripping square wave voltammetry

Structural changes of the iron(II)-tris-bipyridine ( Fe II bpy 3 2 ) complex induced by ultrashort pulse excitation and population of its short-lived ( 0:6 ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined analysis of the x-rayabsorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 Å in the quintet state compared to the singlet ground state.

Synchrotron-based X-ray Tomographic Microscopy (SRXTM) is nowadays a powerful technique for non-destructive, high-resolution investigations of a broad kind of materials. High-brilliance and high-coherence third generation synchrotron radiation facilities allow micrometer and sub-micrometer, quantitative, three-dimensional imaging within very short time and extend the traditional absorption imaging technique to edge-enhanced and phase-sensitive measurements. At the Swiss Light Source TOMCAT, a new beamline for TOmographic Microscopy and Coherent rAdiology experimenTs, has been recently built and started regular user operation in June 2006. The new beamline get photons from a 2.9 T superbend with a critical energy of 11.1 keV. This makes energies above 20 keV easily accessible. To guarantee the best beam quality (stability and homogeneity), the number of optical elements has been kept to a minimum. A Double Crystal Multilayer Monochromator (DCMM) covers an energy range between 8 and 45 keV with a bandwidth of a few percent down to 10 −4 . The beamline can also be operated in white-beam mode, providing the ideal conditions for real-time coherent radiology. This article presents the beamline design, its optical components and the endstation. It further illustrates two recently developed phase contrast techniques and finally gives an overview of recent research topics which make intense use of SRXTM.

L(2,3) X-ray absorption spectra of aqueous [Ru(II)(bpy)3]2+ have been recorded in its ground and excited states, 50 ps after short pulse laser excitation. Significant changes in both the XANES (X-ray Near-Edge Absorption Structure) and the EXAFS (Extended X-ray Absorption Fine Structure) regions of the excited state complex are detected. The XANES line shapes have been quantitatively simulated using a crystal field multiplet code in trigonal symmetry. In addition, spectral changes in the EXAFS region of both ground and excited states are analyzed in order to extract structural parameters of their corresponding molecular structures. We obtain a Ru-N bond contraction by approximately 0.03 angstroms in the excited-state complex, as compared to the ground-state compound. This contraction results from electrostatic and polarization contributions, limited by steric constraints on the bpy ligands.

Accurate determination of the transient electronic structures, which drive photochemical reactions, is crucial in chemistry and biology. We report the detection of transient chemical changes on the picosecond time scale by x-ray-absorption near-edge structure of photoexcited aqueous [Ru(bpy)(3)](2+). Upon ultrashort laser pulse excitation a charge transfer excited state having a 300 ns lifetime is formed. We detect the change of oxidation state of the central Ru atom at its L3 and L2 edges, at a temporal resolution of 100 ps with the zero of time unambiguously determined.

We present a static and picosecond X-ray absorption study at the Cu K-edge of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) ([Cu(dmp)2](+); dmp = 2,9-dimethyl-1,10-phenanthroline) dissolved in acetonitrile and dichloromethane. The steady-state photoluminescence spectra in dichloromethane and acetonitrile are also presented and show a shift to longer wavelengths for the latter, which points to a stronger stabilization of the excited complex. The fine structure features of the static and transient X-ray spectra allow an unambiguous assignment of the electronic and geometric structure of the molecule in both its ground and excited (3)MLCT states. Importantly, the transient spectra are remarkably similar for both solvents, and the spectral changes can be rationalized using the optimized ground- and excited-state structures of the complex. The proposed assignment of the lifetime shortening of the excited state in donor solvents (acetonitrile) to a metal-centered exciplex is not corroborated here. Molecular dynamics simulations confirm the lack of complexation; however, in both solvents the molecules come close to the metal but undergo rapid exchange with the bulk. The shortening of the lifetime of the title complex and nine additional related complexes can be rationalized by the decrease in the (3)MLCT energy. Deviations from this trend may be explained by means of the effects of the dihedral angle between the ligand planes, the solvent, and the (3)MLCT-(1)MLCT energy gap.

We report the implementation of a method which can yield the 3D distribution of the phase (refractive index) of a weakly absorbing object from a single tomographic data set. In order to reduce the residual absorption artifact (due to the fact that only one data set is used) the original algorithm presented by A. V. Bronnikov is amended by adding in the filter a factor found by using a semi empirical approach. The quality of the reconstruction is largely sufficient for optimal segmentation and further postprocessing even though the filter correction is based on assumption of constant absorption. This one step approach allows keeping radiation dose to the minimum. Spatial resolution is comparable to the conventional absorption based technique. The performance of the method is validated by using an established phase contrast technique.