V. T. Pham

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

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.

Structural analysis of ultrafast extended x-ray absorption fine structure with subpicometer spatial resolution: Application to spin crossover complexes

We present a novel analysis of time-resolved extended x-ray absorption fine structure (EXAFS) spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments. We apply it to the analysis of picosecond EXAFS data on aqueous [Fe(II)(bpy)(3)](2+), which undergoes a light induced conversion from its low-spin (LS) ground state to the short-lived (tau approximately 650 ps) excited high-spin (HS) state. A series of EXAFS spectra were simulated for a collection of possible HS structures from which the ground state fit spectrum was subtracted to generate transient difference absorption (TA) spectra. These are then compared with the experimental TA spectrum using a least-squares statistical analysis to derive the structural change. This approach reduces the number of required parameters by cancellation in the differences. It also delivers a unique solution for both the fractional population and the extracted excited state structure. We thus obtain a value of the Fe-N bond elongation in the HS state with subpicometer precision (0.203+/-0.008 A).

X-ray absorption spectroscopy of ground and excited rhenium-carbonyl-diimine complexes: evidence for a two-center electron transfer.

Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)(3)(bpy)](n+), X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L(3)- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re-C-O scattering paths are dominant due to the so-called focusing effect. For the excited-state spectrum, the Re-X bond undergoes one of the largest changes but still remains a weak contribution to the photoinduced changes of the EXAFS spectrum.

Time-resolved x-ray absorption spectroscopy: Watching atoms dance

The introduction of pump-probe techniques to the field of x-ray absorption spectroscopy (XAS) has allowed the monitoring of both structural and electronic dynamics of disordered systems in the condensed phase with unprecedented accuracy, both in time and in space. We present results on the electronically excited high-spin state structure of an Fe(II) molecular species, [FeII(bpy)3]2+, in aqueous solution, resolving the Fe-N bond distance elongation as 0.2 A. In addition an analysis technique using the reduced χ2 goodness of fit between FEFF EXAFS simulations and the experimental transient absorption signal in energy space has been successfully tested as a function of excited state population and chemical shift, demonstrating its applicability in situations where the fractional excited state population cannot be determined through other measurements. Finally by using a novel ultrafast hard x-ray slicing source the question of how the molecule relaxes after optical excitation has been successfully resolved using femtosecond XANES.

Ultrafast x-ray spectroscopy for structural dynamics studies in chemistry and biology

Time-resolved x-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resolution is a new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). We present key experiments on charge transfer reactions as well as spin-crossover processes in coordination chemistry compounds next to solvation dynamics studies of photogenerated atomic radicals.

Solvation Dynamics Using Ultrafast X-Ray Absorption Spectroscopy

Solvation dynamics aims to incorporate the active role of the solvent in chemical and biochemical reactions and its relevance stems from the fact that most of chemistry and biology takes place in liquids. A full description of the electronic and molecular changes associated with these reactions includes time-resolved structural information, which can be obtained using transient X-ray absorption spectroscopy.

Femtosecond X-Ray Absorption Spectroscopy of a Photoinduced Spin-Crossover Process

We present ultrafast x-ray absorption studies of photoexcited aqueous iron trisbipyridine with 160 fs and with 70 ps temporal resolution to monitor the structural evolution in this spin-crossover complex.

Picosecond and femtosecond X-ray absorption studies of the photoinduced spin change in Fe complexes

We developed a combined imaging and dynamic light scattering (DLS) system for routine measurements in droplets of multi-well plates as well as in gel tubes used for protein crystallization. The system is of high value for rapid identification of good crystallization conditions. Today automated methods to crystallize macromolecules are widely used and can easily generate thousands of crystallization droplets. Nevertheless the evaluation of crystallization experiments to find optimal growth conditions remains a bottleneck. Therefore we have investigated methods to improve the process of evaluating results and finding crystal growth conditions. One method is DLS, the second is the use of combined white/UV illumination for determination of whether crystal-like objects are biomolecular and identification of crystals in crystallisation set-ups. Up to now, it has been impossible to determine the particle size directly in protein solution droplets because of size and configuration constraints. We have developed a CCD camera-based imaging instrument and combined a laser source and a detector to perform DLS measurements in situ. The plate-screening system allows to monitor and evaluate the entire crystallization process in an automated way. For example the stages of nucleation and the progress of crystal growth without disrupting the course of equilibration can be analyzed. The data provide information to understand in greater detail the process of crystal initiation and growth and will allow further optimisation, thereby leading to better crystals. Finally we will also describe a method to support the identification of protein crystals, exploiting the fact that most proteins and other biomolecules fluoresce when illuminated with UV light.