Femtosecond X-ray Absorption Spectroscopy at a Hard X-ray Free Electron Laser: Application to Spin Crossover Dynamics

Abstract

X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼10 12 photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved Xray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2′-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs. ■ INTRODUCTIONChemical reactions involve the dynamic evolution of strongly coupled electrons and nuclei on ultrafast time scales from few to a few hundred femtoseconds. Disentangling this complex dynamics has motivated a wide range of experimental and theoretical studies to obtain a more complete picture of the elementary steps underlying these processes. 1 Despite the progresses in ultrafast optical spectroscopies that can resolve dynamics of the valence shell electrons with femtosecond time resolution and in theoretical calculations of many-body dynamics, our understanding of the early chemical transformations still remains incomplete due to the lack of direct structural measurements visualizing both electronic structure and nuclear movements in real time. Information on nuclear dynamics has been obtained with time-resolved scattering with a time resolution of 100 ps on a variety of system from solutions 2,3 to single crystals. 4−6 Recently, femtoseconds studies have been published. 7−9 When the surrounding of specific atoms is of interest, recent advances have demonstrated the utility of time-resolved X-ray absorption spectroscopy (XAS) as a reliable monitor of nuclear and electronic structure during chemical reactions on both the picosecond 10,11 and, more recently, the femtosecond time scale. 12−14 Yet, most of the synchrotron-based experiments are limited to insufficient time resolution (∼100 ps in normal operating conditions) or very low flux (∼10 photons/pulse at kilohertz rate, when operated in femtosecond laser slicing mode 15 ). The development of new X-ray facilities such as X-ray free electron lasers (XFELs) provide unprecedented capabilities in terms of both X-ray pulse durations (down to tens of femtoseconds) and single pulse intensities of more than 10 10 photons when a 1 eV bandwidth monochromator is used. In the present study we exploit XFEL radiation from the Linac Coherent Light Source (LCLS), the first hard XFEL, 16 and present an experimental study of the photoexcited spin transition dynamics in aqueous ferreous bipyridine ([Fe-(bpy) 3 ] 2+ ), which proves the feasibility of femtosecond XAS at the LCLS.Transition metal complexes exhibiting photoinduced spin crossover (SCO) transition provide a good example of the electronic dynamics coupled with nuclear motions. In a

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…The experiment demonstrated the feasibility of ultrafast XAS at X-FELs with sub-100 fs time resolution [284]. Finally, the same Fe-complex was selected to successfully test the feasibility of hard X-ray emission spectroscopy, along with resonant inelastic X-ray scattering using the pump-and-probe scheme [285].…”

X-ray absorption spectroscopy using a self-seeded soft X-ray free-electron laser

“…The ultra-bright femtosecond (fs) x-ray pulses produced at XFELs allow not only the studies of ultrafast dynamics [2][3][4][5][6] and materials in extreme short-lived conditions [7], but also they can probe a sample faster than the onset of x-ray damage [8][9][10][11]. The seeded FERMI FEL at Elettra Synchrotron Trieste, Italy (~12-300 eV) also allows well-controlled experiments based on nonlinear techniques [12] and coherent control [13] mainly in the vacuum ultraviolet (VUV) spectral region.…”

Charge transfer to solvent dynamics in iodide aqueous solution studied at ionization threshold

“…Due to the fact that free electrons do not have discrete energy levels, in principle the emitted photon energy can be set with infinite variability over the range from microwaves to hard X-rays. The first measurements investigating the ultrafast dynamics of a solution, aqueous [Fe(bpy) 3 ] 2+ , have been conducted at an X-ray FEL (XFEL) in 2013 via femtosecond time-resolved X-ray Absorption Near-Edge Spectroscopy (XANES) [149]. Here jitter of the electronic signals for synchronization of the pump laser and the XFEL probe exceeds the necessary requirements for femtosecond resolution.…”