Wojciech Gawelda

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.

Tracking excited-state charge and spin dynamics in iron coordination complexes

Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2′-bipyridine)3]2+, where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2′-bipyridine)3]2+ on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.

Guest–host interactions investigated by time-resolved X-ray spectroscopies and scattering at MHz rates: Solvation dynamics and photoinduced spin transition in aqueous Fe(bipy)3 2+

We have studied the photoinduced low spin (LS) to high spin (HS) conversion of [Fe(bipy)(3)](2+) in aqueous solution. In a laser pump/X-ray probe synchrotron setup permitting simultaneous, time-resolved X-ray diffuse scattering (XDS) and X-ray spectroscopic measurements at a 3.26 MHz repetition rate, we observed the interplay between intramolecular dynamics and the intermolecular caging solvent response with better than 100 ps time resolution. On this time scale, the initial ultrafast spin transition and the associated intramolecular geometric structure changes are long completed, as is the solvent heating due to the initial energy dissipation from the excited HS molecule. Combining information from X-ray emission spectroscopy and scattering, the excitation fraction as well as the temperature and density changes of the solvent can be closely followed on the subnanosecond time scale of the HS lifetime, allowing the detection of an ultrafast change in bulk solvent density. An analysis approach directly utilizing the spectroscopic data in the XDS analysis effectively reduces the number of free parameters, and both combined permit extraction of information about the ultrafast structural dynamics of the caging solvent, in particular, a decrease in the number of water molecules in the first solvation shell is inferred, as predicted by recent theoretical work.

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 resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

Dynamics of plasma formation, relaxation, and topography modification induced by femtosecond laser pulses in crystalline and amorphous dielectrics

We have studied plasma formation and relaxation dynamics along with the corresponding topography modifications in fused silica and sapphire induced by single femtosecond laser pulses (800 nm and 120 fs). These materials, representative of high bandgap amorphous and crystalline dielectrics, respectively, require nonlinear mechanisms to absorb the laser light. The study employed a femtosecond time-resolved microscopy technique that allows obtaining reflectivity and transmission images of the material surface at well-defined temporal delays after the arrival of the pump pulse which excites the dielectric material. The transient evolution of the free-electron plasma formed can be followed by combining the time-resolved optical data with a Drude model to estimate transient electron densities and skin depths. The temporal evolution of the optical properties is very similar in both materials within the first few hundred picoseconds, including the formation of a high reflectivity ring at about 7 ps. In contrast, at longer delays (100 ps–20 ns) the behavior of both materials differs significantly, revealing a longer lasting ablation process in sapphire. Moreover, transient images of sapphire show a concentric ring pattern surrounding the ablation crater, which is not observed in fused silica. We attribute this phenomenon to optical diffraction at a transient elevation of the ejected molten material at the crater border. On the other hand, the final topography of the ablation crater is radically different for each material. While in fused silica a relatively smooth crater with two distinct regimes is observed, sapphire shows much steeper crater walls, surrounded by a weak depression along with cracks in the material surface. These differences are explained in terms of the most relevant thermal and mechanical properties of the material. Despite these differences the maximum crater depth is comparable in both material at the highest fluences used (16 J/cm2). The evolution of the crater depth as a function of fluence can be described taking into account the individual bandgap of each material.

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses.

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Amorphization dynamics of Ge2Sb2Te5 films upon nano- and femtosecond laser pulse irradiation

Summary form only given. The aim of this work is to study the amorphization dynamics upon pulsed laser irradiation (ns and fs) with highest temporal resolution (ns and fs). The pump laser used was a femtosecond-seeded regeneratively amplified laser system operating at 800 nm central wavelength with a pulse duration that could be switched from 120 fs to 8 ns by blocking the seed laser. The reflectivity evolution was measured in real-time with ns resolution by focusing a cw probe laser at 532 nm onto the center of the region irradiated by the pump laser and measuring the reflection with a fast photodiode. The sputter-deposited was a 40 nm thick, crystalline Ge2Sb2Te5 film on a Si wafer that was covered with a 10 nm thick SiO2 layer.

Optimized Finite Difference Method for the Full-Potential XANES Simulations: Application to Molecular Adsorption Geometries in MOFs and Metal–Ligand Intersystem Crossing Transients

Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal-organic frameworks and (ii) light induced dynamics of spin crossover in metal-organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni-CO configuration was found to be linear, while Ni-NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3](2+) with a 100 ps delay we identified the Fe-N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

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).

Plasma formation and structural modification below the visible ablation threshold in fused silica upon femtosecond laser irradiation

We have investigated the temporal and spatial evolution of the ablation process induced in fused silica upon irradiation with single 120fs laser pulses at 800nm. Time-resolved microscopy images of the surface reflectivity at 400nm reveal the existence of a transient plasma distribution with annular shape surrounding the visible ablation crater. The material in this annular zone shows an increased reflectivity after irradiation, consistent with a local refractive index increase of approximately 0.01. White light interferometry measurements indicate a shallow surface depression in this outer region, most likely due to material densification.