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Data repository for "Disentangling the Evolution of Electrons and Holes in photoexcited ZnO nanoparticles", submitted to Structural Dynamics (#SDY23-AR-XAS2023-00063). Data are divided in 4 numbered folders, organized as follows: Folder "1 - ZnO_PL_scans": This folder contains the raw data of ultrafast photoluminescence measurement. ZnO_full_time.pxp file (Igor Pro by WaveMetrics) is the ultrafast photoluminescence(PL) data obtained upon pumping at 4.66eV at different fluence :0.8, 2.3, 4.7, 5.4 (unit: mJ*cm-2), which are plotted in Figure 1, Figure 2, Figure S3, Figure S4, and Figure S5. time_0312cuts.txt, zno1812corr_wn_233nm.txt, PL_cut_fit2exp.ipynb are the time delay, time trace cut at PL peak, and python code for exponential fitting, respectively, which is plotted in Figure S7. ZnO Raman_rise.pxp contains the Raman data of water in comparison of rising of ZnO PL data, which is plotted in Figure S6. Folder "2 - ZnO_XANES_transient": This folder contains the raw data of Zn K-edge fs-XANES transients data measured in SACLA (2ps), which are plotted in Figure 3 and Figure 4. In the SACLA_APS_comparison_XANES.txt file: the headers "allDiff_N", "allscaler1S3_N", "fluonorm_diff_norm", indicates the transient data measured in APS (100ps), ground-state XAS, and the transient fs-XANES data measured in SACLA(2ps), repectively. Folder "3 - ZnO_XANES_timescan":This folder contains the fs-XANES time scan along with the fitting python code, which is plotted in Figure 5 and Figure S8. Folder "4 - ZnO_MD_Simulation" - This folder contains the result of ab-initio molecular dynamics simulations, which is plotted in Figure 6.

Photochemical reactions in solution are governed by a complex interplay between transient intramolecular electronic and nuclear structural changes and accompanying solvent rearrangements. State-of-the-art time-resolved X-ray solution scattering has emerged in the last decade as a powerful technique to observe solute and solvent motions in real time. However, disentangling solute and solvent dynamics and how they mutually influence each other remains challenging. Here, we simultaneously measure femtosecond X-ray emission and scattering to track both the intramolecular and solvation structural dynamics following photoexcitation of a solvated copper photosensitizer. Quantitative analysis assisted by molecular dynamics simulations reveals a two-step ligand flattening strongly coupled to the solvent reorganization, which conventional optical methods could not discern. First, a ballistic flattening triggers coherent motions of surrounding acetonitrile molecules. In turn, the approach of acetonitrile molecules to the copper atom mediates the decay of intramolecular coherent vibrations and induces a further ligand flattening. These direct structural insights reveal that photoinduced solute and solvent motions can be intimately intertwined, explaining how the key initial steps of light harvesting are affected by the solvent on the atomic time and length scale. Ultimately, this work takes a step forward in understanding the microscopic mechanisms of the bidirectional influence between transient solvent reorganization and photoinduced solute structural dynamics.

We report femtosecond Fe K-edge absorption (XAS) and nonresonant X-ray emission (XES) spectra of ferric cytochrome C (Cyt c) upon excitation of the haem (>300 nm) or mixed excitation of the haem and tryptophan (

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I

Photo-induced oxidation-enhancement in biomimetic bridged Ru(ii)-Mo(vi) photo-catalyst is unexpectedly photo-activated in ps timescales. One-photon absorption generates an excited state where both photo-oxidized and photo-reduced catalytic centres are activated simultaneously and independently.

An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d4-d7 transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays.

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I−(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine–electron recombination model without the need for a long-lived (I0:e−) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine–oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I−) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.

Time‐resolved X‐ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X‐ray transitions with insights from high‐level ab initio calculations. The specific target has been a heteroleptic [IrIII(ppy)2(bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high‐resolution Ir L‐edge X‐ray absorption spectroscopy on the picosecond time scale and restricted active space self‐consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance., The light induced dynamics of an Iridium‐based photosensitizer for hydrogen generation were studied by picosecond‐resolved X‐ray absorption spectroscopy (XAS) during the primary steps of the photocatalytic cycle. We used total fluorescence yield and high‐resolution energy detection (HERFD) XAS spectra to follow the oxidation state of the central metal of an iridium‐based photosensitizer subject to photoinduced electron transfer. The detailed analysis of the main X‐ray transitions from the ground and excited states are guided by high‐level ab initio calculations.

Chemistry - a European journal 27(38), 9905 - 9918 (2021). doi:10.1002/chem.202100766, A new base metal iron-cobalt dyad has been obtained by connection between a heteroleptic tetra-NHC iron(II) photosensitizer combining a 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine with 2,6-bis(3-methyl-imidazol-2-ylidene)-4,4���-bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)-cobalt(III) assembly has been extensively characterized by ground- and excited-state methods like X-ray crystallography, X-ray absorption spectroscopy, (spectro-)electrochemistry, and steady-state and time-resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited-state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal-to-ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the $^{3}$MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a $^{3}$MC state. Attachment of cobaloxime to the iron photosensitizer increases the $^{3}$MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two-component system, consisting of the iron photosensitizer and Co(dmgH)$_2$(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems., Published by Wiley-VCH, Weinheim

The structure-function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe K-alpha and K-beta X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

The photoinduced dynamics of ferric Cytochrome c was investigated by ultrafast non-resonant X-ray emission (XES) and X-Ray Absorption (XAS) spectroscopies, and a cascade through high spin states accompanied by heme doming are observed for the first time.

Femtosecond electronic and nuclear dynamics in [Cu(dmp)2]+ complex upon 550 nm photoexcitation are studied with X-ray emission spectroscopy and X-ray solution scattering, revealing pseudo Jahn-Teller distortion (~410 fs) coupled with coherent vibrational motion.

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+ (where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1 crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2 state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2 state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2 state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1 ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules-in particular, transition metal complexes-that are difficult if not impossible to obtain by other means.

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)

Nature Communications 10(1), 3606 (2019). doi:10.1038/s41467-019-11499-w, Disentangling the strong interplay between electronic and nuclear degrees of freedom is essential to achieve a full understanding of excited state processes during ultrafast nonadiabatic chemical reactions. However, the complexity of multi-dimensional potential energy surfaces means that this remains challenging. The energy flow during vibrational and electronic relaxation processes can be explored with structural sensitivity by probing a nuclear wavepacket using femtosecond time-resolved X-ray Absorption Near Edge Structure (TR-XANES). However, it remains unknown to what level of detail vibrational motions are observable in this X-ray technique. Herein we track the wavepacket dynamics of a prototypical [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ complex using TR-XANES. We demonstrate that sensitivity to individual wavepacket components can be modulated by the probe energy and that the bond length change associated with molecular breathing mode can be tracked with a sub-Angstrom resolution beyond optical-domain observables. Importantly, our results reveal how state-of-the-art TR-XANES provides deeper insights of ultrafast nonadiabatic chemical reactions., Published by Nature Publishing Group UK, [London]

X-ray emission and X-ray Raman scattering spectroscopy are powerful tools to investigate the local electronic and atomic structure of high and low Z elements in situ. Notably, these methods can be applied for in situ spectroscopy at high pressure and high temperature using resistively or laser-heated diamond anvil cells in order to achieve thermodynamic conditions which appear in the Earth's interior. We present a setup for combined X-ray emission and X-ray Raman scattering studies at beamline P01 of PETRA III using a portable wavelength-dispersive von Hamos spectrometer together with the permanently installed multiple-analyzer Johann-type spectrometer. The capabilities of this setup are exemplified by investigating the iron spin crossover of siderite FeCO 3 up to 49.3 GPa by measuring the Fe M 2,3 -edge and the Fe Kβ 1,3 emission line simultaneously. With this setup, the Fe valence-to-core emission can be detected together with the Kβ 1,3 emission line providing complementary information on the sample's electronic structure. By implementing a laser-heating device, we demonstrate the strength of using a von Hamos type spectrometer for spin state mapping at extreme conditions. Finally, we give different examples of low Z elements' absorption edges relevant for application in geoscience that are accessible with the Johann-type XRS spectrometer. With this setup new insights into the spin transition and compression mechanisms of Earth's mantle materials can be obtained of importance for comprehension of the macroscopic physical and chemical properties of the Earth's interior.

Femtosecond spectroscopy and electron diffraction are used to characterize spin crossover in single crystal iron(II)-tris(bipyridine)-bis(hexafluorophosphate). The high-spin lifetime is reduced compared to in solution. Preliminary electron diffraction experiments show evidence of ultrafast Fe-N bond elongation associated with spin crossover and the subsequent molecular reorganization resulting from vibrational cooling.

The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir2(dimen)4]2+, where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute–solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis., Interactions between reactive excited states of molecular photocatalysts and surrounding solvent can dictate reaction pathways, but are not readily accessible to conventional spectroscopic methods. Here the authors use diffuse X-ray scattering and theory to study the atomistic solvation dynamics of a photoexcited di-iridium complex in acetonitrile.

In liquid phase chemistry dynamic solute-solvent interactions often govern the path, ultimate outcome, and efficiency of chemical reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramolecular electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermolecular dynamics of a model chromophore, aqueous [Fe(bpy)3](2+), with complementary X-ray tools in a single experiment exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering analysis. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramolecular structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can extract new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent density increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio molecular dynamics simulations. Structural changes in the aqueous solvent associated with density and temperature changes occur with ∼1 ps time constants, characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute molecules well before the solvent contributions become dominant.

The journal of chemical physics 152(21), 214301 (2020). doi:10.1063/1.5138641 special issue: "ltrafast molecular sciences by femtosecond photons and electrons", Oligonuclear complexes of d4–d7 transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the FeII centers in the low-spin state are independently photoexcited. The Franck–Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)2]2+ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching., Published by American Institute of Physics, Melville, NY

Time-resolved X-ray methods are widely used for monitoring transient intermediates over the course of photochemical reactions. Ultrafast X-ray absorption and emission spectroscopies as well as elastic X-ray scattering deliver detailed electronic and structural information on chemical dynamics in the solution phase. In this work, we describe the opportunities at the Femtosecond X-ray Experiments (FXE) instrument of European XFEL. Guided by the idea of combining spectroscopic and scattering techniques in one experiment, the FXE instrument has completed the initial commissioning phase for most of its components and performed first successful experiments within the baseline capabilities. This is demonstrated by its currently 115 fs (FWHM) temporal resolution to acquire ultrafast X-ray emission spectra by simultaneously recording iron Kα and Kβ lines, next to wide angle X-ray scattering patterns on a photoexcited aqueous solution of [Fe(bpy)3]2+, a transition metal model compound.

The Femtosecond X-ray Experiments scientific instrument is a versatile instrument for ultrafast pump–probe X-ray absorption, emission and scattering experiments at the European XFEL; the instrumentation, commissioning results and operation performance status are presented., The European X-ray Free-Electron Laser (EuXFEL) delivers extremely intense (>1012 photons pulse−1 and up to 27000 pulses s−1), ultrashort (

Heteroleptic iron based complexes bearing the 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine motif and a polypridine ligand have been synthesized and characterized in their ground and excited state. This series of complexes includes a first example of a hetero-bimetallic complex connecting an iron N-heterocyclic carbene photosensitizer with a cobalt dimethylglyoxime fragment. Focus is set on the influence of the linker and cobalt center as second ligand at the iron center on the photophysics. While electronic absorption spectroscopy and cyclic voltammetry reveal a weak mutual influence of the single ligands in the heteroleptic complexes, an increasing MLCT lifetime with larger π-accepting abilities was found by time-resolved transient absorption spectroscopy, with maximum lifetime in the case of the hetero-bimetallic dyad. Concurrently the MC lifetimes were observed to decrease. The reported results will allow to develop guidelines for designing bimetallic devices, which may allow electron transfer from the photosensitizer fragment to a catalytically active center. (Less)

The journal of physical chemistry / C 121(5), 2620 - 2626 (2017). doi:10.1021/acs.jpcc.6b12940, We probe the dynamics of valence electrons in photoexcited [Fe(terpy)$_2$]$^{2+}$ in solution to gain deeper insight into the Fe−ligand bond changes. We use hard X-rayemission spectroscopy (XES), which combines element specificity and high penetration with sensitivity to orbital structure, making it a powerful technique for molecular studies in a wide variety of environments. A picosecond-time-resolved measurement of the complete 1s X-ray emission spectrum captures the transient photoinduced changes and includes the weak valence-to-core (vtc) emission lines that correspond to transitions from occupied valence orbitals to the nascent core-hole. Vtc-XES offers particular insight into the molecular orbitals directly involved in the light-driven dynamics; a change in the metal− ligand orbital overlap results in an intensity reduction and a blue energy shift in agreement with our theoretical calculations and more subtle features at the highest energies reflect changes in the frontier orbital populations., Published by ACS, Washington, DC

We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)3 with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.

We study the structural dynamics of photoexcited [Co(terpy)_{2}]^{2+} in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.

International audience; Finite difference method (FDM) implemented in the FDMNES software [Phys. Rev. B, 2001, 63, 125120] was revised. Thorough analysis shows, that the calculated diagonal in the FDM matrix consists of about 96% zero elements. Thus a sparse solver would be more suitable for the problem instead of traditional Gaussian elimination for the diagonal neighbourhood. We have tried several iterative sparse solvers and the direct one MUMPS solver with METIS ordering turned out to be the best. Compared to the Gaussian solver present method is up to 40 times faster and allows XANES simulations for complex systems already on personal computers. We show applicability of the software for metal-organic [Fe(bpy) 3 ] 2+ complex both for low spin and high spin states populated after laser excitation.

Solvated iron(II)-tris(bipyridine) ([FeII(bpy)3]2+) has been extensively studied with regard to the spin crossover (SCO) phenomenon. Herein, the ultrafast spin transition dynamics of single crystal [FeII(bpy)3](PF6)2 was characterized for the first time using femtosecond transient absorption (TA) spectroscopy. The single crystal environment is of interest for experiments that probe the nuclear motions involved in the SCO transition, such as femtosecond X-ray and electron diffraction. We found that the TA at early times is very similar to what has been reported in solvated [FeII(bpy)3]2+, whereas the later dynamics are perturbed in the crystal environment. The lifetime of the high-spin state is found to be much shorter (100 ps) than in solution due to chemical pressure exerted by the lattice. Oscillatory behavior was observed on both time scales. Our results show that single crystal [FeII(bpy)3](PF6)2 serves as an excellent model system for localized molecular spin transitions.

The technical implementation of a multi-MHz data acquisition scheme for laser–X-ray pump–probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high-repetition rates of X-ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60-bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X-ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12-bit resolution. These traces are then processed to deliver an integrated value for each recorded single X-ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single-shot values are averaged over ∼107 pulses to deliver a mean TFY value with its standard error for each data point,e.g.at a given X-ray probe energy. The sensitivity reaches down to the shot-noise limit, and signal-to-noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse−1and is only technically limited by the utilized APD.

We have performed femtosecond X-ray spectroscopy measurements after UV photoexcitation of a colloidal solution of ZnO nanoparticles. The results indicate sub-ps hole trapping at oxygen vacancies with shallowly-trapped electrons in the conduction band.

Ultrafast X-ray techniques serve as a direct probe of a long-lived pseudo-octahedral high-valent iron(V) complex. Its photo-induced formation from an azido-iron(III) precursor is observed in real-time with sub-picosecond resolution at SACLA XFEL.

We report about the development and implementation of a new setup for time-resolved X-ray absorption fine structure spectroscopy at beamline P11 utilizing the outstanding source properties of the low-emittance PETRA III synchrotron storage ring in Hamburg. Using a high intensitymicrometer-sized X-ray beam in combination with two positional feedback systems, measurements were performed on the transition metal complex fac-Tris[2-phenylpyridinato-C2,N]iridium(III) alsoreferred to as fac-Ir(ppy)$_3$. This compound is a representative of the phosphorescent iridium(III) complexes, which play an important role in organic light emitting diode (OLED) technology. The experiment could directly prove the anticipated photoinduced charge transfer reaction. Our results further reveal that the temporal resolution of the experiment is limited by the PETRA III X-ray bunch length of ∼ 103 ps full width at half maximum (FWHM).

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.