Development of an experimental apparatus to observe ultrafast phenomena by tender X-ray absorption spectroscopy at PAL-XFEL.

Understanding the ultrafast dynamics of molecules is of fundamental importance. Time-resolved X-ray absorption spectroscopy (TR-XAS) is a powerful spectroscopic technique for unveiling the time-dependent structural and electronic information of molecules that has been widely applied in various fields. Herein, the design and technical achievement of a newly developed experimental apparatus for TR-XAS measurements in the tender X-ray range with X-ray free-electron lasers (XFELs) at the Pohang Accelerator Laboratory XFEL (PAL-XFEL) are described. Femtosecond TR-XAS measurements were conducted at the Ru L3-edge of well known photosensitizer tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) in water. The results indicate ultrafast photoinduced electron transfer from the Ru center to the ligand, which demonstrates that the newly designed setup is applicable for monitoring ultrafast reactions in the femtosecond domain.

UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge.

We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH2S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

Synthesis of Ultra-Small Palladium Nanoparticles Deposited on CdS Nanorods by Pulsed Laser Ablation in Liquid: Role of Metal Nanocrystal Size in the Photocatalytic Hydrogen Production.

It is imperative to suppress the rate of recombination of photogenerated carriers to improve the semiconductor-catalyzed solar-driven production of hydrogen. To this end, photocatalysts comprising active sunlight-harvesting photo-absorbers and stable metal co-catalysts have attracted significant attention. However, the size, clean surface, and highly dispersed nature of the metal co-catalysts are crucial factors affecting catalyst performance and reaction rate. Nevertheless, most of the available metal nanocrystals have been synthesized by complex procedures using harmful organic templates and stabilizers, affording high-purity compounds with difficulty and high cost. To overcome these problems, in this study, the pulsed laser ablation in liquid approach was utilized to generate palladium and bimetallic palladium-platinum nanoparticles with an average size and distribution by adjusting the laser wavelength and fluence. A high rate of evolution of hydrogen of 130.33 mmol g-1 h-1 was obtained by using the optimized CdS-PdPt catalyst under simulated sunlight irradiation. This value is 51.31 times greater than that observed for bare CdS nanostructures. Furthermore, the amount of hydrogen evolved was significantly better than that obtained by using several other noble-metal co-catalysts deposited on CdS. This proposed strategy is thought to open new avenues for the design of advanced photocatalytic materials for efficient solar-driven production of hydrogen.

Dynamic three-dimensional structures of a metal-organic framework captured with femtosecond serial crystallography.

Crystalline systems consisting of small-molecule building blocks have emerged as promising materials with diverse applications. It is of great importance to characterize not only their static structures but also the conversion of their structures in response to external stimuli. Femtosecond time-resolved crystallography has the potential to probe the real-time dynamics of structural transitions, but, thus far, this has not been realized for chemical reactions in non-biological crystals. In this study, we applied time-resolved serial femtosecond crystallography (TR-SFX), a powerful technique for visualizing protein structural dynamics, to a metal-organic framework, consisting of Fe porphyrins and hexazirconium nodes, and elucidated its structural dynamics. The time-resolved electron density maps derived from the TR-SFX data unveil trifurcating structural pathways: coherent oscillatory movements of Zr and Fe atoms, a transient structure with the Fe porphyrins and Zr6 nodes undergoing doming and disordering movements, respectively, and a vibrationally hot structure with isotropic structural disorder. These findings demonstrate the feasibility of using TR-SFX to study chemical systems.

Observing ice structure of micron-sized vapor-deposited ice with an x-ray free-electron laser.

The direct observation of the structure of micrometer-sized vapor-deposited ice is performed at Pohang Accelerator Laboratory x-ray free electron laser (PAL-XFEL). The formation of micrometer-sized ice crystals and their structure is important in various fields, including atmospheric science, cryobiology, and astrophysics, but understanding the structure of micrometer-sized ice crystals remains challenging due to the lack of direct observation. Using intense x-ray diffraction from PAL-XFEL, we could observe the structure of micrometer-sized vapor-deposited ice below 150 K with a thickness of 2-50 µm grown in an ultrahigh vacuum chamber. The structure of the ice grown comprises cubic and hexagonal sequences that are randomly arranged to produce a stacking-disordered ice. We observed that ice with a high cubicity of more than 80% was transformed to partially oriented hexagonal ice when the thickness of the ice deposition grew beyond 5 µm. This suggests that precise temperature control and clean deposition conditions allow µm-thick ice films with high cubicity to be grown on hydrophilic Si3N4 membranes. The low influence of impurities could enable in situ diffraction experiments of ice nucleation and growth from interfacial layers to bulk ice.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy.

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)3]2+ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L3-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (3MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the 3MC state. A quarter of the 3MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge.

We demonstrate for the case of photoexcited [Ru(2,2'-bipyridine)3]2+ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal-ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.

Shot noise limited soft x-ray absorption spectroscopy in solution at a SASE-FEL using a transmission grating beam splitter.

X-ray absorption near-edge structure (XANES) spectroscopy provides element specificity and is a powerful experimental method to probe local unoccupied electronic structures. In the soft x-ray regime, it is especially well suited for the study of 3d-metals and light elements such as nitrogen. Recent developments in vacuum-compatible liquid flat jets have facilitated soft x-ray transmission spectroscopy on molecules in solution, providing information on valence charge distributions of heteroatoms and metal centers. Here, we demonstrate XANES spectroscopy of molecules in solution at the nitrogen K-edge, performed at FLASH, the Free-Electron Laser (FEL) in Hamburg. A split-beam referencing scheme optimally characterizes the strong shot-to-shot fluctuations intrinsic to the process of self-amplified spontaneous emission on which most FELs are based. Due to this normalization, a sensitivity of 1% relative transmission change is achieved, limited by fundamental photon shot noise. The effective FEL bandwidth is increased by streaking the electron energy over the FEL pulse train to measure a wider spectral window without changing FEL parameters. We propose modifications to the experimental setup with the potential of improving the instrument sensitivity by two orders of magnitude, thereby exploiting the high peak fluence of FELs to enable unprecedented sensitivity for femtosecond XANES spectroscopy on liquids in the soft x-ray spectral region.

Heterostructured WS2 -MoS2 Ultrathin Nanosheets Integrated on CdS Nanorods to Promote Charge Separation and Migration and Improve Solar-Driven Photocatalytic Hydrogen Evolution.

Solar-driven photocatalytic hydrogen evolution is important to bring solar-energy-to-fuel energy-conversion processes to reality. However, there is a lack of highly efficient, stable, and non-precious photocatalysts, and catalysts not designed completely with expensive noble metals have remained elusive, which hampers their large-scale industrial application. Herein, for the first time, a highly efficient and stable noble-metal-free CdS/WS2 -MoS2 nanocomposite was designed through a facile hydrothermal approach. When assessed as a photocatalyst for water splitting, the CdS/WS2 -MoS2 nanostructures exhibited remarkable photocatalytic hydrogen-evolution performance and impressive durability. An excellent hydrogen evolution rate of 209.79 mmol g-1 h-1 was achieved under simulated sunlight irradiation, which is higher than the values for CdS/MoS2 (123.31 mmol g-1 h-1 ) and CdS/WS2 nanostructures (169.82 mmol g-1 h-1 ) and the expensive CdS/Pt benchmark catalyst (34.98 mmol g-1 h-1 ). The apparent quantum yield reached 51.4 % at λ=425 nm in 5 h. Furthermore, the obtained hydrogen evolution rate was better than those of several noble-metal-free catalysts reported previously. The observed high rate of hydrogen evolution and remarkable stability may be a result of the ultrafast separation of photogenerated charge carriers and transport between the CdS nanorods and the WS2 -MoS2 nanosheets, which thus increases the number of electrons involved in hydrogen production. The proposed designed strategy is believed to potentially open a door to the design of advanced noble-metal-free photocatalytic materials for efficient solar-driven hydrogen production.

Hierarchical BiOI nanostructures supported on a metal organic framework as efficient photocatalysts for degradation of organic pollutants in water.

Semiconductor-based photocatalysis is a green method for the removal of toxic organic pollutants by decomposition into harmless products. However, traditional single-component semiconductors are unable to reach high degradation efficiencies due to excessive photo charge carrier recombination. The use of hybrid nanocomposite photocatalysts is a promising strategy for overcoming this problem by reducing recombination as well as ensuring that large amounts of solar energy are harvested. Herein, a novel visible-light-active hybrid nanocomposite, BiOI/MIL-88B(Fe), was successfully synthesized through a simple precipitation method. In the BiOI/MIL-88B(Fe) composite, both BiOI and MIL-88B(Fe) have improved charge carrier separation and reduced recombination via a simple Z-scheme mechanism. Photocatalytic degradation of the pollutant RhB was carried out during irradiation of the as-synthesized composites with simulated solar light, and the BiOI/MIL-88B(Fe) (2 wt%) composite was found to exhibit the highest photocatalytic activity among the composites. In addition, colorless phenol and ciprofloxacin (CIP) degradation experiments were also performed to confirm the visible light photocatalytic performance of the BiOI/MIL-88B(Fe) hybrid nanocomposite. Scavenger experiments, PL analysis, NBT transformations, and TA-PL experiments all supported the proposed Z-scheme mechanism of the BiOI/MIL-88B(Fe) composite photocatalyst. Moreover, simple separation from solution provides this 3D composite with good reusability and long-term stability.

Synthesis and detailed spectroscopic characterization of various hydroxy-functionalized fluorescent chalcones: A combined experimental and theoretical study.

Four different bright yellow to orange hydroxy-substituted chalcones (i.e., 2',4-di-hydroxy (1), 2',3',4-trihydroxy (2), 2',3',4'-trihydroxy (3), and 2'-hydroxy-4-methoxy (4) chalcones) were synthesized and characterized by LC-MS, FT-IR, FT-Raman, and fluorescence spectroscopy and thermogravimetric analysis. UV-visible absorption spectroscopy was also used. The experimental (theoretical) bandgaps of 1, 2, 3, and 4 are 2.89 (2.90), 2.93 (2.95), 3.04 (3.09), and 3.01 (2.91) eV, respectively. The hydroxy-substituted chalcones exhibited strong dual emissions as a consequence of the locally excited states followed by internal charge transfer processes. The molecular structures, lowest energy transitions, vibrational frequencies, and spectroscopic information were calculated using density functional theory and time-dependent density functional theory methods at the B3LYP/6-31G(d,p) theoretical level. The experimental and theoretical data were compared and the relationship between them was briefly discussed.