Ru-Pan Wang

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L ₃ -edge was used to identify metal-centered (MC) character in the S ₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S ₁ state, but these probes lack a clear signature of the S ₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes, but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L ₃ -edge. Comparison of the L ₃ -edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S ₁ state is primarily a MC excited state. The sub-20 µOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally-occurring and synthetic transition metal complexes.

Time-resolved X-ray absorption spectroscopy (TRXAS) at the Co L ₃ -edge was used to identify metal-centered (MC) character in the S ₁ excited state of cyanocobalamin (CNCbl). Cobalamins have UV/visible spectra that are dominated by intense corrin-based excitations, but these ligand-centered states energetically overlap with charge transfer and MC excited states that may be populated following photoexcitation. Ultrafast optical and hard X-ray spectroscopy have shown that CNCbl forms a structurally distorted S ₁ state, but these probes lack a clear signature of the S ₁ electronic identity, which theory has suggested is a ligand-to-metal charge transfer (LMCT) state. Femtosecond soft X-ray TRXAS offers greater state-selectivity than many optical or hard X-ray probes, but has, so far, been limited to highly concentrated (≥100 mM) samples. A new experimental setup at the European X-ray Free Electron Laser (EuXFEL) that enables studies of sub-10 mM samples and provides ∼100 fs time-resolution is used to measure the TRXAS of CNCbl at the Co L ₃ -edge. Comparison of the L ₃ -edge XAS spectrum measured at 0.8 ps with ligand field multiplet simulations indicates that the S ₁ state is primarily a MC excited state. The sub-20 µOD detection sensitivity achieved in this study demonstrates the possibility of applying this method to a wide range of naturally-occurring and synthetic transition metal complexes.

Transient X-ray absorption spectroscopy at the sulfur K edge probes the ultrafast internal conversion and intersystem crossing of thiopyridone isomers in solution.

Photoexcited states of tungsten trioxide (WO₃) were observed using femtosecond high-energy-resolution fluorescence detection X-ray Absorption Spectra (HERFD-XAS) and resonant X-ray emission spectra (RXES). In the initial state of the photoexcitation (∼100 fs), the W L₃ edge XAS shifts to lower energy and the energy levels of the t_2g and e_g peaks are modulated due to the photoexcited electrons in the conduction band. The electronic state of the photoexcited W atoms is modified by 500 fs. The crystal field splitting between the t_2g and e_g peaks shrinks by 500 fs, which indicates local structural changes around the W atoms due to the formation of polarons. HERFD-XAS and RXES provide more details about the early state of the photoexcited states of WO₃. This work demonstrates that the detailed dynamics of 5d elements in the femtosecond range can be addressed with HERFD-XAS/RXES.

We have investigated the photochemistry of dimethyl disulfide (DMDS) and glutathione disulfide (GSSG) in solution upon ultraviolet (UV) excitation in comparison to L‐cystine by femtosecond X‐ray absorption spectroscopy at the sulfur K‐edge. For these aliphatic disulfides, the very similar differential absorption spectra reveal unequivocally a pair of geminate thiyl radicals as the single primary reaction product with a formation time constant of about 100 fs. We find size‐dependent majority yields for disulfide recombination with DMDS reforming ten‐fold slower and at lower quantum yield than L‐cystine while GSSG recombines even faster than L‐cystine at a yield of nearly 80 % within 2 ps after excitation. We attribute the much lower recombination rates and yields of DMDS to larger product separation after homolytic S−S bond cleavage due to the smaller thiyl radical mass and correspondingly larger mutual acceleration. Furthermore, we observe the delayed formation of at least one additional reaction product for these disulfides that we attribute to perthiyl radicals. The ultrafast recombination of L‐cystine and GSSG at majority yield in condensed phase suggests a dynamic S−S bond integrity which may be exploited in natural systems to mitigate UV radiation damage, and which could be of interest in materials design.

Time-resolved absorption spectroscopy and magnetic circular dichroism with circularly polarized soft x-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. By employing these methods, groundbreaking results have been obtained, for instance, for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free-electron laser facility FLASH, key capabilities for ultrafast XAS and XMCD experiments have recently improved. In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft x-ray pulses with a duration of a few tens of femtoseconds covering the L3,2-edges of the important 3d transition metal elements with pulse energies of several μJ. Here, we present first experimental results with such ultrashort x-ray pulses from the FL23 beamline employing XMCD at the L3,2-edges of the 3d metals, Co, Fe, and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump-laser setup, FLASH can offer important and efficient experimental instrumentation for ultrafast demagnetization studies and other investigations of ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.

The use of advanced x-ray sources plays a key role in the study of dynamic processes in magnetically ordered materials. The progress in x-ray free-electron lasers enables the direct and simultaneous observation of the femtosecond evolution of electron and spin systems through transient x-ray absorption spectroscopy and x-ray magnetic circular dichroism, respectively. Such experiments allow us to resolve the response seen in the population of the spin-split valence states upon optical excitation. Here, we utilize circularly polarized ultrashort soft x-ray pulses from the new helical afterburner undulator at the free-electron laser FLASH in Hamburg to study the femtosecond dynamics of a laser-excited CoPt alloy at the Co L3-edge absorption. Despite employing a weaker electronic excitation level, we find a comparable demagnetization for the Co 3d-states in CoPt compared to previous measurements on CoPd. This is attributed to the distinctly different spin–orbit coupling between 3d and 4d vs 3d and 5d elements in the corresponding alloys and multilayers.

We report a combined experimental and theoretical investigation of the ultrafast internal conversion (IC) and intersystem crossing (ISC) dynamics of two thiopyridone isomers in solution. Our study used sulfur K-edge ultrafast transient x-ray absorption spectroscopy (XAS), in conjunction with electronic excited state surface hopping molecular dynamics and simulations of the excited state XAS, to investigate the impact of the functional group substitution pattern and solvent on the dynamics of IC and ISC. The combination of the localized x-ray probe and the simulation results enables, in part, the differentiation between ππ∗ and nπ∗ character excited states, as well as singlet and triplet states. Access to nπ∗ character excitations has particular value since they often prove challenging to assess with optical spectroscopy. The results point to the key role of these nπ∗ intermediates in mediating the intersystem crossing of these systems. Varying the substitution pattern of the molecule can stabilize or destabilize these intermediates leading to an increase in the rate of ISC in the ortho isomer as compared to the para isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.

To understand the electronic-structure change of LiCoO ₂ during charge–discharge, we performed ex situ resonant soft X-ray emission spectroscopy of the Co L ₃ edge in combination with charge-transfer multiplet calculations.