A zone-plate-based two-color spectrometer for indirect X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La_0.6Sr_0.4MnO₃ by analysis of three emission lines simultaneously at the detector, namely the O 2p-1s, Mn 3s-2p and Mn 3d-2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s-2p and Mn 3d-2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.

Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering

Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time-resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.

Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)₂(CN)₂], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)₂(CN)₂] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)₂(CN)₂] complement prior measurement performed on [Fe(bpy)₃]²⁺ and [Fe(bpy)(CN)₄]^2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)_N(CN)_6-2N]^2N-4, where N = 1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

Optical and X-ray free-electron laser measurements reveal ligand substitution in an Fe(ii)-centered complex extends its MLCT lifetime.

Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover – the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN^–) ligands and one 2,2′-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)₄(bpy)]^2–. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)₄(bpy)]^2– decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2′-bipyridine)₃]²⁺ by more than two orders of magnitude.

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.

Resonant and near-threshold photoionization cross sections of Fe14+

Photoionization (PI) of Fe14+ in the range from 450 to 1100 eV was measured at the BESSY II storage ring using an electron beam ion trap achieving high target-ion area densities of 10(10)  cm(-2). Photoabsorption by this ion is observed in astrophysical spectra and plasmas, but until now cross sections and resonance energies could only be provided by calculations. We reach a resolving power E/ΔE of at least 6500, outstanding in the present energy range, which enables benchmarking and improving the most advanced theories for PI of ions in high charge states.