Grigory Smolentsev

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV-9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.

X-ray Emission Spectroscopy to Study Ligand Valence Orbitals in Mn Coordination Complexes

We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H(2)O)(6)](2+), [Mn(H(2)O)(5)OH](+), and [Mn(H(2)O)(5)NH(3)](2+). An application of the method, with comparison between theory and experiment, is presented for the solvated Mn(2+) ion in water and three Mn coordination complexes, namely [LMn(acac)N(3)]BPh(4), [LMn(B(2)O(3)Ph(2))(ClO(4))], and [LMn(acac)N]BPh(4), where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B(2)O(3)Ph(2) represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.

Quantitative local structure refinement from XANES: multi-dimensional interpolation approach.

A new method to determine local structure in terms of a few structural parameters is proposed and realised in FitIt software. It is based on fitting of X-ray absorption near-edge structure (XANES) spectra using the combination of full multiple-scattering calculations, and multi-dimensional interpolation of spectra as a function of structural parameters. The procedure is divided into two steps: the construction of an interpolation polynomial, and the fitting of experimental spectra using the interpolation polynomial. During the construction of the polynomial, multiple-scattering calculations for certain sets of structural parameters are needed. The strategy for the selection of the most important expansion terms and corresponding sets of structural parameters is proposed. Fitting of the spectrum using multi-dimensional interpolation is very fast (a few seconds) because multiple-scattering calculations are unnecessary during this step. Also, this approach allows the development of a visual interface with the possibility of seeing the spectrum that corresponds to any set of structural parameters immediately. Thus, using a very limited number of multiple-scattering calculations, which are most time-consuming, it is possible to fit XANES. The interpolation polynomial construction procedure for three model molecules, FeS4, FeO6 and Ni(CN)4, is demonstrated. An additional test has been performed for the latter most-complex example to check the assumption that a minimum of discrepancy between theoretical and experimental spectra corresponds only to the correct structure of the complex. A comparison with another XANES fitting software, MXAN, is given.

Palladium versus Platinum: The Metal in the Catalytic Center of a Molecular Photocatalyst Determines the Mechanism of the Hydrogen Production with Visible Light

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

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.

Influence of Ligand Substitution on Excited State Structural Dynamics in Cu(I) Bisphenanthroline Complexes

This study explores the influences of steric hindrance and excited state solvent ligation on the excited state dynamics of Cu(I) diimine complexes. Ultrafast excited state dynamics of Cu(I)bis(3,8-di(ethynyltrityl)-1,10-phenanthroline) [Cu(I)(detp)(2)](+) are measured using femtosecond transient absorption spectroscopy. The steady state electronic absorption spectra and measured lifetimes are compared to those of Cu(I)bis(1,10-phenanthroline), [Cu(I)(phen)(2)](+), and Cu(I)bis(2-9-dimethyl-1,10-phenanthroline), [Cu(I)(dmp)(2)](+), model complexes to determine the influence of different substitution patterns of the phenanthroline ligand on the structural dynamics associated with the metal to ligand charge transfer excited states. Similarities between the [Cu(I)(detp)(2)](+) and [Cu(I)(phen)(2)](+) excited state lifetimes were observed in both coordinating and noncoordinating solvents and attributed to the lack of steric hindrance from substitution at the 2- and 9-positions. The solution-phase X-ray absorption spectra of [Cu(I)(detp)(2)](+), [Cu(I)(phen)(2)](+), and [Cu(I)(dmp)(2)](+) are reported along with finite difference method calculations that are used to determine the degree of ground state dihedral angle distortion in solution and to account for the pre-edge features observed in the XANES region.

Triplet Excited State Distortions in a Pyrazolate Bridged Platinum Dimer Measured by X-ray Transient Absorption Spectroscopy.

The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(μ-(t)Bu(2)pz)](2) (ppy = 2-phenylpyridine; (t)Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal-metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt-Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 Å in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.

Cr local environment by valence-to-core X-ray emission spectroscopy.

Valence-to-core X-ray emission spectra have been measured for a number of chromium compounds such as oxides, carbides, borides, nitrides, phosphides etc. The experimental spectra are in good agreement with the calculated ones. They form the basis for a detailed analysis of the first coordination shell of Cr. The unique application of this technique for material chemistry and environmental science is illustrated by two very different examples. In the former case the presence of Cr–C bonds in the bulk structure of electrochemically deposited amorphous chromium coatings was confirmed. In the latter one we have shown that a contaminated soil sample most probably contains Cr carbides and/or phosphides from anthropogenic origin.

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts

Building a detailed understanding of the structure-function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.

Three-dimensional local structure of photoexcited Cu diimine complex refined by quantitative XANES analysis.

The structural details of [Cu(dmp) 2] (+) (dmp = 2,9-dimethyl-1,10-phenanthroline) at its metal-to-ligand charge-transfer (MLCT) excited-state in acetonitrile were extracted using quantitative analysis of Cu K-edge X-ray adsorption near edge structure (XANES). The study combines two techniques: fitting experimental XANES spectra with a multidimensional interpolation approximation, and calculating theoretical XANES spectra with molecular potentials beyond the muffin-tin approximation. The results of the study show that the best fit of the experimental XANES data must include a solvent molecule binding to the Cu with a short Cu-N distance of 2.00 A. This confirms that the formation of an exciplex is responsible for the excited-state quenching in coordinating solvents, such as acetonitrile. Moreover, the calculations suggest that the formation of this exciplex state is accompanied by significant rocking distortions of the dmp ligands resulting in a 108 degrees angle between the N(solvent)-Cu bond and the C 2 symmetry axis of the dmp ligand. This combined approach allows us to extract molecular configurations that would otherwise be missed in a conventional qualitative XANES analysis.