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Speciation of Ru Molecular Complexes in a Homogeneous Catalytic System: Fingerprint XANES Analysis Guided by Machine Learning

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During the last two decades, remarkable and often spectacular progress has been made in the methodological and instrumental aspects of x-ray absorption and emission spectroscopy. This progress includes considerable technological improvements in the design and production of detectors especially with the development and expansion of large-scale synchrotron reactors All this has resulted in improved analytical performance and new applications, as well as in the perspective of a dramatic enhancement in the potential of x-ray based analysis techniques for the near future. This comprehensive two-volume treatise features articles that explain the phenomena and describe examples of X-ray absorption and emission applications in several fields, including chemistry, biochemistry, catalysis, amorphous and liquid systems, synchrotron radiation, and surface phenomena. Contributors explain the underlying theory, how to set up X-ray absorption experiments, and how to analyze the details of the resulting spectra. X-Ray Absorption and X-ray Emission Spectroscopy: Theory and Applications: Combines the theory, instrumentation and applications of x-ray absorption and emission spectroscopies which offer unique diagnostics to study almost any object in the Universe. Is the go-to reference book in the subject for all researchers across multi-disciplines since intense beams from modern sources have revolutionized x-ray science in recent years. Is relevant to students, postdocurates and researchers working on x-rays and related synchrotron sources and applications in materials, physics, medicine, environment/geology, and biomedical materials.
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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.
Chapter
The rising interest in XANES is due to the experimental evidence that information on coordination geometry and bonding angles, not given by EXAFS, can indeed be extracted from XANES [1,2]. In this paper I would like to point out that certain features of XANES, being sensitive to interatomic distances, can also be exploited to determine first coordination bond lengths to a few percent accuracy (5% or less) [3]. This aspect of XANES is particularly useful whenever EXAFS modulations are too weak to be measured, as in the case of low 7 backscatterers. For the sake of concreteness, I shall limit myself to K-edge absorption spectra of simple molecules containing C-C, C-N, C-O type of bonds and of two series of Mn and V oxides (see Tables I and II). These cases are general enough to serve as an illustration of other cases.
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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 calcula
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The effects of hydrogen in Pd nanoparticles on the features of Pd K-edge X-ray absorption near edge structure (XANES) are studied. Simulations indicate a linear dependence of the intensities of XANES maxima upon the concentration of hydrogen in the bulk of Pd clusters at low H-concentrations and stepwise increase of the first near edge peak at high concentrations. The effects of the diffusion of H atoms over the interstitial sites are considered. The application of the obtained results to the experimental spectra measured in PdH nanoparticles of sizes from 1.3nm to 10.5nm indicates low H-concentrations and high mobility of H-atoms in the bulk of the nanoparticles.
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This paper gives a recent overview on numerical aspects of multivariate interpolation and approximation by radial basis functions. It comments on the correct choice of the basis function. It discusses the reduction of complexity by different methods as well as the problem of ill-conditioning. It is aimed to be a user’s guide to an efficient employment of radial basis functions for the reconstruction of multivariate functions. For the entire collection, see [K. Jetter (ed.), M. Buhmann (ed.), W. Haussmann (ed.), R. Schaback (ed.), J. Stoeckler (ed.), Topics in multivariate approximation and interpolation. Studies in Computational Mathematics 12. Amsterdam: Elsevier. (2005; Zbl 1205.41002)].
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