Keisuke Hatada

X-ray absorption spectroscopy: state-of-the-art analysis.

State-of-the-art techniques for analysing X-ray absorption spectra are reviewed, with an eye to biological applications. Recent attempts to perform full spectral fitting of the XANES energy region and beyond for the purpose of structural analysis have met with encouraging success. The present paper analyses the theoretical motivations behind this success and indicates routes for future improvements. The theoretical background is not entirely new, although the point of view is, and some sections and appendices present material that the Authors believe has never been published before. The aim of this paper is to provide a theoretical analysis that is as self-contained as possible.

EIS: the scattering beamline at FERMI

The Elastic and Inelastic Scattering (EIS) beamline at the free-electron laser FERMI is presented. It consists of two separate end-stations: EIS-TIMEX, dedicated to ultrafast time-resolved studies of matter under extreme and metastable conditions, and EIS-TIMER, dedicated to time-resolved spectroscopy of mesoscopic dynamics in condensed matter. The scientific objectives are discussed and the instrument layout illustrated, together with the results from first exemplifying experiments.

Full-potential multiple scattering for x-ray spectroscopies

We present a full-potential multiple scattering (FPMS) scheme for the interpretation of several x-ray spectroscopies that is a straightforward generalization of the more conventional muffin-tin version. Like this latter, it preserves the intuitive description of the physical process under consideration and overcomes some of the limitations of the existing FPMS codes. It hinges on a fast and efficient method for solving the single-cell scattering problem that avoids the convergence drawbacks of the angular momentum expansion of the cell shape function; it relies on an alternative derivation of the multiple scattering equations that allows us to work reliably with only one truncation parameter, i.e., the number of local basis functions in the expansion of the global scattering function determined by the classical relation

Direct Deconvolution of Two-State Pump-Probe X-ray Absorption Spectra and the Structural Changes in a 100 ps Transient of Ni(II)-tetramesitylporphyrin

{l}_{\mathrm{max}}\ensuremath{\sim}kR

Full-potential multiple scattering for core electron spectroscopies

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Dynamics of Photoelectrons and Structural Changes of Tungsten Trioxide Observed by Femtosecond Transient XAFS

Full multiple scattering (FMS) Minuit XANES (MXAN) has been combined with laser pump-probe K-edge X-ray absorption spectroscopy (XAS) to determine the structure of photoexcited Ni(II)tetramesitylporphyrin, Ni(II)TMP, in dilute toluene solution. It is shown that an excellent simulation of the XANES spectrum is obtained, excluding the lowest-energy bound-state transitions. In ground-state Ni(II)TMP, the first-shell and second-shell distances are, respectively, d(Ni-N) = (1.93 +/- 0.02) A and d(Ni-C) = (2.94 +/- 0.03) A, in agreement with a previous EXAFS result. The time-resolved XANES difference spectrum was obtained (1) from the spectra of Ni(II)TMP in its photoexcited T(1) state and its ground state, S(0). The XANES difference spectrum has been analyzed to obtain both the structure and the fraction of the T(1) state. If the T(1) fraction is kept fixed at the value (0.37 +/- 0.10) determined by optical transient spectroscopy, a 0.07 A elongation of the Ni-N and Ni-C distances [d(Ni-N) and d(Ni-C)] is found, in agreement with the EXAFS result. However, an evaluation of both the distance elongation and the T(1) fraction can also be obtained using XANES data only. According to experimental evidence, and MXAN simulations, the T(1) fraction is (0.60 +/- 0.15) with d(Ni-N) = (1.98 +/- 0.03) A (0.05 A elongation). The overall uncertainty of these results depends on the statistical correlation between the distances and T(1) fraction, and the chemical shift of the ionization energy because of subtle changes of metal charge between the T(1) and S(0) states. The T(1) excited-state structure results, independently obtained without the excited-state fraction from optical transient spectroscopy, are still in agreement with previous EXAFS investigations. Thus, full multiple scattering theory applied through the MXAN formalism can be used to provide structural information, not only on the ground-state molecules but also on very short-lived excited states through differential analysis applied to transient photoexcited species from time-resolved experiments.

Multiple scattering theory for non-local and multichannel potentials

We present a rigorous derivation of a real space full-potential multiple-scattering theory (FP-MST), valid both for continuum and bound states, that is free from the drawbacks that up to now have impaired its development, in particular the need to use cell shape functions and rectangular matrices. In this connection we give a new scheme to generate local basis functions for the truncated potential cells that is simple, fast, efficient, valid for any shape of the cell and reduces to the minimum the number of spherical harmonics in the expansion of the scattering wavefunction. This approach provides a straightforward extension of MST in the muffin-tin (MT) approximation, with only one truncation parameter given by the classical relation l(max) = kR(b), where k is the photo-electron wavevector and R(b) the radius of the bounding sphere of the scattering cell. Some numerical applications of the theory are presented, both for continuum and bound states.

Nanoclusters Synthesized by Synchrotron Radiolysis in Concert with Wet Chemistry

The dynamics of the local electronic and geometric structures of WO3 following photoexcitation were studied by femtosecond time-resolved X-ray absorption fine structure (XAFS) spectroscopy using an X-ray free electron laser (XFEL). We found that the electronic state was the first to change followed by the local structure, which was affected within 200 ps of photoexcitation.

Progresses in the MXAN Fitting Procedure

Methodological advances in multiple scattering theory (MST) in both wave and Greens function versions are reported for the calculation of electronic ground and excited state properties of condensed matter systems with an emphasis on core-level photoemission and absorption spectra. Full-potential MST is reviewed and extended to non-local potentials. Multichannel MST is reformulated in terms of the multichannel density matrix whereby strong electron correlation of atomic multiplet type can be accounted for in both ground and excited states.

Scissors mode and dichroism in an anisotropic crystal

Wet chemical reduction of metal ions, a common strategy for synthesizing metal nanoparticles, strongly depends on the electric potential of the metal, and its applications to late transition metal clusters have been limited to special cases. Here, we describe copper nanoclusters grown by synchrotron radiolysis in concert with wet chemistry. The local structure of copper aggregates grown by reducing Cu(II) pentanedionate using synchrotron x-ray beam was studied in situ by x-ray absorption spectroscopy. A detailed analysis of the XANES and EXAFS spectra, compared with DFT calculations and full-potential non-muffin-tin multiple scattering calculations, identified the nanocluster as Cu13 with icosahedral symmetry. The novel “charged” nanoclusters tightly bound to electron-donating amido molecules, which formed as a result of photo-induced deprotonation of ligand amines, were stabilized by irradiation. Monodispersive deposition of nanoclusters was enabled by controlling the type and density of “monomers”, in remarkable contrast to the conventional growth of metallic nanoparticles.