G. Jennings

An energy dispersive x-ray absorption spectroscopy beamline, X6A, at NSLS

An energy dispersive x‐ray absorption spectroscopy instrument has been built at the X6A beam port of the x‐ray ring at the National Synchrotron Light Source (NSLS). This instrument allows the collection of extended x‐ray‐absorption fine structure and/or x‐ray absorption near‐edge structure spectra for many elements on the millisecond time scale. The beamline employs a four‐point crystal bender and a rectangular Si 220 crystal to access incident energies between 6.5 and 21 keV. Because the polychromator focuses the synchrotron beam to a narrow 100‐μm line, this experimental apparatus is ideal for x‐ray absorption spectroscopy experiments in special environments such as at high pressures, for in situ experiments, and/or for very small samples. In this manuscript we will describe the instrument design and present data with which to evaluate the instrument. This beamline is available through the NSLS user proposal system.

Excited-state molecular structures captured by X-ray transient absorption spectroscopy: a decade and beyond.

Transient molecular structures along chemical reaction pathways are important for predicting molecular reactivity, understanding reaction mechanisms, as well as controlling reaction pathways. During the past decade, X-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated X-ray absorption spectroscopy), analogous to the commonly used optical transient absorption spectroscopy, has been developed. XTA uses a laser pulse to trigger a fundamental chemical process, and an X-ray pulse(s) to probe transient structures as a function of the time delay between the pump and probe pulses. Using X-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution-solid interfaces. Several examples from the studies at the Advanced Photon Source in Argonne National Laboratory are summarized, including excited-state metalloporphyrins, metal-to-ligand charge transfer (MLCT) states of transition metal complexes, and charge transfer states of metal complexes at the interface with semiconductor nanoparticles. Recent developments of the method are briefly described followed by a future prospective of XTA. It is envisioned that concurrent developments in X-ray free-electron lasers and synchrotron X-ray facilities as well as other table-top laser-driven femtosecond X-ray sources will make many breakthroughs and realise dreams of visualizing molecular movies and snapshots, which ultimately enable chemical reaction pathways to be controlled.

Occurrence of van Hove singularities in YBa2Cu4O8 and YBa2Cu3O6.9

Abstract We report seeing a saddle point in the energy band, known as a van Hove singularity. We measure an exact position of the singularity in both YBa 2 Cu 4 O 8 (Y124) and YBa 2 Cu 3 O 6.9 (Y123). In Y123 the van Hove singularity is at binding energy of less then 10 meV, closer ot E F , than in Y124, which has a singularity at 19 meV. The singularity has and extended character along the Γ-Y direction. Another clearly resolved band exhibits a saddle point singularity at binding energy of 115 meV. Similar singularities are observed in the vicinity of the X point, extended along the Γ-X direction. In order to study these features the energy resolution of the system was improved to better than 10 me V.

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.

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.

Application of a multi-element Ge detector in laser pump/x-ray probe time-domain x-ray absorption fine structure

The characterization and application of a multi-element Ge solid state detector in time-domain laser pump, x-ray probe (LPXP) x-ray absorption fine structure (XAFS) using a third-generation synchrotron source (Advanced Photon Source or APS) are described. In particular, the following problems are discussed: (1) proper handling of high numbers of x-ray photons within a single pulse or pulse cluster, (2) optimizing the incident number of photons, (3) synchronization of the laser pulse, the x-ray pulse and the detector readout, and (4) shaping time and timing requirements of the detector. Based on the study, we estimate the detector efficiency for LPXP-XAFS experiments and identify the problems and possible solutions.

X-ray snapshots for metalloporphyrin axial ligation

Axial ligation mechanisms of a metalloporphyrin, nickel(II) tetramesitylporphyrin (NiTMP), were investigated by static and transient X-ray absorption spectroscopy at Ni K-edge (8.333 keV). A surprisingly broad (i.e. ∼1.4 eV) linewidth for the 1s → 3dx2-y2 transition in the ground state was attributed to strong geometry dependent 3d molecular orbital (MO) energies due to coexisting conformers in solution. The broad distribution of 3d MO energy levels enables transient degeneracy of the 3dz2 and 3dx2-y2 MOs to produce a temporary vacancy in the 3dz2 MO which favors axial ligation. Photoexcitation also induces the vacancy in the 3dz2 MO, leading to a more than two-fold enhancement in the axial ligated species. Therefore, a unified axial ligation mechanism for both the ground and excited state is proposed based on the elucidation of the excited state structural dynamics, which will have a broad impact in understanding and controlling axial ligation in enzymatic reactions and molecular catalysis involving transient axial ligation.

Photoemission measurement of Fermi surface, band dispersion and quasiparticle lifetime in YBa2Cu3O6.9

Abstract High resolution (20 meV) angle-resolved photoemission measurements on single-crystals of YBa 2 Cu 3 O 6.9 were analyzed to obtain band dispersion and quasiparticle lifetimes. The mass enhancement is 1.2 for a Cu-O plane band mass and 1.0 for the pdπ chain band. Fitting measured energy distribution curves to a model spectrum that includes self-energy corrections, we find that the imaginary part of the self-energy varies quadratically in the immediate vicinity of E F , and becomes linear above ~20 meV.

A new method for x‐ray powder diffraction studies

A new method has been developed which results in a major improvement in count rate of Bragg–Brentano‐type diffractometers. The method uses a focusing exit beam monochromator and a multichannel detector to achieve this improvement. The method is very efficient in eliminating background and can have very high resolution. Existing diffractometers can be easily modified to take advantage of this technique.

Multipurpose monochromator for the Basic Energy Science Synchrotron Radiation Center Collaborative Access Team beamlines at the Advanced Photon Source x‐ray facility

The Basic Energy Science Synchrotron Radiation Center (BESSRC) Collaborative Access Team (CAT) will construct x‐ray beamlines at two sectors of the Advanced Photon Source facility. In most of the beamlines the first optical element will be a monochromator, so that a standard design for this critical component is advantageous. The monochromator is a double‐crystal, fixed exit scheme with a constant offset designed for ultrahigh vacuum windowless operation. In this design, the crystals are mounted on a turntable with the first crystal at the center of rotation. Mechanical linkages are used to correctly position the second crystal and maintain a constant offset. The main drive for the rotary motion is provided by a vacuum compatible Huber goniometer isolated from the main vacuum chamber. The design of the monochromator is such that it can accommodate water, gallium, or liquid‐nitrogen cooling for the crystal optics.