Photosynthetic water oxidation is a fundamental process in the biosphere that results in the sunlight-driven formation of O₂ from water. Biological photosynthesis encompasses a series of complicated processes involving several transition states and intermediates that scientists continue to investigate. Mimicking this reaction in a man-made device will allow for sunlight-to-chemical energy conversion, with water providing electrons and protons for the formation of oxygen and reduced chemicals(1-2). Such processes are best suited for sustainable and clean generation of H₂. The first synthetic catalyst designed to mimic the portion of biological photosynthesis involved in water oxidation, i.e. the catalyzed evolution of O₂ from H₂O, was the ruthenium-based compound commonly referred to as blue dimer. Although the water-oxidizing capabilities of blue dimer were first reported around three decades ago, several aspects of this catalytic process remained hidden. A variety of spectroscopic techniques namely stopped-flow UV-Vis Spectroscopy, Electron Paramagnetic Resonance, X-Ray Absorption Spectroscopy and Resonance Raman are used to probe the catalytic process of blue dimer as well as single site monomeric ruthenium complexes with higher turnover rate.

EPR, Raman and XAS characterization of the electronic structure and molecular geometry of peroxo intermediates in blue dimer as well as single-site water-oxidizing complexes are reported. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts(3). However, such intermediates were never isolated and their structural and electronic characterization has not been reported. The intermediates described here are direct products of the O-O bond formation step in the studied catalysts. The combination of all these techniques enabled identification of the critical requirements for catalytic water oxidation for the design of new economical and efficient catalysts.

1. Esper B, Badura A, & Rogner M (2006) Photosynthesis as a power supply for (bio) hydrogen production. Trends in Plant Science 11.
2. Moonshiram D, et al. (2012) Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis. J.Am.Chem.Soc. 134(10):4625-4636.
3. Concepcion JJ, Jurss JW, Templeton JL, & Meyer TJ (2008) One Site is Enough. Catalytic Water Oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+. J Am Chem Soc 130(49):16462-16463.
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Powder diffraction is a useful tool for examining a number of compounds that do not form single crystals for a variety of reasons. Unlike with single crystals, structure determination with powders is not a routine task, especially when external knowledge about the composition and structural makeup of the material are limited. I will present two interesting families of cyanide-based compounds, which highlight interesting and unique structural features and properties.

Cs2MnII[MnII(CN)6] has the archetypal Prussian blue structure with cations in the cubic voids. Substitution with smaller alkali ions lead to structural distortions and a marked increase in ordering temperatures with increasing distortions. On the other hand, substitution of larger cations, NMe4+ and NEt4+ drive a rearrangement of the Mn-CN-Mn network and produce several previously unobserved Mn(II) coordination geometries and very different structural motifs.

Zn(CN)2 forms an interpenetrated diamondoid structure and undergoes a number of transitions upon the elevation of pressure. The structures of the four new crystalline phases have been resolved through ab-initio structural determination by synchrotron powder diffraction. The specific transition depends on the hydrostatic fluid used, and surprisingly three of these new phases involve a close to 2-fold expansion of volume. This counter-intuitive expansion is due to minimization of the solid and fluid volume, rather than just the solid volume.
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Coherent x-ray diffraction imaging in the reflection geometry is desirable since it offers many advantages, including (1) lensless technique without need of object lenses, (2) few restrictions on sample size, chambers or environments, (3) suitable for studies of thin films grown on any substrates, (4) no need of pin holes near samples as in holography-based techniques, and (5) easily expandable to resonant coherent x-ray imaging.

We demonstrate by numerical simulation that atomic structures on single crystal surface can be reconstructed using the ptychography coherent x-ray diffraction imaging in the reflection geometry. Our approach is based on the concept of crystal truncation rod. We can obtain the highest surface sensitivity at anti-Bragg condition, and achieve a phase contrast up to from a single atomic step. Ptychograhy scanning scheme allows us to overcome the stringent requirement for isolated samples in typical CDI experiments. We will show experimental results from real platinum (001) surfaces and their ptychography reconstructions. This technique can be readily applied to buried interfaces under catalytic and electrochemical conditions and to nanoscience applications, such as nanowire with stacking faults.
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Attosecond transient absorption (ATA) studies provide a way to push our understanding of the energy transfer between electromagnetic fields and matter to the sub femtosecond time scale. ATA is an all-optical attosecond metrology that complements methods based on the measurement of charged particles, such as attosecond streaking and electron interferometry. In this talk I will review the basics of ATA and summarize recent results in this fast growing field. I will discuss calculations from our attosecond theory group on ATA in laser-dressed atoms that highlight the extent to which attosecond dynamics can be extracted from these measurements.
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Partially coherent radiations generated by high-brilliance third generation synchrotron sources have allowed measurements of slow dynamics in various soft and hard material systems. We have recently developed an X-ray photon correlation spectroscopy (XPCS) technique in the ultra-small angle scattering regime, which fills an existing gap between the accessible Q ranges of dynamic light scattering and pinhole-based XPCS and enables studies of low-frequency equilibrium and nonequilibrium dynamics in optically opaque materials. This technique, based on the Bonse-Hart ultra-small angle X-ray scattering instrument at the Advanced Photon Source, requires modifications to the beamline configuration and instrument operations for dynamic measurements. We will review the basic features of this technique, and will discuss in details the optimizations that we made to meet the needs of signal-to-noise limited XPCS studies. We will present the data analysis approaches that we established to quantify the dynamic time scales of measured equilibrium or nonequilibrium processes. Finally, we will use a few examples to illustrate the applications of this technique in understanding of equilibrium dynamics of soft materials and nonequilibrium behavior of both soft and hard materials.
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Perovskite oxides are a fascinating class of materials - they exhibit a huge range of properties such as magnetic ordering, ferroelectricity, metal to insulator transitions, piezoelectricity, just to name a few. These properties are extremely sensitive to the position, bonding, and electronic state of the central atom. As a result, small strains or structural changes can have large effects on the functional properties. The perovskites present an exciting opportunity to gain insight into these phenomena by pairing structural characterization with measurements of the macroscopic properties.

In this talk, I will discuss how synchrotron x-ray diffraction can be used to study ferroelectricity, piezoelectricity, and charge disproportionation in perovskite thin films. Ferroelectric domains in (Pb,Zr)TiO₃ thin films were written using PFM. X-ray nanodiffraction was used to simultaneously image the domains and probe the structure. By comparing our results with PFM imaging, we find that the strain induced by the writing process is responsible for certain polarizations being less stable than others. In piezoelectric thin films, the film is macroscopically attached (clamped) to its substrate and the average piezoelectric distortion along the film-substrate interface must be zero. We observe very different results at the microscopic level. Using time-resolved x-ray microdiffraction, we find that the piezoelectric response of individual domains in BiFeO₃ thin films is not zero and varies on a domain-by-domain basis. Finally, we combine electronic transport measurements with x-ray diffraction from charge disproportionation in La1/3Sr2/3FeO3 thin films. The temperature dependence of the resistivity and intensity and correlation lengths of the charge disproportionation reflections give insight into the nature of this phase transition.
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Electron correlations can be found in various fields of physics whenever we have to go beyond the independent particle model. A convenient method to study electron correlations in atoms and molecules is to measure the probability to remove TWO electrons simultaneously with a single photon (called double photoionization) from the sample. Because a single photon can interact with only one electron, the removal of two electrons is due to electron correlations. I will present our recent results on double photoionization over a broad range of photon energies for several aromatic molecules. Our goal is to find systematic trends as the molecular structure of our different samples changes. Questions that will be addressed in the talk are: How does the structure of a molecule affect the double-photoionization process? Which mechanisms contribute to double photoionization?
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The glass forming ability (GFA) of metallic alloys is widely varied. Bulk metallic glasses (BMGs) have been identified in a number of alloy systems but far more compositions can be formed only when their liquids are rapidly quenched. Understanding the differences between these systems remains one of the most important problems in condensed matter physics. Understanding the structural evolution of metallic liquids as they are supercooled and quenched into glasses is critically important, not only for providing insight into the nature of the glass transition, but also for understanding technical aspects of glass formation and the thermal stability of the glassy solid. In this talk, we discuss the results of high energy X-ray diffraction studies on Cu-Zr and Ni-Nb liquids and glasses. The temperature dependence of the X-ray structure factors has been measured in the glass from room temperature to above the glass transition temperature by means of stationary diffraction in a ! capillary while data in the equilibrium and supercooled liquid state were acquired using the Beamline Electrostatic Levitation technique. As will be shown, both the structure factors as well as the calculated total pair correlation functions display an anomalous evolution indicating a rapid acceleration of short-range order above the glass transition temperature. This behavior contrasts sharply with that observed in high glass forming ability metallic alloys suggesting a structural fragility metric distinguishing good glass formers from poor ones. We discuss the implications for this observation on our fundamental understanding of glass formation.
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The anticipated doubling of the worlds energy consumption within the next 50 drives the need for efficient and clean energy. Advanced electrical energy storage systems and nuclear energy are technologies that are crucial to achieving this goal. However, key materials challenges must be overcome before these technologies become viable:

• For electrical energy storage systems to become practical for transportation and electrical grid application, higher capacity/power and faster recharge times are needed. Gaps in our understanding of the atomic- and molecular-scale processes that control performance and failure of these systems must be addressed by fundamental research.


• For safe nuclear energy industry, the challenges lay in the effective disposal of the radioactive waste generated by the nuclear fuel cycle. Many solutions involve "multiple-barrier" geological depositories which use a wide range of sequestration agents. Understanding of the structural and chemical properties of these agents, and their interaction with radioactive elements, is key to the fidelity of these facilities.

In both electrical energy storage and nuclear energy, the complexity of the systems and the often metastable and/or disordered nature of the materials pose a significant characterization challenge. This challenge can be addressed through judicious selection and, where necessary, development of incisive characterization tools. We have recently designed an operando electrochemical cell, optimized for a variety of hard X-ray methodologies, and used it to probe next-generation battery materials with an unprecedented level of precision. First-of-the-kind operando pair distribution function measurements have provided a detailed understanding of high-performance electrode materials. In situ time-resolved powder X-ray diffraction data, analyzed through Rietveld refinement, combined with X-ray absorption spectroscopy, scanning electron microscopy and other analyses, have been used to evaluate the use of hydroxylapatite as radionuclide sequestration agent and solid nuclear waste form.
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I will be talking on the recently published calculation of RIXS response in the Falicov-Kimball model. The Falicov-Kimball model is exactly solvable under single site dynamical mean field theory approximation and the RIXS response in this model can also be accurately calculated up to a local background correction. We find that on resonance the RIXS response is greatly enhanced. The response systematically evolves from a single peak structure, arising due to relaxation processes within the lower Hubbard band, to a two peak structure, arising due to relaxation processes within the upper Hubbard band as well as across the Mott gap into the lower Hubbard band, as we vary the incident photon frequency to allow excitations from the lower Hubbard band to the upper Hubbard band. The charge transfer excitations are found to disperse monotonically as we go from the center of the Brillouin zone towards the zone corner. These correlation induced features are found to be robust and survive even for large Auger lifetime broadening effects which can mask the many-body effects by smearing out spectral features. As a comparison, we also calculate the dynamic structure factor for this model, which is proportional to the nonresonant part of the response, and does not show these specific signatures.
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The Sub-micron Resolution X-ray spectroscopy beamline (SRX), whose commissioning phase will start in July 2014, is one of the seven project beamlines of NSLS-II at Brookhaven National Laboratory. Operating at energies ranging from 4.65 to 23 keV, SRX will address a wide variety of scientific applications probing heterogeneous complex systems from the meso to the nanoscale. Combined with state of the art optics, the ultralow emittance and stability of the NSLS-II source is particularly suitable for micro- and nanoprobes like SRX. The SRX main optical components consist of a horizontally focusing mirror creating a secondary source whose size is adjustable with slits, an ultra-stable horizontally deflecting monochromator and two sets of Kirkpatrick-Baez mirrors in two inline stations as focusing optics for operations requiring either high flux (microprobe) or high resolution (nanoprobe). The presentation will focus first on the beamline layout that has been optimized with FEA, ray-tracing and wave front propagation simulations using respectively Shadow and SRW. Results demonstrate that the SRX micro- and nanoprobes will provide 1013 and 1012 ph/s respectively in a sub-micron and a 50 nanometer spot, with an excellent energy resolution (close to the Darwin width of the selected crystals). In a second time, the end-station design will be described with highlights on interferometry tests performed on sample stage assemblies as well as considerations to handle high throughput fluorescence signal. One will conclude on possible early science experiments that are expected by the end of the year 2014.
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The energy dispersive EXAFS (EDE) setup at ODE beamline Soleil Synchrotron as well as the recent development of microsecond (µs) time-resolved XAFS data collection and its application to film continuously the electronic and geometric structural kinetics in a thermolysis reaction will be presented. The proposal of a laser pump and XAS probe development using EDE setup will be introduced briefly. In the second part of the talk, the optical pump and X-ray probe setup and ultrafast solution scattering on small molecules studied in ID09B ESRF will be presented, the photofragmentation reaction of triruthenium dodecacarbonyl Ru3(CO)12 in cyclohexane will be used as an example to show the data analysis process and the complementary nature of ultrafast X-ray scattering and ultrafast spectroscopy in the determination of transient molecular structures and chemical reaction mechanisms.
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Recent efforts of x-ray microscopy research and development are pushing the resolution of state-of-the-art x-ray microscopes toward single digit of nanometer region. Among all the x-ray focusing optics, Fresnel zone plates are superior considering their nature of two-dimensional focusing and a resolution close to diffraction limit. However, high-resolution zone plates operated at x-ray wavelengths, especially in hard x-ray region, require a high-aspect-ratio geometry for their concentric metal zones to efficiently diffract lightwave, a challenge to nanofabrication researchers. In this talk, I will discuss three different approaches for fabricating high-aspect-ratio metallic nanostructures, including methods utilizing stressless electroforming molds, double-patterning, and zone-doubling techniques. Using these techniques we demonstrated hard-x-ray zone plates with 19:1 aspect-ratio and 30 nm outermost zone width, as well as a platinum resolution test pattern featuring 17-nm-wide, 255-nm-tall gratings.
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The Helmholtz-Zentrum Berlin (HZB) operates a full-field transmission X-ray microscope (TXM) in the soft X-ray photon energy range at the undulator beamline U₄₁-FSGM at the BESSY II electron storage ring. Due to the optical setup the HZB-TXM permits cryo X-ray tomographic as well as spectromicroscopic applications in material and life sciences. Some examples of these applications will be shown. The spatial resolution of the full-field microscope is limited by the numerical aperture of the zone plate objective and the wavelength used for imaging. The resolving power can be improved by decreasing the outermost zone width or imaging in higher orders of diffraction. At HZB, we work in our nanotechnology lab on both approaches. Recent results on the nanofabrication of high resolution zone plate objectives will be presented. In addition, the achievable resolution of the HZB-TXM will be discussed.
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X-ray phase imaging has been of interest in the last couple of decades for potential applications in medical imaging and material characterization. A key aspect in phase imaging is the well known “phase problem” which involves detangling the absorption and phase effects from intensity measurements. In this talk, I will make a topical review of the phase retrieval methods and present a novel single-step method to simultaneously retrieve x-ray absorption and phase images which is valid for a broad range of imaging energies and material properties. The method relies on the availability of spectrally resolved intensity measurements, which is now possible using semiconductor x-ray photon counting detectors. I will discuss the potential and utility of the method and address the engineering challenges in x-ray detection with the currently available spectral detectors.
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The presentation reviews the main achievements in zone plates fabrication for X-ray focusing, with accents on how the existent knowledge can be extended to reach the goal of 20 nm resolution at 25 keV photon energy. Several approaches will be discussed, revealing that traditional nanofabrication ways will not work, while new, combined approaches will do the job. Among the discussed methods: sequential height increases by soft x-ray exposure and electroforming, layered depositions and nanolaminates, zone doubling and resolution increase by adding higher order zones, stacking and composing zone plates by MEMS-enabled alignment, diamond Fresnel lenses, and others. Combining the positives of these approaches leads to a heavily-engineered concept of zone plates fabrication, but with good chances of success, and even a perspective for higher photon energies.
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Abstract: It is becoming apparent that nanoscale fluctuations in the charge state are important to the physics of transition metal oxides. The temperature evolution of structural modulation associated with charge (co) and spin order (so) in La1.67Sr0.33NiO4 has been investigated using neutron powder diffraction [1]. For the first time, we report an anomalous shrinking of the “a” lattice parameter that correlates with Tco, at the temperature where long-range stacking order of charge stripes disappears. We show that this response may be explained as resulting from the energy contributed by the interlayer electrostatic interaction leading to shrinkage of c/a. In addition, we report anomalies in the temperature evolution of the refined atomic displacement parameters (ADP) and local structural parameters in the atomic pair distribution function (PDF), which show quite different temperature dependence consistent with the persistence of localized charges, presumably in the form of short range ordered stripe domains, over a broad temperature range above Tco.
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Progress in understanding and exploiting the properties of complex oxide heterostructures requires advances in state-of-the-art growth and characterization techniques for these materials, as atomic control of their synthesis is demonstrably inferior to that of their semiconductor counterparts. Here we report significant improvements in the atomic design of perovskite interfaces made possible by advances in in situ control of the surface by reflection high-energy electron diffraction (RHEED) during growth that reveal the surface termination and characteristic octahedral distortions in the surface layer as it is being deposited. This RHEED approach applies generally to growth of polar and nonpolar perovskite unit cells when a desorption-controlled growth regime is not utilized. As an example, we demonstrate its use in the optimization of atomically-designed manganite/titante interfaces that eliminates cation intermixing and anomalous unit cell dilations that have previously been observed. Careful analysis of the crystal structure shows an unusual evolution of the octahedral distortions that include both J-T type and rotations near the interface that are not seen in bulk. These new results should be included in electronic structure calculations modeling the properties of real heterointerfaces.
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Materials often contain fundamental properties that are determined by structures on nanometer length scales. An example of this is the nanometer size pores in synthetic zeolites crystals. The shapes of the pores can define molecular sieves and the composition of the walls can be tailored to catalytic applications. Occlusions in the pores will significantly restrict the function of the material and if occurring below the surface may be difficult to detect. The x-ray diffraction Bragg peaks of crystals contain a wealth of information regarding the structure of the sample. If the x-ray beam used to study a sample is spatially and temporally coherent, the scattered x-rays in the vicinity of the Bragg peak contains local structural information on potentially nanometer length scales. Images are formed by computational inversion from reciprocal space to direct space using phase retrieval algorithms. These images contain both morphological and strain information on tens of nanometer length scales in three dimensions.

This talk will describe the technique of coherent x-ray diffraction (CXD) imaging in the Bragg geometry and show recent results from 34-ID-C, including a study of zeolites being tailored for catalysis of pollutant gases, the response of a nanocrystal to multi gigapascal pressure, and time resolved phonon modes in nanocrystals.
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Ever wonder where your x-rays are? How many photons you have? Where they‘re going? A diamond based x-ray beam position monitor may be for YOU! Advances in CVD diamond growth have made this material an attractive option for x-ray diagnostics. Diamond is radiation hard, solar blind, and has very small leakage current (sub pA). The low Z of diamond makes it attractive for transmission diagnostics, while the high thermal conductivity makes it attractive for high flux applications. This talk will focus on recently demonstrated monitors for both white and monochromatic beams, along with a discussion of goals for future development.
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The development of zone plate lenses is crucial for the progress of high-resolution x-ray microscopy. Since the resolution achievable in imaging is proportional to the outermost zone width of the lens, narrow zones have to be nanofabricated. In addition, high aspect ratios are required to achieve high diffraction efficiencies, thus rendering zone plate nanofabrication challenging. At the Biomedical and X-Physics group at KTH (Stockholm, Sweden) the development and improvement of nanofabrication processes for high-resolution high-diffraction-efficiency soft x-ray optics is a key aspect. Different approaches, such as cold development, cryogenic RIE, or the concept of nickel-germanium zone plates have been investigated and applied, resulting in soft x-ray zone plates with zone widths down to 12 nm and diffraction efficiencies up to 9.6% (at λ=2.48 nm).
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An unusual decomposition mechanism of highly vibrationally excited molecules, dubbed the “roaming mechanism,” has recently been discovered and is now a very active area of investigation. In these reactions, a molecule undergoes partial dissociation to radical fragments by simple bond fission. When the fragments separate to 3-4 Å, “roaming” reorientation becomes feasible as the kinetic energy is low and the angular forces may be comparable to the radial forces. If this leads the system to access a distinct reactive domain, intramolecular abstraction may take place giving unexpected products often with with large vibrational excitation. This pathway may deviate substantially from the nominal minimum energy path, and in some cases appears to avoid the normal transition state geometry entirely. Many of the details have come to light through high-resolution ion imaging studies of formaldehyde, in concert with quasi-classical trajectory calculations from Bowman and coworkers. Many other examples of roaming dynamics have now been reported, both in experiment and theory. I will emphasize recent results documenting “roaming-mediated isomerization” which appears to be a general feature of the decomposition of nitro compounds. In addition, I will introduce new universal probes of reaction dynamics affording isomer and even conformer selectivity.
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The invention of x-ray free-electron lasers (XFELs) opens the pathway to transfer powerful spectroscopic techniques from the optical to the x-ray region, to ultimately study the interplay of coherent electronic and vibrational dynamics on the timescale of chemical reactions. Resonant Inelastic X-ray Scattering (RIXS) is a viable spectroscopic tool to study the valence-electronic environment of a target atom, which can be selectively addressed by the resonantly tuned x-ray beam. The cross section for this process is, however, small, so that even with powerful XFEL pulses the recording of single-shot RIXS spectra of dilute samples or molecules in the gas phase are challenging. This hampers the application of RIXS in optical pump x-ray probe experiments, which would, for example, be one method of choice to study light-induced chemical reactions. Here we demonstrate, how this bottleneck can be overcome by stimulating the RIXS process. We present results of a recent experiment o! n stimulated RIXS at the Linac Coherent Light Source XFEL in a gas sample of atomic neon, thereby amplifying the RIXS signal by 6-7 orders of magnitude. Using broadband XFEL pulses, which are characterized by a stochastic, spiky substructure of their spectrum, both “pump” and “dump” photons can be provided in a single pulse, thereby stimulating the scattering process and driving an exponential amplification of the signal. Despite the overall broad bandwidth, high-resolution RIXS spectra can be achieved by statistical analysis of a series of stochastic single-shot spectra. The energy resolution is determined by the spectral coherence of the XFEL source, and in principle allows for vibrational resolution. These findings open up a new class of experiments at XFEL sources, with a colossal increase of the RIXS signal. Results of our recent experiment will be presented, along with a perspective and theoretical feasibility study of stimulated RIXS measurements in small molecules.
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Since the late 1990s, ultrafast x-ray pulses have been used to probe impulsive strains propagating in the bulk of optically opaque materials. Time-resolved x-ray diffraction has been proven to be a very powerful tool for visualizing transient one-dimensional crystalline strains, ranging from crystal growth to shockwave production.

In this presentation, I will describe a series of time-resolved x-ray diffraction experiments that visualize transient strain formation from nanometer-scaled laser-excited metallic films. Utilizing a table top picosecond x-ray source in conjunction with a high-power optical laser system, the resulting optical pump/x-ray probe spectra reveal that the spatiotemporal structure of the transient acoustic pulse is bipolar with acoustic wave-vectors up to inverse of the film thickness. In addition, I will also discuss the real-world constraints that place limits on the validity of the reconstructed transient acoustic pulse.
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I will discuss the scientific motivation for, and technical details of, the recently completed conceptual design for the LERIX-2 spectrometer as part of the APS-U. In the conceptual design, we expand to 144 analyzers while taking the most successful mechanical and operational features of the LERIX-1 and LERIX-1B instruments. Among the issues that need to be resolved for the final design are the selection of detectors and the selection and reliable mass-fabrication of spherically-bent crystal analyzers (SBCA's). If in-house, rather than commercial, fabrication of the needed highly efficient SBCA's is possible, it will result in a factor of two cost savings for this instrument.
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Ultrafast XUV and X-ray light sources offer new opportunities to unravel fundamental electronic and nuclear dynamics in matter. The Chemical Dynamics program at the Ultrafast X-ray Science Laboratory is focused on the application of emerging laboratory- and accelerator-based X-ray techniques to monitor the flow of charge, mass, and energy in molecules, clusters, and interfacial systems. A brief overview of the program will be followed by the discussion of two particular showcase examples of ongoing research: The study of photoinduced charge-transfer dynamics in dye-sensitized nanocrystals by time-resolved X-ray photoelectron spectroscopy and the investigation of quantized fluid dynamics in nano- to micron-scale superfluid helium droplets by single-shot coherent diffractive imaging.

Interfacial charge transfer studies are performed at the Linac Coherent Light Source (LCLS, SLAC National Accelerator Laboratory) and the Advanced Light Source (ALS, Berkeley). Recent LCLS results demonstrate the potential of time-resolved X-ray photoelectron spectroscopy (TRXPS) to monitor charge migration in complex interfacial systems with femtosecond time resolution, chemical sensitivity and element specificity. The visible light induced transient oxidation state of N3 dye molecules adsorbed to nanocrystalline ZnO is characterized in a concerted effort of TRXPS experiments and ab-initio calculations of the interfacial electronic structure.

The superfluid nature of helium droplets presents a rare opportunity to study the onset of macroscopic quantum phenomena in sub-micron scale systems. Pure and doped helium droplets are studied by coherent diffractive imaging (CDI) using femtosecond X-ray pulses from the LCLS. Single-shot CDI data provide the most direct access yet to droplet size- and shape-distributions as well as fundamental dynamics inside the clusters. The results will be discussed in the context of the onset of quantum vorticity in finite three-dimensional quantum systems.
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The year 2013 marks the 100th anniversary of the publication of Bragg’s Law by W. L. Bragg and the publication of the first crystal structure determination by x-rays, by W.H. Bragg and W.L. Bragg. This presentation will include a brief introductory discussion regarding the knowledge of the generation and scattering of x-rays in this time period, some personal background regarding the Braggs, and how Laue’s famous photograph set the stage for this father and son team to not only win the Nobel Prize for Physics in 1915 "For their services in the analysis of crystal structure by means of X-rays," but to change the world of science for many years to come through their development of x-ray crystallography.
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The Joint Center for Energy Storage Research (JCESR) develops concepts and technologies for portable electricity storage for transportation and stationary electric storage for the electricity grid. Electrified transportation replaces foreign oil with a host of domestic electricity sources such as gas, nuclear, wind and solar, and utility scale electric storage enables the grid to bridge the peaks and valleys of variable wind and solar generation and consumer demand. JCESR looks beyond Li-ion technology to new materials and phenomena to achieve the factor of five increases in performance needed to realize these transformational societal outcomes. JCESR will leave three legacies: a library of fundamental scientific knowledge of materials and phenomena needed for next-generation batteries, demonstration of battery prototypes suitable for scale up to manufacturing for transportation and the grid, and a new end-to-end integrated operational paradigm for battery research and development spanning discovery research, design, and demonstration.
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The ability of coordination compounds to catalyze chemical reactions and absorb visible radiation makes them appealing targets for the development of photocatalysts. One of the attributes that makes transition metals excellent catalysts – a high density of frontier orbitals – can also lead to ultrafast quenching of electronic excited states. Understanding the properties of coordination complexes that dictate the electronic relaxation dynamics has practical, as well as fundamental importance. Most successful photosensitizers and photocatalysts have utilized 4d and 5d metal centers. The significant cost and low abundance of many 4d and 5d metals has inspired attempts to substitute high cost atoms with isoelectronic 3d metal complexes. But the exchange iron for ruthenium increases the charge transfer relaxation rate by roughly a factor of one million. The huge distinction in lifetimes has generally been attributed to differences in the ligand field excite state energies. But we currently still lack a detailed understanding of how ligand field excited states and charge transfer excited states interact and how this depends upon nuclear and electronic structure. We investigated the role of ligand field excited states in the relaxation dynamics of photogenerated charge transfer states in a series of iron(II) coordination compounds with hard x-ray emission spectroscopy (XES). The tremendous sensitivity of XES to the charge and spin state of the transition metal centers make these techniques ideally suited to investigating the electron dynamics in coordination chemistry. By studying mixed cyanide and bipyridine ligands with advanced x-ray spectroscopy, we discovered that the excited state decay pathway can be controlled and adjusted by systematically tuning the ligand field splitting. We demonstrated that changing the iron ligands lend to a 100-fold increase in the charge transfer excited state lifetime, a critical metric for earth abundant photosensitizers.
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Nano-structured materials and their assemblies have generated considerable scientific and industrial interest as a result of new chemical and physical interactions as their size is reduced and they are positioned into well-defined spatial arrangements. Indeed, a grand challenge in nanotechnology is to construct materials comprised of positionally encoded elements (i.e. nanoparticles) with fine control over spacing, symmetry, and composition, with single- or sub-nanometer precision and registry. The ability to exercise such control over multiple length scales and in three dimensions for a single system would, in principle, provide researchers with a route to fabrication “materials by design”, in which one could design and build a functional system with programmed chemical and physical properties, useful in material synthesis, optics, biomedicine, energy, and catalysis. In this talk, I will discuss recent progress towards this goal, by using DNA as a programmable ligand to direct the assembly of nanoparticles into crystalline arrays. DNA is ideally suited for this purpose, as synthetically tunable variations in nucleotide sequence allow for precise engineering of the nanoparticle’s hydrodynamic radius and binding properties. These factors, in turn, dictate the crystallographic symmetry and lattice parameters of the assembly. By further employing a DNA-functionalized substrate, thin-film nanoparticle superlattices can be grown in a layer-by-layer fashion with fine control over the number of particle layers in the assembly (i.e. film thickness). Importantly, the judicious choice of DNA substrate-particle interconnects allows one to tune the interfacial energy between various crystal planes and the substrate, and thereby control crystal orientation. A theoretical framework to understand these results is presented. These nanoparticle superlattices can further be patterned in arbitrary locations on a substrate using molecular printing techniques such as dip-pen nanolithography (DPN) and polymer pen lithography (PPL). The principles developed in this work represent a major advance in the bottom-up synthesis of nanomaterials and a major step towards the integration of nanoscale materials into functional device architectures. Lastly, ultrafast pump-probe studies of third-generation materials for future photovoltaics will be presented. One such novel photovoltaic material uses heavy O doping of ZnTe to generate the formation of an intermediate band within the forbidden gap, in order to improve the matching of semiconductor absorption and solar spectra. This approach is believed to become useful for realization of single junction solar cells with very high efficiencies. However, the implementation of such devices requires advanced characterization techniques. Multiphoton optical pulse excitations are demonstrated to induce multiband charge transfer dynamics in ZnTe:O films as revealed when monitoring the time-resolved photoluminescence signals.
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Presently there is great interest in the application of light in the X-ray regime, produced by high-order harmonics, to investigate novel coherent X-ray optical phenomena. Loh et al. [1] report the observation of EIT-like behavior in the extreme ultraviolet (XUV) by coherent coupling of 2s2p and 2p2 doubly excited states in He, probing with laser-produced high-order harmonics. The EIT-like phenomenon observed in their work is characterized solely by an increase in transmission over the entire unperturbed lineshape. It is the aim of our work [2] to extend the phenomenological theoretical treatment of this effect included in [1]. We present calculations based on the solution of the time-dependent Schrodinger equation in the LS-coupling configuration interaction basis set. The absorbing boundary is represented by the complex absorbing potential and we present here the analysis of the ionization yield obtained. This approach allows for more accurate treatment of the ionization continuum than presented in [1].

[1] Z.H. Loh, C.H. Greene and S.R. Leone, Chem. Phys. 350, 7 (2008).
[2] M. Tarana, C.H. Greene, Phys. Rev. A 85, 013411 (2012).
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A variety of locally written software is in use on Sectors 11 and 12 at the APS. I will describe the available software and some of the organizing principles behind its design in the hope that it may be more widely useful at other beamlines.

If time/audience interest permits I will also present an overview of a number of relevant features of the Qt framework as related to the software I have written.

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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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X-ray microscopy has become a commonly available and often used tool for element specific investigations on a nanometer scale. However, with a constantly growing user community, the demand for a more flexible sample environment grows as well. In my talk, I will describe several approaches that were realized at the SSRL and the ALS to push the limits of x-ray microscopy, e.g. how to improve the resolution of PEEM microscopy without any changes to the microscope, how to measure in large magnetic fields with sub 10ps time resolution, or how to follow chemical reactions in situ.
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The ultrashort-pulse photoexcitation and measurement techniques are of tremendous interest due to their capability to uncover the ultrafast transient response of materials. Among light-based pump-probe techniques, an original approach employing low-power fs-fiber-lasers was developed to acquire, manipulate and modify a wideband spectrum of photoexcitations in thin films and nanostructures.

In epitaxial ferromagnetic films, coherent spin waves are generated with femtosecond laser pulses via thermal excitation mediated by magnon-electron and magneto-elastic coupling. The propagation speeds and attenuation lengths of exchange spin wave modes are determined during the propagation and reflection at the film boundaries, consistent with their dispersion relation. Moreover, photo-thermal excitation could be used to achieve coherent control of the magnetization vector. An optically-induced spin reorientation transition of first-order is revealed and provides a new route to coherent magnetization switching.

Another experimental effort has been focused on phonon dynamics and thermoelectric transport studies. The coherent optical phonon spectroscopy was employed during the fs laser-induced nanostructuring in binary semiconductors such as Sb₂Te₃ and InSb. Nanostructure fabrication process optimization resulted in highly ordered periodic nanostructures without the adverse effects of residual phase separation. In another case, pump-probe measurements are used to understand the behavior of acoustically mismatched thin films to further assist the design of high-Q acoustic resonators at GHz frequencies.

Lastly, ultrafast pump-probe studies of third-generation materials for future photovoltaics will be presented. One such novel photovoltaic material uses heavy O doping of ZnTe to generate the formation of an intermediate band within the forbidden gap, in order to improve the matching of semiconductor absorption and solar spectra. This approach is believed to become useful for realization of single junction solar cells with very high efficiencies. However, the implementation of such devices requires advanced characterization techniques. Multiphoton optical pulse excitations are demonstrated to induce multiband charge transfer dynamics in ZnTe:O films as revealed when monitoring the time-resolved photoluminescence signals.
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Abstract: The strong-field picture of ionization describes the physics of how an isolated atom interacts with an intense ultra-fast laser field. The basic strong-field picture is described as tunnel ionization, which is characterized by the rapid burst of an electron wave packet into the continuum, followed by the classical motion of a quasi-free electron in a strong laser field and recollision with the parent ion. Recollision physics is at the very heart of what makes strong-field science an exciting tool for probing matter on ultrafast time scales. It offers a mechanism to create Attosecond (1 as = 10-18 s) laser pulses through High-Harmonic Generation and it offers a method for controlling electron-ion collisions on sub-femtosecond (1 fs = 10-15 s) time scales.

In my talk I will discuss how wavelength scaling has offered a more robust description of the strong-field picture. In particular, long wavelength lasers provide deep access to tunnel ionization and high energy electrons (several hundred eV) for studying electron recollision. I will discuss two separate aspects of my contributions that have helped to extend the strong-field picture. First, I will discuss inelastic laser driven scattering, or non-sequential ionization, in the long-wavelength limit of a 3.6 μm laser field. Here, large recollision energies (up to 400 eV) driven at modest field strengths result in the impact ionization of charge states up to Xe6+. The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distributions for (e,ne) electron impact ionization. Then, I will discuss how wavelength scaling has made possible extending the strong-field picture of ionization to condensed phase systems. Here, we have observed evidence of a dramatic new mechanism for High Harmonic Generation that is unique to crystals yet closely parallels the semi-classical analysis of the strong-field atomic picture.
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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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Bent crystals have been widely used as optical elements (e.g., monochromators, focusing optics and spectrometers) of high energy synchrotron radiation beamlines. The effects of bending on the reflectivity of the crystal are discussed within the dynamical theory with a full treatment of the crystal anisotropy and biaxial bending. Such knowledge and its combination with ray tracing and wave propagation are essential in the beamline design process. Two particular examples are presented to illustrate the usage of bent crystals for modern synchrotron radiation beamlines: the design of the X-ray Powder Diffraction (XPD) beamline at NSLS-II and the optimization of high luminosity spectrometers at ESRF and XFEL.

The XPD beamline uses a sagittally bent double-Laue crystal monochromator to provide horizontally focused x-ray beam over a large energy range (30-70 keV). A multi-lamellar model is introduced and implemented in the ray tracing of the monochromator. The instrumental resolution function of the beamline is also described.

Bent crystals are also utilized for high luminosity X-ray emission detection. This presentation will compare various concepts of dispersive/non-dispersive spectrometers with different crystal geometries by means of ray tracing.
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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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