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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Tailoring Panchromatic Absorption and Excited-State Dynamics of Tetrapyrrole–Chromophore (Bodipy, Rylene) Arrays—Interplay of Orbital Mixing and Configuration Interaction

Here, whree sets of tetrapyrrole–chromophore arrays have been examined that exhibit panchromatic absorption across large portions of the near-ultraviolet (NUV) to near-infrared (NIR) spectrum along with favorable excited-state properties for use in solar-energy conversion. The arrays vary the tetrapyrrole (porphyrin, chlorin, bacteriochlorin), chromophore (boron-dipyrrin, perylene, terrylene), and attachment sites (meso-position, β-pyrrole position). In all, seven dyads, one triad, and nine benchmarks in toluene and benzonitrile were studied using steady-state and time-resolved absorption and fluorescence spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

UV-Photochemistry of the Disulfide Bond: Evolution of Early Photoproducts from Picosecond X-ray Absorption Spectroscopy at the Sulfur K-Edge

We have investigated dimethyl disulfide as the basic moiety for understanding the photochemistry of disulfide bonds, which are central to a broad range of biochemical processes. Picosecond time-resolved X-ray absorption spectroscopy at the sulfur K-edge provides unique element-specific insight into the photochemistry of the disulfide bond initiated by 267 nm femtosecond pulses. We observe a broad but distinct transient induced absorption spectrum which recovers on at least two time scales in the nanosecond range. We then employed RASSCF electronic structure calculations to simulate the sulfur-1s transitions of multiple possible chemical species, and identified the methylthiyl and methylperthiyl radicals as the primary reaction products. In addition, we identify disulfur and the CH 2 S thione as the secondary reaction products of the perthiyl radical that are most likely to explain the observed spectral and kinetic signatures of our experiment. Our study underscores the importance of elemental specificity and the potential of time-resolved X-ray spectroscopy to identify short-lived reaction products in complex reaction schemes that underlie the rich photochemistry of disulfide systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

In Situ X-ray Absorption Spectroscopy of a Synergistic Co–Mn Oxide Catalyst for the Oxygen Reduction Reaction

Identifying the catalytically active site(s) in the oxygen reduction reaction (ORR), under real-time electrochemical conditions, is critical to the development of fuel cells and other technologies. We have employed in situ synchrotron-based X-ray absorption spectroscopy (XAS) to explore the synergistic interaction of a Co–Mn oxide catalyst which exhibits impressive ORR activity in alkaline fuel cells. X-ray absorption near edge structure (XANES) was used to track the dynamic structural changes of Co and Mn under both steady state (constant applied potential) and nonsteady state (potentiodynamic cyclic voltammetry, CV). Under steady state conditions, both Mn and Co valences decreased at lower potentials, suggesting the conversion from Mn-(III,IV) and Co(III) to Mn(II,III) and Co(II), respec-tively. Rapid X-ray data acquisition, combined with a slow sweep rate in CV, enabled a 3 mV resolution in the applied potential, approaching a nonsteady (potentiody-namic) state. Changes in the Co and Mn valence states were simultaneous and exhibited periodic patterns that tracked the cyclic potential sweeps. To the best of our knowledge, this represents the first study, using in situ XAS, to resolve the synergistic catalytic mechanism of a bimetallic oxide. Strategies developed/described herein can offer an enticing approach to unveil the reaction mechanism for other multimetallic electrocatalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Monitoring the Formation and Reactivity of Organometallic Alkane and Fluoroalkane Complexes with Silanes and Xe Using Time-Resolved X-ray Absorption Fine Structure Spectroscopy

Complexes with weakly coordinating ligands are often formed in chemical reactions and can play key roles in determining the reactivity, particularly in catalytic reactions. Using time-resolved X-ray absorption fine structure (XAFS) spectroscopy in combination with time-resolved IR (TRIR) spectroscopy and tungsten hexacarbonyl, W(CO) 6 , we are able to structurally characterize the formation of an organometallic alkane complex, determine the W–C distances, and monitor the reactivity with silane to form an organometallic silane complex. Experiments in perfluorosolvents doped with xenon afford initially the corresponding solvated complex, which is sufficiently reactive in the presence of Xe that we can then observe the coordination of Xe to the metal center, providing a unique insight into the metal–xenon bonding. These results offer a step toward elucidating the structure, bonding, and chemical reactivity of transient species by X-ray absorption spectroscopy, which has sensitivity to small structural changes. The XAFS results indicate that the bond lengths of metal–alkane (W–H–C) bond in W(CO) 5 (heptane) as 3.07 (±0.06) Å, which is longer than the calculated W–C (2.86 Å) for binding of the primary C–H, but shorter than the calculated W–C (3.12 Å) for the secondary C–H. Here, a statistical average of the calculated W–C alkane bond lengths is 3.02 Å, and comparison of this value indicates that the value derived from the XAFS measurements is averaged over coordination of all C–H bonds consistent with alkane chain walking. Photolysis of W(CO) 6 in the presence of HSiBu 3 allows the conversion of W(CO) 5 (heptane) to W(CO) 5 (HSiBu 3 ) with an estimated W–Si distance of 3.20 (±0.03) Å. Time-resolved TRIR and XAFS experiments following photolysis of W(CO) 6 in perfluoromethylcyclohexane (PFMCH) allows the characterization of W(CO) 5 (PFMCH) with a W–F distance of 2.65 (±0.06) Å, and doping PFMCH with Xe allows the characterization of W(CO) 5 Xe with a W–Xe bond length of 3.10 (±0.02) Å.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electron transport and visible light absorption in a plasmonic photocatalyst based on strontium niobate

Semiconductor compounds are widely used for photocatalytic hydrogen production applications, where photogenerated electron–hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr 1-x NbO 3 , 0.03$<$x$<$0.20) were reported to show competitive photocatalytic efficiencies under visible light, which was attributed to interband absorption. This discovery expanded the range of materials available for optimized performance as photocatalysts. Here we study epitaxial thin films of SrNbO 3+δ and find that their bandgaps are ~4.1 eV. Surprisingly, the carrier density of the conducting phase exceeds 10 22 cm -3 and the carrier mobility is only 2.47 cm 2 V -1 s -1 . Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the plasmon resonance, arising from the large carrier density. We propose that the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. Furthermore, high-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Mechanistic insight into copper cation exchange in cadmium selenide semiconductor nanocrystals using X-ray absorption spectroscopy

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The main processes governing CE can be explained by considering hard/soft acid/base theory, but the detailed mechanism of CE, however, has been debated and has been attributed to both diffusion and vacancy processes. In this work, we have performed in situ x-ray absorption spectroscopy to further understand the mechanism of the CE of copper in solution phase CdSe NCs. The x-ray data indicates clear isosbestic points, suggestive of cooperative behavior as previously observed via optical spectroscopy. Examination of the extended x-ray absorption fine structure data points to the observation of interstitial impurities during the initial stages of CE, suggesting the diffusion process is the fundamental mechanism of CE in this system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

X-ray transient absorption reveals the 1Au (nπ*) state of pyrazine in electronic relaxation

Abstract Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1 B 2u (ππ*) (S 2 ) and 1 B 3u (nπ*) (S 1 ) states, the participation of the optically dark 1 A u (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1 A u (nπ*) and 1 B 3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1 A u (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Extended X-ray absorption fine structure of dynamically-compressed copper up to 1 terapascal

Abstract Large laser facilities have recently enabled material characterization at the pressures of Earth and Super-Earth cores. However, the temperature of the compressed materials has been largely unknown, or solely relied on models and simulations, due to lack of diagnostics under these challenging conditions. Here, we report on temperature, density, pressure, and local structure of copper determined from extended x-ray absorption fine structure and velocimetry up to 1 Terapascal. These results nearly double the highest pressure at which extended x-ray absorption fine structure has been reported in any material. In this work, the copper temperature is unexpectedly found to be much higher than predicted when adjacent to diamond layer(s), demonstrating the important influence of the sample environment on the thermal state of materials; this effect may introduce additional temperature uncertainties in some previous experiments using diamond and provides new guidance for future experimental design.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Tunable band gaps and optical absorption properties of bent MoS2 nanoribbons

Abstract The large tunability of band gaps and optical absorptions of armchair MoS 2 nanoribbons of different widths under bending is studied using density functional theory and many-body perturbation GW and Bethe–Salpeter equation approaches. We find that there are three critical bending curvatures, and the non-edge and edge band gaps generally show a non-monotonic trend with bending. The non-degenerate edge gap splits show an oscillating feature with ribbon width n , with a period $$\Delta n=3$$ Δ n = 3 , due to quantum confinement effects. The complex strain patterns on the bent nanoribbons control the varying features of band structures and band gaps that result in varying exciton formations and optical properties. The binding energy and the spin singlet–triplet split of the exciton forming the lowest absorption peak generally decrease with bending curvatures. The large tunability of optical properties of bent MoS 2 nanoribbons is promising and will find applications in tunable optoelectronic nanodevices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Self-absorption correction on 2D X-ray fluorescence maps

Abstract X-ray fluorescence mapping (XRF) is a highly efficient and non-invasive technique for quantifying material composition with micro and nanoscale spatial resolutions. Quantitative XRF analysis, however, confronts challenges from the long-lasting problem called self-absorption. Moreover, correcting two-dimensional XRF mapping datasets is particularly difficult because it is an ill-posed inverse problem. Here we report a semi-empirical method that can effectively correct 2D XRF mapping data. The correction error is generally less than 10% from a comprehensive evaluation of the accuracy in various configurations. The proposed method was applied to quantify the composition distribution around the grain boundaries in an electrochemically corroded stainless steel sample. Highly localized Cr enrichment was found around the crack sites, which was invisible before the absorption correction.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Single-shot X-ray absorption spectroscopy at X-ray free electron lasers

X-ray Absorption Spectroscopy (XAS) is a widely used X-ray diagnostic method for studying electronic and structural properties of matter. At first glance, the relatively narrow bandwidth and the highly fluctuating spectral structure of X-ray Free Electron Lasers (XFEL) sources seem to require accumulation over many shots to achieve high data quality. To date the best approach to implementing XAS at XFEL facilities has been using monochromators to scan the photon energy across the desired spectral range. While this is possible for easily reproducible samples such as liquids, it is incompatible with many important systems. Here, we demonstrate collection of single-shot XAS spectra over 10s of eV using an XFEL source, with error bars of only a few percent. We additionally show how to extend this technique over wider spectral ranges towards Extended X-ray Absorption Fine Structure measurements, by concatenating a few tens of single-shot measurements. Our results pave the way for future XAS studies at XFELs, in particular those in the femtosecond regime. This advance is envisioned to be especially important for many transient processes that can only be initiated at lower repetition rates, for difficult to reproduce excitation conditions, or for rare samples, such as those encountered in high-energy density physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Measurement of coherent vibrational dynamics with X-ray Transient Absorption Spectroscopy simultaneously at the Carbon K- and Chlorine L 2,3 - edges

X-ray Transient Absorption Spectroscopy (XTAS) is a powerful probe for ultrafast molecular dynamics. The evolution of XTAS signal is controlled by the shapes of potential energy surfaces of the associated core-excited states, which are difficult to directly measure. Here, we study the vibrational dynamics of Raman activated CCl 4 with XTAS targeting the C 1s and Cl 2p electrons. The totally symmetric stretching mode leads to concerted elongation or contraction in bond lengths, which in turn induce an experimentally measurable red or blue shift in the X-ray absorption energies associated with inner-shell electron excitations to the valence antibonding levels. The ratios between slopes of different core-excited potential energy surfaces (CEPESs) thereby extracted agree very well with Restricted Open-Shell Kohn-Sham calculations. The other, asymmetric, modes do not measurably contribute to the XTAS signal. The results highlight the ability of XTAS to reveal coherent nuclear dynamics involving < 0.01 Å atomic displacements and also provide direct measurement of forces on CEPESs.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

High-throughput computational X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) is a widely-used materials characterization technique. In this work we present a database of computed XAS spectra, using the Green's formulation of the multiple scattering theory implemented in the FEFF code. With more than 500,000 K-edge X-ray absorption near edge (XANES) spectra for more than 40,000 unique materials, this database constitutes the largest existing collection of computed XAS spectra to date. The data is openly distributed via the Materials Project, enabling researchers across the world to access it for free and use it for comparisons with experiments and further analysis.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Unusual infrared absorption increases in photo-degraded organic films

Degradation is among the most pressing problems facing organic materials, occurring through ingress of moisture and oxygen, and light exposure. We determine the nanoscale pathways underlying degradation by light-soaking organic films in an environmental chamber, and performing infrared spectroscopy, to identify atomic bonding changes. We utilize as a prototype the low band gap PTB7-PCBM blend. Films light-soaked in the presence of oxygen show unusual increased absorption at 1727 cm –1 attributable to increased C=O modes, and a broad increase at 3240 cm –1 attributable to hydroxyl (O–H) groups bonded within the organic matrix. Films exposed to oxygen in the dark, or light-soaked in an inert atmosphere, do not exhibit significant absorption changes, suggesting simultaneous exposure of oxygen and light that creates singlet excited oxygen is the detrimental factor. Our ab initio electronic structure simulations interpret these by oxidation at the α-C site of the alkyl chains in PTB7, with an irreversible rupture of the alkyl chain and formation of new C=O and C–O–H conformations at the α-C. Infrared spectroscopy coupled with ab initio simulation can provide a powerful tool for quantifying photo-structural atomic bonding changes. As a result, understanding nanoscale light-induced structural changes will open avenues to designing more stable organic materials for organic electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection–absorption spectroscopy

Here we review the recent development of extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy. This method combines the benefits of X-ray absorption spectroscopy, such as element, oxidation, and spin state specificity, with surface sensitivity and ultrafast time resolution, having a probe depth of only a few nm and an instrument response less than 100 fs. Using this technique we investigated the ultrafast electron dynamics at a hematite (α-Fe 2 O 3 ) surface. Surface electron trapping and small polaron formation both occur in 660 fs following photoexcitation. These kinetics are independent of surface morphology indicating that electron trapping is not mediated by defects. Instead, small polaron formation is proposed as the likely driving force for surface electron trapping. We also show that in Fe 2 O 3 , Co 3 O 4 , and NiO, band gap excitation promotes electron transfer from O 2p valence band states to metal 3d conduction band states. In addition to detecting the photoexcited electron at the metal M 2,3 -edge, the valence band hole is directly observed as transient signal at the O L 1 -edge. The size of the resulting charge transfer exciton is on the order of a single metal–oxygen bond length. Spectral shifts at the O L 1 -edge correlate with metal–oxygen bond covalency, confirming the relationship between valence band hybridization and the overpotential for water oxidation. Furthermore, these examples demonstrate the unique ability to measure ultrafast electron dynamics with element and chemical state resolution using XUV-RA spectroscopy. Accordingly, this method is poised to play an important role to reveal chemical details of previously unseen surface electron dynamics.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

The stability, electronic structure, and optical absorption of boron-nitride diamondoids predicted with first-principles calculations

Although diamondoids are broadly studied for their fundamental properties and applications, boron-nitride-based diamondoids are scarcely explored.In this report we predict the stability, electronic structure, and optical absorption spectra of six boron-nitride (BN) diamondoids with first-principles methods based on pseudopotential density functional theory and many-body perturbation methods implemented with a real-space formalism. We find that four of them are thermodynamically stable at room temperature, while B10N8H24 and B6N4H16 show thermodynamic instability in molecular dynamics simulations. With the GW approximation, we predicted the ionization energies and electron affinities of BN-diamondoids and find that the evolution of the electronic structure with size does not follow the same trend as diamondoids, owing to the unbalanced numbers of boron and nitrogen atoms. We show strong photoabsorption of BN-triamantane and BN-adamantane in the infrared and visible ranges and analyze the features of low-energy absorption by examining the characteristics of related orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Hot Electron-driven Photocatalysis and Transient Absorption Spectroscopy in Plasmon Resonant Grating Structures

Plasmon resonant grating structures provide an effective platform for distinguishing between the effects of plasmon resonant excitation and bulk metal absorption via interband transitions. By simply rotating the polarization of the incident light, we can switch between resonant excitation and non-resonant excitation, while keeping all other parameters of the measurement constant. With light polarized perpendicular to the lines in the grating (i.e., TE-polarization), the photocatalytic reaction rate (i.e., photocurrent) is measured as the angle of the incident laser light is tuned through the resonance with the grating. Here, hot holes photoexcited in the metal are used to drive the oxygen evolution reaction (OER), producing a measurable photocurrent. Using TE-polarized light, we observe sharp peaks in the photocurrent and sharp dips in the photoreflectance at approximately 9° from normal incidence, which corresponds to the conditions under which there is good wavevector matching between the incident light and the lines in the grating. With light polarized parallel to the grating (i.e., TM), we excite the grating structure non-resonantly and there is no angular dependence in the photocurrent or photoreflectance. In order to quantify the lifetime of these hot carriers, we performed transient absorption spectroscopy of these plasmon resonant grating structures. Here, we observe one feature in the spectra corresponding to interband transitions and another feature associated with the plasmon resonant mode in the grating. Both features decay over a time scale of 1–2 ps. Finally, the spectral responses of grating structures fabricated with Ag, Al, and Cu are also presented.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗