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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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1,638 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

The ultrafast Kerr effect in anisotropic and dispersive media

The ultrafast optical Kerr effect (OKE) is widely used to investigate the structural dynamics and interactions of liquids, solutions, and solids by observing their intrinsic nonlinear temporal responses through nearly collinear four-wave mixing. Non-degenerate mixing schemes allow for background free detection and can provide information on the interplay between a material’s internal degrees of freedom. Here, we show a source of temporal dynamics in the OKE signal that is not reflective of the internal degrees of freedom but arises from a group index and momentum mismatch. It is observed in two-color experiments on condensed media with sizable spectral dispersion, a common property near an optical resonance. In particular, birefringence in crystalline solids is able to entirely change the character of the OKE signal via the off-diagonal tensor elements of the nonlinear susceptibility. We develop a detailed description of the phase-mismatched ultrafast OKE and show how to extract quantitative information on the spectrally resolved birefringence and group index from time-resolved experiments in one and two dimensions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Lessons from combined experimental and theoretical examination of the FTIR and 2D-IR spectroelectrochemistry of the amide I region of cytochrome c</em

Amide I difference spectroscopy is widely used to investigate protein function and structure changes. In this article, we show that the common approach of assigning features in amide I difference signals to distinct secondary structure elements in many cases may not be justified. Evidence comes from Fourier transform infrared (FTIR) and 2D-IR spectroelectrochemistry of the protein cytochrome c in the amide I range, in combination with computational spectroscopy based on molecular dynamics (MD) simulations. This combination reveals that each secondary structure unit, such as an alpha-helix or a beta-sheet, exhibits broad overlapping contributions, usually spanning a large part of the amide I region, which in the case of difference absorption experiments (such as in FTIR spectroelectrochemistry) may lead to intensity-compensating and even sign-changing contributions. We use cytochrome c as the test case, as this small electron-transferring redox-active protein contains different kinds of secondary structure units. Upon switching its redox-state, the protein exhibits a different charge distribution while largely retaining its structural scaffold. Our theoretical analysis suggests that the change in charge distribution contributes to the spectral changes and that structural changes are small. However, in order to confidently interpret FTIR amide I difference signals in cytochrome c and proteins in general, MD simulations in combination with additional experimental approaches such as isotope labeling, the insertion of infrared labels to selectively probe local structural elements will be required. In case these data are not available, a critical assessment of previous interpretations of protein amide I 1D- and 2D-IR difference spectroscopy data is warranted.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Transparent window 2D IR spectroscopy of proteins

Proteins are complex, heterogeneous macromolecules that exist as ensembles of interconverting states on a complex energy landscape. A complete, molecular-level understanding of their function requires experimental tools to characterize them with high spatial and temporal precision. Infrared (IR) spectroscopy has an inherently fast time scale that can capture all states and their dynamics with, in principle, bond-specific spatial resolution. Two-dimensional (2D) IR methods that provide richer information are becoming more routine but remain challenging to apply to proteins. Spectral congestion typically prevents selective investigation of native vibrations; however, the problem can be overcome by site-specific introduction of amino acid side chains that have vibrational groups with frequencies in the “transparent window” of protein spectra. This Perspective provides an overview of the history and recent progress in the development of transparent window 2D IR of proteins.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Broadband 2DES detection of vibrational coherence in the S x state of canthaxanthin

The nonadiabatic mechanism that mediates nonradiative decay of the bright S 2 state to the dark S 1 state of carotenoids involves population of a bridging intermediate state, S x , in several examples. The nature of S x remains to be determined definitively, but it has been recently suggested that S x corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S 2 potential surface. Here, the electronic and vibrational dynamics accompanying the formation of S x in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of S x in <20 fs following excitation of the main absorption band. S x is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN’s isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that S x undergoes displacements from the Franck–Condon S 2 state along out-of-plane coordinates as it passes to the S 1 state. The results are consistent with the conclusion that CAN’s carbonyl-substituted β-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from S x to S 1 compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S 1 state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the S x state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Excited-state vibronic coherences and intramolecular charge transfer dynamics of the photoinactive cyanobacteriochrome NpF2164g5

We have characterized the excited-state structural dynamics that follow optical excitation of the photoinactive cyanobacteriochrome (CBCR) NpF2164g5 to determine the first events along the photoisomerization reaction coordinate of the phycocyanobilin (PCB) chromophore in red-to-green photoactive CBCRs. Within 25 fs of photoexcitation to the first excited singlet state, S1, a cross peak begins to develop below the diagonal of the broadband two-dimensional electronic spectrum (2DES) owing to the formation of a twisted conformation of the PCB chromophore with an enhanced intramolecular charge-transfer (ICT) character. Excited-state coherent wavepacket motions, including a torsion of the methine bridge between rings C and D and the carbon–hydrogen out-of-plane (HOOP) wagging vibration, are rapidly damped as the cross peak forms. This finding supports an assignment of the torsional and HOOP modes to the reaction coordinate of a coherent nonadiabatic mechanism. The ICT process is accompanied by an ultrafast Stokes shift and rapidly damped oscillations at the torsional mode’s frequency, which are sensed by measurements of the energy gap of the cross peak using its first moment with respect to the detection frequency axis of the 2DES spectrum. Furthermore, these results have the further implication that polar side chains of nearby amino acid residues in the binding site of the PCB chromophore can manipulate the barrier height for photoisomerization in red-to-green photoactive CBCRs and in phytochromes by redistributing the π-electron density along the methine bridge between the C and D rings.

Charge transfer↗
DOE OSTI2018

Primary processes in the bacterial reaction center probed by two-dimensional electronic spectroscopy

In the initial steps of photosynthesis, reaction centers convert solar energy to stable charge-separated states with near-unity quantum efficiency. The reaction center from purple bacteria remains an important model system for probing the structure–function relationship and understanding mechanisms of photosynthetic charge separation. Here we perform 2D electronic spectroscopy (2DES) on bacterial reaction centers (BRCs) from two mutants of the purple bacterium Rhodobacter capsulatus , spanning the Q y absorption bands of the BRC. We analyze the 2DES data using a multiexcitation global-fitting approach that employs a common set of basis spectra for all excitation frequencies, incorporating inputs from the linear absorption spectrum and the BRC structure. We extract the exciton energies, resolving the previously hidden upper exciton state of the special pair. We show that the time-dependent 2DES data are well-represented by a two-step sequential reaction scheme in which charge separation proceeds from the excited state of the special pair (P*) to P + H A − via the intermediate P + B A − . When inhomogeneous broadening and Stark shifts of the B* band are taken into account we can adequately describe the 2DES data without the need to introduce a second charge-separation pathway originating from the excited state of the monomeric bacteriochlorophyll B A *.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Decoding ultrafast polarization responses in lead halide perovskites by the two-dimensional optical Kerr effect

Significance Optoelectronic devices, such as photovoltaics or light-emitting diodes, are based on the interaction of light with charges in the active material. Lead halide perovskites (LHPs) have emerged as excellent materials for optoelectronics, but mysteries remain as to how these materials interact with light. Here, we develop a spectroscopic method to observe the nearly instantaneous responses of LHPs to propagating light fields. This method has allowed us to precisely trace a nonlinear material response resulting from the mixing of different light components overtaking each other during propagation. The resulting oscillatory signals could be easily confused with collective motions of ions or molecules, but are quantitatively accounted by strong nonlinear mixing and polarization-dressed light propagation in single-crystal LHPs.

14 SOLAR ENERGY↗
DOE OSTI2018

Characterization of the state of nanoparticle aggregation in non-equilibrium plasma synthesis systems

Non-equilibrium plasmas offer a unique environment for nanoparticle synthesis. Particles are homogeneously nucleated and grow at near room temperature as a result of non-thermal decomposition of vapor precursors by electrons and other plasma-excited species. Despite their widespread use, several features regarding particle growth in these systems remain poorly understood. In particular, particle aggregation (the formation of non-spherical entities composed of primary particles) is assumed to be negligible because of unipolar particle charging and subsequent Coulombic repulsion, which would hinder collisional growth. Here, we apply ion mobility-mass spectrometry (IM-MS) to a non-thermal, atmospheric pressure DC microplasma to study the state of aggregation of as-synthesized nanoparticles. Under all examined synthesis conditions, we find the presence of highly branched, chain-like aggregates at the reactor outlet, with a primary particle radius below 10 nm that is relatively insensitive to synthesis conditions. The aggregates are polydisperse, with mean masses and mobility diameters increasing with both increasing precursor concentration and increasing flow residence time within the system. TEM structural characterization shows that the aggregates can be described by a quasifractal model, with a fractal dimensions in the 1.6–2.0 range. The mass-mobility relationship inferred from IM-MS and TEM agrees well with Langevin dynamics simulations where coulomb interactions are not considered. We suggest that particle aggregation occurs either in the plasma volume due to the scavenging of smaller neutral or positively charged particles by growing aggregates or outside the reactor where the plasma density is lower and electrons are not available to maintain high levels of unipolar charge. Furthermore, the methods applied here additionally demonstrate the potential of IM-MS and TEM structural characterization in analyzing gas-phase nanoparticle production processes.

42 ENGINEERING↗
DOE OSTI2016

Problematic projection to the in-sample subspace for a kernelized anomaly detector

We examine the properties and performance of kernelized anomaly detectors, with an emphasis on the Mahalanobis-distance-based kernel RX (KRX) algorithm. Although the detector generally performs well for high-bandwidth Gaussian kernels, it exhibits problematic (in some cases, catastrophic) performance for distances that are large compared to the bandwidth. By comparing KRX to two other anomaly detectors, we can trace the problem to a projection in feature space, which arises when a pseudoinverse is used on the covariance matrix in that feature space. Here, we show that a regularized variant of KRX overcomes this difficulty and achieves superior performance over a wide range of bandwidths.

58 GEOSCIENCES↗
DOE OSTI2023

Charge separation in the photosystem II reaction center resolved by multispectral two-dimensional electronic spectroscopy

The photosystem II reaction center (PSII RC) performs the primary energy conversion steps of oxygenic photosynthesis. While the PSII RC has been studied extensively, the similar time scales of energy transfer and charge separation and the severely overlapping pigment transitions in the Qy region have led to multiple models of its charge separation mechanism and excitonic structure. Here, we combine two-dimensional electronic spectroscopy (2DES) with a continuum probe and two-dimensional electronic vibrational spectroscopy (2DEV) to study the cyt b559-D1D2 PSII RC at 77 K. This multispectral combination correlates the overlapping Qy excitons with distinct anion and pigment-specific Q x and mid-infrared transitions to resolve the charge separation mechanism and excitonic structure. Through extensive simultaneous analysis of the multispectral 2D data, we find that charge separation proceeds on multiple time scales from a delocalized excited state via a single pathway in which Pheo D1 is the primary electron acceptor, while Chl D1 and P D1 act in concert as the primary electron donor.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

Neutron diffraction: a primer

Because of the neutron’s special properties, neutron diffraction may be considered one of the most powerful techniques for structure determination of crystalline and related matter. Neutrons can be released from nuclear fission, from spallation processes, and also from low-energy nuclear reactions, and they can then be used in powder, time-of-flight, texture, single crystal, and other techniques, all of which are perfectly suited to clarify crystal and magnetic structures. With high neutron flux and sufficient brilliance, neutron diffraction also excels for diffuse scattering, for in situ and operando studies as well as for high-pressure experiments of today’s materials. For these, the wave-like neutron’s infinite advantage (isotope specific, magnetic) is crucial to answering important scientific questions, for example, on the structure and dynamics of light atoms in energy conversion and storage materials, magnetic matter, or protein structures. In this primer, we summarize the current state of neutron diffraction (and how it came to be), but also look at recent advances and new ideas, e.g., the design of new instruments, and what follows from that.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Two-Dimensional Electronic Spectroscopies for Probing Electronic Structure and Charge Transfer: Applications to Photosystem II

Photosystem II (PSII) is the only known natural enzyme that uses solar energy to split water, making the elucidation of its design principles critical for our fundamental understanding of photosynthesis and for our ability to mimic PSII’s remarkable properties. This report discusses progress towards addressing key open questions about the PSII RC. It describes new spectroscopic methods that were developed to answer these questions, and summarizes the outcomes of applying these methods to study the PSII RC. Using 2D electronic spectroscopy and 2D electronic Stark spectroscopy, models for the PSII RC were tested and refined. Work is ongoing to use the collected data to elucidate the charge separation mechanism in the PSII RC. Coherent dynamics were also observed in the PSII RC for the first time. Through extensive characterization and modeling we have assigned these coherences as vibronic in nature, and believe that they reflect resonances between key vibrational pigment modes and electronic energy gaps that may facilitate charge separation. Work is ongoing to definitively test the functional relevance of electronic-vibrational resonances.

14 SOLAR ENERGY↗
NASA NTRS2012

A Localized Ensemble Kalman Smoother

Numerous geophysical inverse problems prove difficult because the available measurements are indirectly related to the underlying unknown dynamic state and the physics governing the system may involve imperfect models or unobserved parameters. Data assimilation addresses these difficulties by combining the measurements and physical knowledge. The main challenge in such problems usually involves their high dimensionality and the standard statistical methods prove computationally intractable. This paper develops and addresses the theoretical convergence of a new high-dimensional Monte-Carlo approach called the localized ensemble Kalman smoother.

recursive estimation↗
NASA NTRS2011

Resolution Assessment in Dynamic Image Formation

Remote sensing and astronomical image formation is often complicated by deficiencies in measurement quality, density, or diversity. Penalized likelihood methods can incorporate additional first-principles physical prior knowledge and improve the image reconstructions, but a systematic bias is unavoidable as a consequence. This work derives theory to understand the bias and develops a computational tool to probe its effect on the reconstructed image and bound resolution limits. Though the focus is on image formation, the contributions of this paper apply to any inference problem that can be expressed under the linear state-space signal model.

multidimensional signal processing↗
NASA NTRS1994

VLSI Neuroprocessors

Electronic and optoelectronic hardware implementations of highly parallel computing architectures address several ill-defined and/or computation-intensive problems not easily solved by conventional computing techniques.

computing↗