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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Mechanisms of the Cu(I)-Catalyzed Intermolecular Photocycloaddition Reaction Revealed by Optical and X-ray Transient Absorption Spectroscopies

The [2 + 2] photocycloaddition provides a simple, single-step route to cyclobutane moieties that would otherwise be disfavored or impossible due to ring strain and/or steric interactions. We have used a combination of optical and X-ray transient absorption spectroscopies to elucidate the mechanism of the Cu(I)-catalyzed intermolecular photocycloaddition reaction using norbornene and cyclohexene as model substrates. We find that for norbornene the reaction proceeds through an initial metal-to-ligand charge transfer (MLCT) state that persists for 18 ns before the metal returns to the monovalent oxidation state. The Cu K-edge spectrum continues to evolve until ~5 μs and then remains unchanged for the 50 μs duration of the measurement, reflecting product formation and ligand dissociation. We hypothesize that the MLCT transition and reverse electron transfer serve to sensitize the triplet excited state of one of the norbornene ligands, which then dimerizes with the other to give the product. For the case of cyclohexene, however, we do not observe a charge transfer state following photoexcitation and instead find evidence for an increase in the metal–ligand bond strength that persists for several ns before product formation occurs. This is consistent with a mechanism in which ligand photoisomerization is the initial step, which was first proposed by Salomon and Kochi in 1974 to explain the stereoselectivity of the reaction. Furthermore, our investigation reveals how this photocatalytic reaction may be directed along strikingly disparate trajectories by only very minor changes to the structure of the substrate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Connecting Femtosecond Transient Absorption Microscopy with Spatially Coregistered Time Averaged Optical Imaging Modalities

Multimodal all-optical imaging involving coregistered femtosecond transient absorption microscopy (TAM), time-integrated photoluminescence (PL), and steady-state modalities such as confocal reflectance and transmission offers an appealing approach to gain a comprehensive understanding of complex electronic excited-state phenomena in spatially heterogeneous systems. A unique combination of these modalities allows us to unravel not only the competing electronic excited-state dynamical processes but also the underlying morphological information with simultaneous high temporal and spatial resolution. However, correlating the various images obtained from time-resolved and time-independent modalities is generally nontrivial and particularly challenging when the electronic dynamics under study evolve in both time and space. Here, we demonstrate a new approach for rationally correlating time-resolved microscopy with coregistered time-integrated or steady-state modalities. Specifically, our approach involves an extended global lifetime analysis of the time-resolved microscopic data set to separate distinct dynamical processes taking place on commensurate time scales, and the resulting decay-associated amplitude maps (DAAMs) were applied to explore correlations with the images acquired using time-independent modalities. The feasibility of our approach was validated through analyzing a multimodal data set acquired from a thin film of chloride-containing mixed lead halide perovskites (CH 3 NH 3 PbI 3–x Cl x ) using femtosecond transient absorption, time-integrated PL, and confocal reflectance microscopies. Analysis of the results obtained enable us to gain new insight into the complex ultrafast relaxation dynamics in this highly heterogeneous system.

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

X-ray Transient Absorption Studies of Exciton Self-Trapping

Here, we present optical pump/X-ray absorption probe studies to probe changes in electronic distribution and local structure upon formation of a self-trapped exciton. The experiments were carried out on the mixed-valence halide-bridged transition metal linear chain charge density wave material [Pt(en 2 )][Pt(en 2 )Cl 2 ]·(ClO 4 ) 4 . Photoinduced changes in the Pt L III XANES spectrum were measured on a picosecond time scale following generation of excitons via optical excitation of the intervalence charge transfer transition. Ab initio FEFF9 modeling of the spectra and the associated angular momentum projected density of states was used to interpret the response. We find photoinduced changes reflecting electronic redistribution and local lattice distortions that correspond to strong localization of the self-trapped state.

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

Two-photon absorption in quantum dot semiconductor optical amplifiers-based all-optical XOR gate at 2 Tb/s

The nonlinear effect of two-photon absorption (TPA) on the performance of all-optical XOR gate using quantum dot semiconductor optical amplifiers (QDSOAs)-assisted Mach–Zehnder interferometer is numerically analyzed and investigated at a data rate of 2 Tb/s. The dependence of the quality factor (QF) on the critical parameters of the input signal and QDSOA is examined and assessed. The obtained QFs for the all-optical XOR gate using QDSOAs-MZI at 2 Tb/s with TPA and without TPA are 14 and 8, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Decoupling absorption and radiative cooling in mid-wave infrared bolometric elements

We present a spectrally selective, passively cooled mid-wave infrared bolometric absorber engineered to spatially and spectrally decouple infrared absorption and thermal emission. The structure leverages an antenna-coupled metal–insulator–metal resonance for mid-wave infrared normal incidence photon absorption and a long-wave infrared optical phonon absorption feature, aligned closer to peak room temperature thermal emission. The phonon-mediated resonant absorption enables a strong long-wave infrared thermal emission feature limited to grazing angles, leaving the mid-wave infrared absorption feature undisturbed. The two independently controlled absorption/emission phenomena demonstrate decoupling of the photon detection mechanism from radiative cooling and offer a new design approach enabling ultra-thin, passively cooled mid-wave infrared bolometers.

Ware, Alexander (ORCID:0009000256937727)↗
NASA NTRS2005

X-Ray Absorbed, Broad-Lined, Red AGN and the Cosmic X-ray Background

This award represents the second phase of an X-ray study of QSOs that are heavily obscured in the optical/near-IR. An earlier survey revealed that these active galactic nuclei (AGN) are also typically strongly absorbed at high photon energies, but the enhanced sensitivity of XMM-Newton provided for the first time the opportunity to measure the spectral indices of individual sources and to test the possibility that obscured AGN are responsible for a substantial portion of the cosmic X-ray background (CXRB). The new observations confirm and greatly extend the earlier results. Substantial intervening absorbing material is detected for 3 of the sources, and 2 additional targets show hard power-law continua. In addition, soft X-ray excesses are detected in 3 of sources, indicating the presence of extended regions of ionized gas. This component is-particularly well-defined in the Type 1 source 2MASS2344+1221, allowing analysis and modeling at a level of detail unusual for such distant sources. An important finding that parallels conclusions from optical polarimetry is the lack of dependence of absorption on optical spectral class (Type 1 or 2). The combination of strong polarization and X-ray absorption in sources that show strong, broad emission lines indicates either a very small (nuclear) absorber or a favored viewing angle which allows the X-ray source to be covered but the surrounding broad emission-line region to be largely exposed. The observed spectral also indicate that obscured AGN of both Type 1 and 2 contribute significantly to the CXRB at higher X-ray energies.

Schmidt, Gary D.↗
DOE OSTI2016

THE ERUPTION OF THE CANDIDATE YOUNG STAR ASASSN-15QI

Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star–disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The ∼3.5 mag brightening in the V band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from ∼10,000 K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km s{sup −1}. The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10–20 days. Fluorescent excitation of H{sub 2} is detected in emission from vibrational levels as high as v = 11, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, though the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2018

Correcting for Trace Gas Absorption When Retrieving Aerosol Optical Depth from Satellite Observations of Reflected Shortwave Radiation

Retrieving aerosol optical depth (AOD) from top-of-atmosphere (TOA) satellite measured radiance requires separating the aerosol signal from the total observed signal. Total TOA radiance includes signal from underlying surface and from atmospheric constituents such as aerosols, clouds and gases. Multispectral retrieval algorithms, such as the dark-target (DT) algorithm that operates upon Moderate Resolution Imaging Spectroradiometer (MODIS, onboard Terra and Aqua satellites) and Visible Infrared Imaging Radiometer Suite (VIIRS, onboard Suomi-NPP) sensors, use wavelength bands in "window" regions. However, while small, the gas absorptions in these bands are non-negligible and require correction. In this paper we use High-resolution TRANsmission (HITRAN) database and Line-by-Line Radiative Transfer Model (LBLRTM) to derive consistent gas corrections for both MODIS and VIIRS wavelength bands. Absorptions from H2O, CO2 and O3 are considered, as well as other trace gases. Even though MODIS and VIIRS bands are "similar", they are different enough that applying MODIS specific gas corrections to VIIRS observations results in an underestimate of global mean AOD (by 0.01), but with much larger regional AOD biases up to 0.07. As recent studies are attempting to create a long-term data record by joining multiple satellite datasets, including MODIS and VIIRS, the consistency of gas correction becomes even more crucial.

Satellite Remote Sensing↗
DOE OSTI2021

Laser absorption measurements of electron density in nanosecond-scale atmospheric pressure pulsed plasmas

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as n e = 7 x 10 19 cm -3 , and decay to 1/e of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[ 5 S o ]-3p[ 5 P] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Energy Transfer from Antenna Ligand to Europium(III) Followed Using Ultrafast Optical and X-ray Spectroscopy

A series of highly luminescent europium(III) complexes which exhibit photoluminescence from the Eu(III) center following energy transfer from the UV absorbing organic sensitizer have been investigated using a combination of ultrafast optical transient absorption and Eu L3 X-ray transient absorption techniques. We have previously demon- strated that the latter can be used as a signature of 4f–4f excitation responsible for the photoluminescence in these Eu(III) coordination complexes, but the long time scale of the earlier measurements did not allow direct observation of the ligand-to-metal energy transfer step, preventing a determi- nation of the sensitization mechanism. Here, we provide the first direct experimental verification that Dexter electron exchange from the ligand triplet state is the dominant energy transfer mechanism in these photoluminescent systems. Moreover, the optical transient absorption results obtained herein imply that energy transfer for all three compounds has near unity yield, regardless of differences in the sensitization efficiencies, suggesting that the variations in the sensitization efficiencies are determined almost entirely by differences in the ligand-centered intersystem crossing rates. As a result, the implications for the rational design of more effective photoluminescent lanthanide complexes are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Platform for probing radiation transport properties of hydrogen at conditions found in the deep interiors of red dwarfs

We describe an experimental concept at the National Ignition Facility for specifically tailored spherical implosions to compress hydrogen to extreme densities (up to ∼800× solid density, electron number density ne∼4×1025 cm−3) at moderate temperatures (T∼200 eV), i.e., to conditions, which are relevant to the interiors of red dwarf stars. The dense plasma will be probed by laser-generated x-ray radiation of different photon energy to determine the plasma opacity due to collisional (free–free) absorption and Thomson scattering. The obtained results will benchmark radiation transport models, which in the case for free–free absorption show strong deviations at conditions relevant to red dwarfs. This very first experimental test of free–free opacity models at these extreme states will help to constrain where inside those celestial objects energy transport is dominated by radiation or convection. Moreover, our study will inform models for other important processes in dense plasmas, which are based on electron–ion collisions, e.g., stopping of swift ions or electron–ion temperature relaxation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS

Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination with a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Electron Mobility and Trapping in Ferrihydrite Nanoparticles

Iron is the most abundant transition metal in the Earth's crust, and naturally occurring iron oxide minerals play a commanding role in environmental redox reactions. Although iron oxide redox reactions are well studied, their precise mechanisms are not fully understood. Recent work has shown that these involve electron transfer pathways within the solid, suggesting that overall reaction rates could be dependent on electron mobility. Initial ultrafast spectroscopy studies of iron oxide nanoparticles sensitized by fluorescein derivatives supported a model for electron mobility based on polaronic hopping of electron charge carriers between iron sites, but the constitutive relationships between hopping mobilities and interfacial charge transfer processes has remained obscured. We developed a coarse-grained lattice Monte Carlo model to simulate the collective mobilities and lifetimes of these photoinjected electrons with respect to recombination with adsorbed dye molecules for the essential nanophase ferrihydrite, and tested predictions made by the simulations using pump-probe spectroscopy. We acquired optical transient absorption spectra as a function of particle size and under a variety of solution conditions, and used cryogenic transmission electron microscopy to determine the aggregation state of the nanoparticles. We observed biphasic electron recombination kinetics over timescales that spanned picoseconds to microseconds, the slower regime of which was fit with a stretched exponential decay function. The recombination rates were weakly affected by nanoparticle size and aggregation state, suspension pH, and the injection of multiple electrons per nanoparticle. We conclude that electron mobility indeed limits the rate of interfacial electron transfer in these systems with the slowest processes relating to escape from deep traps, the presence of which outweighs the influence of environmental factors such as pH-dependent surface charge.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

The multi-optics high-resolution absorption x-ray spectrometer (HiRAXS) for studies of materials under extreme conditions

We report the development of a high-resolution spectrometer for extended x-ray absorption fine structure (EXAFS) studies of materials under extreme conditions. A curved crystal and detector in the spectrometer are replaceable such that a single body is employed to perform EXAFS measurements at different x-ray energy intervals of interest. Two configurations have been implemented using toroidal crystals with Ge 311 reflection set to provide EXAFS at the Cu K-edge (energy range 8.9–9.8 keV) and Ge 400 reflection set to provide EXAFS at the Ta L3-edge (9.8–10.7 keV). Key performance characteristics of the spectrometer were found to be consistent with design parameters. The data generated at the National Ignition Facility have shown an ≃3 eV spectral resolution for the Cu K-edge configuration and ≃6 eV for the Ta L3-edge configuration.

Stoupin, S. (ORCID:0000000226225270)↗
DOE OSTI2017

Angle-Selective Reflective Filters for Exclusion of Background Thermal Emission

Selective filtering of spectral and angular optical transmission has recently attracted a great deal of interest. While optical passband and stop-band spectral filters are already widely used, angle-selective transmission and reflection filtering represents a less than fully explored alternative. Nonetheless, this approach can be promising for several applications, including stray radiation minimization and background emission exclusion. In this work, a concept for angle-selective reflection filtering using guided-mode resonance coupling is proposed. Although guided-mode resonance structures are already used for spectral filtering, in this work, a variation of angle-selective reflection filtering using guided-mode resonance coupling is proposed. We investigate angle-dependent properties of such structures for potential use as angle-selective reflective filters. Furthermore, we utilize interference between diffraction modes to provide tunable selectivity with a sufficient angular width. Combining these structures with thermal emitters can exclude selected emission angles for spatially selective thermal emissivity reduction toward sensitive targets, as well as directionally selective emissivity exclusion for suppression of solar heating. We show a very large selective reduction of heat exchange by 99.77% between an engineered emitter and a distant receiver using just a single-groove grating and an emitting substrate in the emitter’s side. Also, we show a selective reduction of heat exchange by approximately 77% between an emitter covered by engineered sets of angle-selective reflective filters and a nearby sensitive target. The suggested angle-selective structure may have applications in excluding background thermal radiation, in particular, thermal emission reduction for daytime radiative cooling, sensitive IR telescope detectors, and high-fidelity thermoluminescent spectroscopy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1975

Optical atmospheric scattering and absorption limitations on offset pointing from Earth Observatory Satellite /EOS/ sensors

The Braslau-Dave atmospheric model which calculates the upward monochromatic light fluxes leaving the top of the atmosphere as a function of viewing angle, sun angle, and ground reflectance was employed to study the effect of atmospheric scattering and attenuation on universal apparent contrast for two EOS remote sensors operated at very large offset or pointing angles: the Thematic Mapper (TM) and the High Resolution Pointable Imager (HRPI). The TM offset off nadir could be plus or minus 20 degrees with an 11 degree scan angle and the HRPI pointing angle off nadir could be plus or minus 45 degrees with a 3 degree scan angle. The reduction of universal apparent contrast of EOS imagery is studied as a function of sun elevation angle, atmospheric aerosol loading, radiation wavelength and sensor look angles.

Egan, W. G.↗