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At least 235 records · Page 13

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NASA NTRS1992

X-ray emission-line spectra of photoionized plasmas - Density sensitivity of the Fe L-shell series

The circumsource environments of accretion-powered X-ray sources are likely to support relatively dense (greater than 10 exp 11/cu cm) photoionized X-ray emission-line regions. The Fe L-shell ions provide a versatile class of plasma diagnostics in this regime, their multielectron structures resulting in diverse spectral phenomena. Attention is given to the spectral response of Fe L-shell ions to variations in electron density over the range 10 exp 11 to 10 exp 16/cu cm. It is found that density-sensitive line ratios exist in the wavelength interval 12-17 A for the ions Fe XVII-XXI. The prominent role of radiative recombination in the population kinetics distinguishes the density-sensitive Fe lines in photoionized plasmas from those which operate in coronal equilibrium plasmas. The results of detailed atomic modeling of these ions are presented and applications to spectroscopic observations of accretion-driven X-ray sources are discussed.

Liedahl, Duane A.↗

DOE OSTI2022

Advanced Pt-Based Core–Shell Electrocatalysts for Fuel Cell Cathodes

Proton-exchange membrane fuel cells (PEMFCs) are highly efficient energy storage and conversion devices. Thus, the platinum group metal (PGM)-based catalysts which are the dominant choice for the PEMFCs have received extensive interest during the past couple of decades. However, the drawbacks in the existing PGM-based catalysts (i.e., high cost, slow kinetics, poor stability, etc.) still limit their applications in fuel cells. The Pt-based core–shell catalysts potentially alleviate these issues through the low Pt loading with the associated low cost and the high corrosion resistance and further improve the oxygen reduction reaction’s (ORR’s) activity and stability. This Account focuses on the synthetic strategies, catalytic mechanisms, factors influencing enhanced ORR performance, and applications in PEMFCs for the Pt-based core–shell catalysts. We first highlight the synthetic strategies for Pt-based core–shell catalysts including the galvanic displacement of an underpotentially deposited non-noble metal monolayer, thermal annealing, and dealloying methods, which can be scaled-up to meet the requirements of fuel cell operations. Subsequently, catalytic mechanisms such as the self-healing mechanism in the Pt monolayer on Pd core catalysts, the pinning effect of nitrogen (N) dopants in N-doped PtNi core–shell catalysts, and the ligand effect of the ordered intermetallic structure in L10-Pt/CoPt core–shell catalysts and their synergistic effects in N-doped L10-PtNi catalysts are described in detail. Additionally, the core–shell structure in the Pt-based catalysts have two main effects for enhanced ORR performance: (i) the interaction between Pt shells and core substrates can tune the electronic state of the surface Pt, thus boosting the ORR activity and stability, and (ii) the outer Pt shell with modest thickness can enhance the oxidation and dissolution resistance of the core, resulting in improved durability. We then review the recent attempts to optimize the ORR performance of the Pt-based core–shell catalysts by considering the shape, composition, surface orientation, and shell thickness. The factors influencing the ORR performance can be grouped into two categories: the effect of the core and the effect of the shell. In the former, PtM core–shell catalysts which use different non-PGM element cores (M) are summarized, and in the latter, Pt-based core–shell catalysts with different shell structures and compositions are described. The modifications of the core and/or shell structure can not only optimize the intermediate-binding energetics on the Pt surface through tuning the strain of the surface Pt, which increases the intrinsic activity and stability, but also offer a significantly decreased catalyst cost. Finally, we discuss the membrane electrode assembly performance of Pt-based core–shell catalysts in fuel cell cathodes and evaluate their potential in real PEMFCs for light-duty and heavy-duty vehicle applications. Even though some challenges to the activity and lifetime in the fuel cells remain, the Pt-based core–shell catalysts are expected to be promising for many practical PEMFC applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Hetero-site Double Core Ionization Energies with Sub-electronvolt Accuracy from Delta-Coupled-Cluster Calculations

Benchmark scalar-relativistic delta-coupled-cluster calculations of hetero-site double core ionization energies of small molecules containing second-row elements are reported. The present study has focused on the high-spin triplet components of two-site double core-ionized states, which are single reference in character and consistent with the use of standard coupledcluster methods. Contributions to computed double core ionization energies from electron-correlation and basis-set effects as well as corrections to the core-valence separation approximation have been analyzed. Furthermore, on the basis of systematic convergence of computational results with respect to these effects, delta-coupled-cluster calculations have been shown to be capable of providing accurate double core ionization energies with remaining errors estimated to be below 0.3 eV, and thus they are recommended for use to facilitate experimental studies of two-site double core-ionized states that are involved in X-ray pump/X-ray probe studies of electronic and molecular dynamics following inner shell ionization or excitation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1994

Radial diffusion models of energetic electrons and Jupiter's synchrotron radiation. 2: Time variability

We used a radial diffusion code for energetic electrons in Jupiter's magnetosphere to investigate variations in Jupiter's radio emission due to changes in the electron phase space density at L shells between 6 and 50, and due to changes in the radial diffusion parameters. We suggest that the observed variations in Jupiter's radio emission are likely caused by changes in the electron phase space density at some boundary L(sub 1) is greater than 6, if the primary mode of transport of energetic electrons is radial diffusion driven by fluctuating electric and/or magnetic fields induced by upper atmospheric turbulence. We noticed an excellent empirical correlation, both in phase and relative amplitude, between changes in the solar wind ram pressure and Jupiter's synchrotron radiation if the electron phase space density at the boundary L(sub 1) (L(sub 1) is approximately equal to 20-50) varies linearly with the square root of the solar wind ram pressure, f is approximately (N(sub s)nu(exp 2 sub s))(exp 1/2). The calculations were carried out with a diffusion coefficient D(sub LL) = D(sub n)L(exp n) with n = 3. The diffusion coefficient which best fit the observed variations in Jupiter's synchrotron radiation D(sub 3) = 1.3 +/- 0.2 x 10(exp -9)/s is approximately 0.041/yr, which corresponds to a lagtime of approximately 2 years. We further show that the observed short term (days-weeks) variations in Jupiter's radio emission cannot be explained adequately when radial diffusion is taken into account.

De Pater, Imke↗

DOE OSTI2024

SN 2021adxl: A luminous nearby interacting supernova in an extremely low-metallicity environment

SN 2021adxl is a slowly evolving, luminous, Type IIn supernova with asymmetric emission line profiles, similar to the well-studied SN 2010jl. We present extensive optical, near-ultraviolet, and near-infrared photometry and spectroscopy covering ∼1.5 years post discovery. SN 2021adxl occurred in an unusual environment, atop a vigorously star-forming region that is offset from its host galaxy core. The appearance of Ly α and O II , as well as the compact core, would classify the host of SN 2021adxl as a “Blueberry” galaxy, analogous to higher redshift, low-metallicity, star-forming dwarf “Green Pea” galaxies. Using several abundance indicators, we find a metallicity of the explosion environment of only ∼0.1 Z ⊙ , the lowest reported metallicity for a Type IIn SN environment. SN 2021adxl reaches a peak magnitude of M r ≈ −20.2 mag and since discovery, SN 2021adxl has faded by only ∼4 magnitudes in the r band with a cumulative radiated energy of ∼1.5 × 10 50 erg over 18 months. SN 2021adxl shows strong signs of interaction with a complex circumstellar medium, seen by the detection of X-rays, revealed by the detection of coronal emission lines, and through multi-component hydrogen and helium profiles. In order to further understand this interaction, we model the H α profile using a Monte Carlo electron scattering code. The blueshifted high-velocity component is consistent with emission from a radially thin spherical shell resulting in the broad emission components due to electron scattering. Using the velocity evolution of this emitting shell, we find that the SN ejecta collide with circumstellar material of at least ∼5 M ⊙ assuming a steady-state mass-loss rate of ∼4 − 6 × 10 −3 M ⊙ yr −1 for the first ∼200 days of evolution. SN 2021adxl was last observed to be slowly declining at ∼0.01 mag d −1 , and if this trend continues, SN 2021adxl will remain observable after its current solar conjunction. Continuing the observations of SN 2021adxl may reveal signatures of dust formation or an infrared excess, similar to that seen for SN 2010jl.

Brennan, S. J. (ORCID:0000000313256235)↗

DOE OSTI2024

On‐Surface Synthesis and Determination of the Open‐Shell Singlet Ground State of Tridecacene**

Abstract The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open‐shell character has so far been missing. Here, we present the on‐surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many‐body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet‐triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

Chemistry↗

NASA NTRS2006

The Spectropolarimetric Evolution of V838 Monocerotis

I review photo-polarimetric and spectropolarimetric observations of V838 Mon, which revealed that it had an asymmetrical inner circumstellar envelope following its 2nd photometric outburst. Electron scattering, modified by pre- or post-scattering H absorption, is the polarizing mechanism in V838 Mon's envelope. The simplest geometry implied by these observations is that of a spheroidal shell, flattened by at least 10% and having a projected position angle on the sky of approx.37deg. Analysis of V838 Mon's polarized flux reveals that this electron scattering shell lies interior to the envelope region in which Ha and Ca I1 triplet emission originates. To date, none of the theoretical models proposed for V838 Mon have demonstrated that they can reproduce the evolution of V838 Mon's inner circumstellar environment, as probed by spectropolarimetry.

Wisniewski, John P.↗

NASA NTRS2007

The Spectropolarimetric Evolution of V838 Mon

I review photo-polarimetric and spectropolarimetric observations of V838 Mon, which revealed that it had an asymmetrical inner circumstellar envelope following its 2nd photometric outburst. Electron scattering, modified by preor post-scattering H absorption, is the polarizing mechanism in V838 Mon's envelope. The simplest geometry implied by these observations is that of a spheroidal shell, flattened by at least 20% and having a projected position angle on the sky of approx. 37 degrees. Analysis of V838 Mon's polarized flux reveals that this electron scattering shell lies interior to the envelope region in which Halpha and Ca II triplet emission originates. To date, none of the theoretical models proposed for V838 Mon have demonstrated that they can reproduce the evolution of V838 Mon's inner circumstellar environment, as probed by spectropolarimetry.

Wisniewski, John P.↗

DOE OSTI2024

Interfacial Engineering Using Covalent Organic Frameworks in Polymer Composites for High‐Temperature Electrostatic Energy Storage

Abstract The use of inorganic nanofillers has been an effective method to improve high‐temperature capacitive performance of dielectric polymers, though there are unmet challenges such as undesirable organic–inorganic compatibility, and low efficiencies and energy densities. Herein, a surface functionalization strategy using covalent organic frameworks (COFs) is employed to address such challenges in realizing high‐performing polymer composites. Specifically, core–shell structured nanoparticles, where ZrO 2 nanoparticles act as the core and a COF material forms the shell, are constructed and composited with the polyetherimide (PEI) matrix. The design leverages the high electron affinity ( E A ) of the outer COF shell to create energy traps, thereby capturing free charges and limiting electrical conduction. Concurrently, the low E A and wide bandgap of the ZrO 2 core introduce energy barriers to impede charge injection and migration. This orchestrated “energy level cascade” results in a marked reduction of leakage current and energy loss. The resulting polymer composite showcases an impressive discharged energy density of 6.21 J cm −3 at an efficiency above 90%, with a maximum discharged energy density reaching 7.43 J cm −3 at 150 °C. These performance metrics position the PEI/ZrO 2 @COF polymer composite to surpass or be on par with state‐of‐the‐art high‐temperature PEI composites and other advanced polymer dielectrics.

Xie, Zongliang↗

DOE OSTI2024

Fluorescence-mediated postcollision interaction in x-ray photoionization of the Xe 𝐾 edge

Postcollision interaction (PCI) in near-threshold photoionization of the K shell of atomic Xe is investigated with hard-x-ray synchrotron radiation. Applying Auger electron spectroscopy, line-shape distortion is monitored in the K-L 2 → L 2 M 4,5 M 4,5 decay channel, where the initial K-L 2 (Kα 2 ) x-ray-emission step mediates the emission of the two continuum electrons. The K-edge result is compared to similar data obtained on direct L 2 M 4,5 M 4,5 decay at the L 2 edge without x-ray-emission. In conclusion, it is found that the evolution of the PCI shift with excess photon energy is very similar at both edges, but a subtle decrease in the PCI shift is observable at the K edge due to the effect of the ultrafast x-ray emission step delaying the PCI energy exchange by a few attoseconds.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2021

High-resolution Laboratory Measurements of K-shell X-Ray Line Polarization and Excitation Cross Sections in Helium-like S XV Ions

We report measurements of electron-impact excitation cross sections for the strong K-shell n = 2 → 1 transitions in S xv, using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well-known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line-emission cross sections of the forbidden line, z.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS2021

High-resolution Laboratory Measurements of K-shell X-ray Line Polarization and Excitation Cross Sections in Helium-like S XV Ions

We report measurements of electron-impact excitation cross sections for the strong K-shell n = 2 -> 1 transitions in S XV, using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well-known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line emission cross sections of the forbidden line, z.

atomic data↗

NASA NTRS1982

Low-energy hot plasma and particles in Saturn's magnetosphere

Results of the low-energy charged particle experiment carried by Voyager 2 in the Saturn magnetosphere are presented. Measurements of ions of energy greater than 28 keV and electrons of energies greater than 22 keV revealed the presence of a region containing an extremely hot (30-50 keV) plasma extending from the orbit of Tethys past the orbit of Rhea, and a low-energy ion mantle inside the dayside and nightside magnetospheres. H, H2, H3, He, C and O at energies greater than 200 keV/n were found to be important constituents of the Saturn magnetosphere, at relative abundances suggestive of a solar wind origin. Low-energy electron flux enhancements were observed between the L shells of Rhea and Tethys which were absent during the Voyager 1 encounter, and persistent asymmetric electron pitch-angle distributions were noted in the outer magnetosphere in conjunction with the hot ion plasma torus. Signatures of the passage of Tethys and Enceladus through the magnetosphere were found, although not at the positions predicted by dipole magnetic field models.

Krimigis, S. M.↗

NASA NTRS1971

Radiative interactions of electrons with matter

Equipment and capabilities developed in investigation of K and L shell ionization cross sections, X ray intensity ratios, bremsstrahlung spectrum, coincidence, and inelastic electron scattering

Quarles, C. A.↗

DOE OSTI2021

Superatomic nature of metal encapsulated dodecahedrane: The case of M@ C 20 H 20 (M = Li, Na, Mg + )

The idea of designing unprecedented materials made of superatomic building blocks, motivated the present study on endohedral M@C 20 H 20 (M = Li, Na, Mg + ) species. Here, ground and excited electronic structures of M@C 20 H 20 (M = Li, Na, Mg + ) were analyzed by means of high-level quantum calculations. In their ground states, one electron occupies a defuse superatomic s-orbital that lies around the C 20 H 20 cage. These entities populate higher angular momentum p-, d-, f-, g-superatomic orbitals in their low-lying electronic states. The proposed superatomic Aufbau shell model for Li@C 20 H 20 and Na@C 20 H 20 is 1s, 1p, 1d, 2s, 1f, 2p, 2d, 1g, 2f slightly different from that of Mg@C 20 H 20 + which is 1s, 1p, 1d, 2s, 1f, 2p, 2d, 1g, 3s, 2f, 2g, 3p. These introduced superatomic orbital series resemble the Aufbau principle of solvated electron precursors.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Direct strain correlations at the single-atom level in three-dimensional core-shell interface structures

Abstract Nanomaterials with core-shell architectures are prominent examples of strain-engineered materials. The lattice mismatch between the core and shell materials can cause strong interface strain, which affects the surface structures. Therefore, surface functional properties such as catalytic activities can be designed by fine-tuning the misfit strain at the interface. To precisely control the core-shell effect, it is essential to understand how the surface and interface strains are related at the atomic scale. Here, we elucidate the surface-interface strain relations by determining the full 3D atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via atomic electron tomography. Full 3D displacement fields and strain profiles of core-shell nanoparticles were obtained, which revealed a direct correlation between the surface and interface strain. The strain distributions show a strong shape-dependent anisotropy, whose nature was further corroborated by molecular statics simulations. From the observed surface strains, the surface oxygen reduction reaction activities were predicted. These findings give a deep understanding of structure-property relationships in strain-engineerable core-shell systems, which can lead to direct control over the resulting catalytic properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Distinguishing erbium dopants in Y 2 O 3 by site symmetry: Ab initio theory of two spin-photon interfaces

Here, we present a first-principles study of the defect formation and electronic structure of erbium (Er)–doped yttria (Y 2 O 3 ). This is an emerging material for spin-photon interfaces in quantum information science due to the narrow-linewidth optical emission from Er dopants at standard telecommunication wavelengths and their potential for quantum memories and transducers. We calculate formation energies of neutral and negatively and positively charged Er dopants and find the charge-neutral configuration to be the most stable, consistent with experiment. Of the two substitutional sites of Er for Y, the C 2 (more relevant for quantum memories) and C 3i (more relevant for quantum transduction), we identify the former as possessing the lowest formation energy. The electronic properties are calculated using the Perdew-Burke-Ernzerhof functional along with the Hubbard U parameter and spin-orbit coupling, which yields a ~ 6 μ B orbital and a ~ 3 μ B spin magnetic moment, and 11 electrons in the Er 4 f shell, confirming the formation of charge-neutral Er 3+ . This standard density functional theory approach underestimates the band gap of the host and lacks a first-principles justification for U . To overcome these issues we performed screened hybrid functional calculations, including a negative U for the 4 f orbitals, with mixing (α) and screening (w) parameters. These produced robust electronic features with slight modifications in the band gap and the 4 f splittings depending on the choice of tuning parameters. We also computed the many-particle electronic excitation energies and compared them with experimental values from photoluminescence.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Shell Thickness and Heterogeneity Dependence of Triplet Energy Transfer between Core–Shell Quantum Dots and Adsorbed Molecules

Quantum dot (QD)-sensitized triplet energy transfer (TET) has found promising applications in photon upconversion and photocatalysis. However, the underlying mechanism of TET in the QD-acceptor complex remains unclear despite the well-developed TET theory for the molecular donor–acceptor systems. Herein, the coupling strength of TET from CdSe/CdS core–shell QDs to 9-anthracene carboxylic acid (ACA) was studied by measuring the TET rate as a function of shell thickness with time-resolved photoluminescence. The change of TET-coupling strength with increasing shell thickness was further compared to those of electron and hole transfers from QDs so that we could test whether QD-sensitized TET is mediated by the charge transfer virtual state and can be considered as simultaneous electron and hole transfers as in molecular donor–acceptor systems. The measured coupling strength of TET from the CdSe/CdS QD decreases exponentially with the CdS shell thickness r: |V|(r) = |V|(0)e –βr , with an exponential decay factor β of 0.19 Å –1 , which is smaller than the sum of the measured decay factors for electron transfer to methyl viologen (0.18 Å –1 ) and hole transfer to phenothiazine (0.29 Å –1 ) from the same QD. This inconsistency is explained by the broadening of QD shell thicknesses in the distance dependence study, which significantly modifies the TET-coupling strength and driving force, resulting in a shallower distance dependence of the TET rate constants. This study sheds light on the fundamental mechanisms of QD-sensitized TET reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗