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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Spectral line-shape in collinear laser spectroscopy after atomic charge exchange

Collinear laser spectroscopy experiments on fast, neutral beams have been extensively used for studies on short-lived radioactive nuclei, taking advantage of its high sensitivity. The resulting resonance line-shape is known to show significant distortion, due to the energy exchange during the charge-exchange neutralization process, which can cause large systematic uncertainty in the determined centroid. A model for the line shape was constructed and simulated to be compared to measured Al, Si, and Ni hyperfine spectra. It is shown that the distortion is caused mainly by the transfer of electron into many different energy levels in the projectile atom and subsequent decays, rather than secondary inelastic collisions, which were often assumed in the line shape analysis before. Furthermore, the model can also be applied to other projectile–alkali pairs, providing a reliable line-shape with less fitting parameters than conventional phenomenological models.

Atom↗
DOE OSTI2025

Satellites, core hole excitations, and spin-resolved electronic structure in the spectroscopy of half-metallic CrO 2

Photoelectron satellites—the structures appearing on the low kinetic or high binding-energy side of the “main” or “elastic” photopeak—betray the complex many-body interactions set in motion by the sudden creation of the core hole. In this work, we demonstrate, using the technologically important ferromagnetic half-metal CrO 2 , how such satellites can manifest themselves in other core-level spectroscopies of the material and how they can reveal important details pertinent to its electronic structure. Specifically, we identify a fluorescence satellite in the Cr 𝐿 3 resonant x-ray-emission spectra that radiates at a constant emission energy across the Cr 𝐿 3 x-ray edge with energy ≈1.3 eV above the ordinary valence fluorescence. Here, we provide evidence that this feature arises from the valence recombination of the Cr 2⁢𝑝 core hole “dressed” by the same shakeup charge-transfer process present in both the Cr x-ray photoelectron and the Cr x-ray absorption spectra with its energy uniquely measuring the exchange splitting of the Cr 3⁢𝑑 level. Further analysis of the x-ray emission data reveals three additional features that radiate at constant loss energy that are attributed to combinations of Cr 3⁢𝑑⁢(𝑡 2⁢𝑔 ) → Cr 3⁢𝑑⁢(𝑡 2⁢𝑔 ), charge-transfer O 2⁢𝑝→Cr 3⁢𝑑, and crystal-field Cr⁢ 3⁢𝑑⁡(𝑡 2⁢𝑔 )→Cr⁢ 3⁢𝑑⁡(𝑒 𝑔 ) excitations. These assignments and their energies are supported by density-functional theory calculations, the accuracy of which we demonstrate by hard x-ray valence-photoemission measurements. Atomic multiplet calculations, which include crystal-field effects, help interpret x-ray photoelectron and x-ray absorption spectra of the covalently mixed Cr ion. Resonant Cr K-𝐿 2,3 ⁢𝐿 2,3 Auger-electron emission spectra support a ligand-to-metal nature of the charge-transfer process while highlighting the charge sensitivity differences between photon-in/electron-out and photon-in/photon-out spectroscopies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Modeling the sequential dissociative double ionization of O 2 by ultrashort intense infrared laser pulses

A density matrix approach for sequential double ionization (DM-SDI) of molecules has been developed recently and was applied to the N 2 molecule. In this article, we extended the DM-SDI model to O 2 , which is a more complicated system to model than N 2 , due to its electronic structures and spin-orbit and laser couplings in the manifold of doubly charged states. We obtained a good agreement on the kinetic energy release spectrum of O + + O + from previous experiments. Thanks to the low computational cost of the model, we explored the mechanism behind the ionization and dissociation dynamics as well as the effects of lasers on the spectrum. Furthermore, this work will pave the way to model sequential dissociative double ionization of larger molecules and to probe molecular dynamics by measuring kinetic energy release spectra from this process.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Tunable Infrared Laser Absorption Spectroscopy of Aluminum Monoxide $A^2\Pi_i $–$X^2\Sigma^+$.

We report the details of an infrared, laser absorption diagnostic capable of quantifying aluminum monoxide temperature and column density at 100 kHz repetition rate. This novel technique employs a near infrared MEMS-VCSEL to measure rotationally resolved optical absorption spectra of aluminum monoxide $A^2\Pi_i$ - $X^2\Sigma^+$ from approximately 7400 –7900 cm -1 . Temperatures and column densities are extracted from model regressions to provide temporally resolved thermochemical information on aluminum oxidation reactions. The measurement capability is demonstrated by performing 100 kHz measurement in the plume of an exploding bridgewire with measured temperatures of 3450–3100 K and column densities of 1– 11 x 10 16 cm -2 . To the authors knowledge, this is the first use of the AlO $A^2\Pi_i$ - $X^2\Sigma^+$ transition to characterize aluminum combustion environments. Details regarding signal extraction and calibration of MEMS-VCSEL spectra are also included. Although unsuccessful, efforts to extract kinetic temperature and column density from simultaneously measured, atomic aluminum 2 P 3/2,1/2 - 2 S 1/2 transitions at 7618 cm -1 and 7602 cm -1 are also described.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Watching Excited State Dynamics with Optical and X-ray Probes: The Excited State Dynamics of Aquocobalamin and Hydroxocobalamin

Femtosecond time-resolved X-ray absorption (XANES) at the Co K-edge, X-ray emission (XES) in the Co Kβ and valence-to-core regions, and broadband UV–vis transient absorption are combined to probe the femtosecond to picosecond sequential atomic and electronic dynamics following photoexcitation of two vitamin B12 compounds, hydroxocobalamin and aquocobalamin. Polarized XANES difference spectra allow identification of sequential structural evolution involving first the equatorial and then the axial ligands, with the latter showing rapid coherent bond elongation to the outer turning point of the excited state potential followed by recoil to a relaxed excited state structure. Time-resolved XES, especially in the valence-to-core region, along with polarized optical transient absorption suggests that the recoil results in the formation of a metal-centered excited state with a lifetime of 2–5 ps. In conclusion, this combination of methods provides a uniquely powerful tool to probe the electronic and structural dynamics of photoactive transition-metal complexes and will be applicable to a wide variety of systems.

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

Chemical and elemental mapping of spent nuclear fuel sections by soft X-ray spectromicroscopy

Soft X-ray spectromicroscopy at the O K -edge, U N 4,5 -edges and Ce M 4,5 -edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub-micrometer scale. To analyze these data, a modification to non-negative matrix factorization (NMF) was developed, in which the data are no longer required to be non-negative, but the non-negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K -edge to distinguish between two components, one present in the bulk of the sample similar to UO 2 and one present at the interface of the sample which is a hyperstoichiometric UO 2+ x species. The species maps are consistent with a model of a thin layer of UO 2+ x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2023

Optimization of the generator coordinate method with machine-learning techniques for nuclear spectra and neutrinoless double- β decay: Ridge regression for nuclei with axial deformation

The generator coordinate method (GCM) is an important tool of choice for modeling large-amplitude collective motion in atomic nuclei. The computational complexity of the GCM increases rapidly with the number of collective coordinates. It imposes a strong restriction on the applicability of the method. In this work, we propose a subspace-reduction algorithm that employs optimal statistical ML models as surrogates for exact quantum-number projection calculations for norm and Hamiltonian kernels. The model space of the original GCM is reduced to a subspace relevant for nuclear low energy spectra and the NME of ground state to ground state 0νββ decay based on the orthogonality condition (OC) and the energy-transition-orthogonality procedure (ENTROP), respectively. For simplicity, the polynomial ridge regression (RR) algorithm is used to learn the norm and Hamiltonian kernels of axially deformed configurations. The efficiency and accuracy of this algorithm are illustrated for 76 Ge and 76 Se by comparing results obtained using the optimal RR models to direct GCM calculations. The low-lying energy spectra of 76 Ge and 76 Se, as well as the 0νββ-decay NME between their ground states, are computed. Furthermore, the results show that the performance of the GCM+OC/ENTROP+RR is more robust than that of the GCM+RR alone, and the former can reproduce the results of the original GCM calculation accurately with a significantly reduced computational cost.

59 ≤ A ≤ 89↗
DOE OSTI2021

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate in this work how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

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

Velocity-Map Imaging and Magnetic-Bottle Photoelectron Spectroscopy of [SeCCH] – : Electronic Properties and Spin–Orbit Splitting

In this work, the recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH] – anion was successfully investigated in the gas phase by high-resolution velocity-map imaging (VMI) and magnetic-bottle (MB) photoelectron spectroscopy coupled with an electrospray ionization source. Both VMI and MB spectra exhibited identical electron affinities (EA, 2.517 ± 0.002 eV), spin–orbit coupling (SOC) splittings (1492 ± 20 cm –1 ), and Se–C stretching frequencies (573 ± 20 cm –1 ) of the corresponding neutral tetra-atomic radical [SeCCH] • with the VMI spectrum possessing six times higher spectral resolution compared with the MB spectrum. These experimental values were well reproduced by calculations at the CCSD(T) level, in which both the isolated [SeCCH] – anion and the [SeCCH] • radical adopted linear geometries. The simulated spectra based on the calculated Franck–Condon factors, the SOC splitting, and the experimental line width matched well with the measured spectra. Furthermore, comparisons of the EA and SOC splitting values with the previously reported isolobal species [SeCN] • are also made and discussed. The decrease in the EA and SOC splitting of [SeCCH] • is ascribed to the differences in the electronegativities between C and N atoms as well as the electron density on the Se atom in its singly occupied molecular orbital (SOMO).

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

Effects of recoil spectra and electronic energy dissipation on defect survival in 3C-SiC

The coincidence of electronic and damage energy dissipation from energetic ions to an atomic lattice can significantly affect damage production along the ion trajectory due to spatial overlap of inelastic and elastic processes. Damage production and disordering in single crystal 3C-SiC from 5 MeV Si and 10 MeV Au ions is investigated using ion-channeling experiments. While defects are created by damage energy dissipation via elastic scattering, electronic energy dissipation via electron-phonon coupling decreases defect survival along the ion trajectory for Si ions. Finally, the more energetic recoil spectrum for 10 MeV Au ions leads to weaker spatial coupling of electronic and damage energy dissipation processes, and damage production is only weakly affected.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Interspecies radiative transition in warm and superdense plasma mixtures

Superdense plasmas widely exist in planetary interiors and astrophysical objects such as brown-dwarf cores and white dwarfs. How atoms behave under such extreme-density conditions is not yet well understood, even in single-species plasmas. Here, we have applied thermal density functional theory to investigate the radiation spectra of superdense iron–zinc plasma mixtures at mass densities of ρ = 250 to 2000 g cm -3 and temperatures of kT = 50 to 100 eV, accessible by double-shell–target implosions. Our ab initio calculations reveal two extreme atomic-physics phenomena—firstly, an interspecies radiative transition; and, secondly, the breaking down of the dipole-selection rule for radiative transitions in isolated atoms. Our first-principles calculations predict that for superdense plasma mixtures, both interatomic radiative transitions and dipole-forbidden transitions can become comparable to the normal intra-atomic Kα-emission signal. These physics phenomena were not previously considered for extreme high-density plasma mixtures at super-high energy densities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Time-dependent density-functional theory of high-order harmonic generation from noble-gas atoms driven by orthogonally polarized two-color laser fields

An orthogonally polarized two-color (OTC) field of frequency ratio 1 : 2 is known to generate both odd and even harmonic spectra in the mutually orthogonal directions of the fundamental and the second-harmonic fields, respectively. Recollision angles of an ionized electron driven back to its parent ion by an OTC field are inferred from the intensity ratio of odd and even harmonics, where the relative phase delays between the two-color fields can be used in time-resolved measurements on a subcycle time scale. Here, we present the ab initio, many-electron calculation of time-resolved recollision angles of noble-gas atoms driven by an OTC field, based on the time-dependent density-functional theory. Our results agree well with experimental data by Shafir et al. in Nat. Phys. 5, 412 (2009) of helium atom and New J. Phys. 12, 073032 (2010) of neon atom.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Rabi spectroscopy of the clock transition in thulium atoms in a one-dimensional optical lattice

The clock transition at a wavelength of 1.14 μm in thulium atoms in an optical lattice in the Lamb – Dick regime is studied. The capture of thulium atoms makes it possible to suppress completely both the transition broadening due to the first-order Doppler effect and the shift of the transition frequency caused by the recoil effect. The excitation spectra of sideband oscillation frequencies and Rabi oscillations of population between the clock transition levels upon resonance excitation are investigated. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Elucidating the Role of Dimensionality on the Electronic Structure of the Van der Waals Antiferromagnet NiPS 3

Abstract The sustained interest in investigating magnetism in the 2D limit of insulating antiferromagnets is driven by the possibilities of discovering, or engineering, novel magnetic phases through layer stacking. However, due to the difficulty of directly measuring magnetic interactions in 2D antiferromagnets, it is not yet understood how intra layer magnetic interactions in insulating , strongly correlated, materials can be modified through layer proximity. Herein, the impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS 3 is explored by measuring electronic excitations in exfoliated samples using Resonant Inelastic X‐ray Scattering (RIXS). The resulting spectra shows systematic broadening of NiS 6 multiplet excitations with decreasing layer count from bulk down to three atomic layers (3L). It is shown that these trends originate from a decrease in transition metal‐ligand and ligand–ligand hopping integrals, and by charge‐transfer energy evolving from Δ = 0.83 eV in the bulk to 0.37 eV in 3L NiPS 3 . Relevant intralayer magnetic exchange integrals computed from the electronic parameters exhibit a decrease in the average interaction strength with thickness. This study underscores the influence of inter layer electronic interactions on intra layer ones in insulating magnets, indicating that magnetic Hamiltonians in few‐layer insulating magnets can greatly deviate from their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2020

On the nature of macroradicals formed upon radiolysis of aqueous poly(N-vinylpyrrolidone) solutions

Herein we have explored the nature of macroradicals formed upon radiolysis of aqueous poly(N-vinylpyrrolidone) (PVP) solutions using pulse radiolysis, density functional theory (DFT) and literature data. On the basis of literature data on site-specific kinetics of hydrogen abstraction from simple amides and spectra corresponding to specific radical sites on the same amides we have assessed the distribution of H-atom abstraction by • OH radicals from different positions on the pyrrolidone ring and the polymer backbone. Pulse radiolysis experiments performed at different doses per pulse and different PVP concentrations demonstrate that the H-abstracting radiolysis products are not quantitatively scavenged by the polymer when the dose per pulse exceeds ≈40 Gy. The implications of this are discussed in the context of radical-initiated crosslinking reactions. At a mass fraction of 0.1% PVP and doses per pulse ranging from 7 Gy to 117 Gy, the overall radical decay observed at 390 nm follows second order kinetics with rate constants on the order of 10 9 dm 3 mol -1 s -1 .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

X-ray Spectroscopy Fingerprints of Pristine and Functionalized Graphene

In this work, we demonstrate how to identify and characterize the atomic structure of pristine and functionalized graphene materials from a combination of computational simulation of X-ray spectra, on the one hand, and computer-aided interpretation of experimental spectra, on the other. Despite the enormous scientific and industrial interest, the precise structure of these 2D materials remains under debate. As we show in this study, a wide range of model structures from pristine to heavily oxidized graphene can be studied and understood with the same approach. We move systematically from pristine to highly oxidized and defective computational models, and we compare the simulation results with experimental data. Comparison with experiments is valuable also the other way around; this method allows us to verify that the simulated models are close to the real samples, which in turn makes simulated structures amenable to several computational experiments. Our results provide ab initio semiquantitative information and a new platform for extended insight into the structure and chemical composition of graphene-based materials.

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

Machine learning dielectric screening for the simulation of excited state properties of molecules and materials

Accurate and efficient calculations of absorption spectra of molecules and materials are essential for the understanding and rational design of broad classes of systems. Solving the Bethe–Salpeter equation (BSE) for electron–hole pairs usually yields accurate predictions of absorption spectra, but it is computationally expensive, especially if thermal averages of spectra computed for multiple configurations are required. We present a method based on machine learning to evaluate a key quantity entering the definition of absorption spectra: the dielectric screening. We show that our approach yields a model for the screening that is transferable between multiple configurations sampled during first principles molecular dynamics simulations; hence it leads to a substantial improvement in the efficiency of calculations of finite temperature spectra. We obtained computational gains of one to two orders of magnitude for systems with 50 to 500 atoms, including liquids, solids, nanostructures, and solid/liquid interfaces. Importantly, the models of dielectric screening derived here may be used not only in the solution of the BSE but also in developing functionals for time-dependent density functional theory (TDDFT) calculations of homogeneous and heterogeneous systems. Overall, our work provides a strategy to combine machine learning with electronic structure calculations to accelerate first principles simulations of excited-state properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Origin of the multiphoton-regime harmonic-generation plateau structure

A physical interpretation is provided for the formation of the multiphoton-regime plateau feature in the spectra of nonlinear and correlated process of high-order harmonic generation (HHG) of two-active-electron atoms interacting with an intense linearly polarized laser field. While in the strong-field tunneling regime the plateau feature is well known to be due to rescattering effects of the freed electron, its counterpart in the multiphoton regime is due to atomic resonance effects involving both singly excited states and doubly excited states. Here, we propose a strategy to uncover which kind of these states are responsible for this phenomenon. By studying the sensitivity of HHG to the initial state in helium, in this work we show that this multiphoton-regime plateau structure mostly originates from singly excited states.

74 ATOMIC AND MOLECULAR PHYSICS↗