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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 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Anisotropic Electron-Photon and Electron-Phonon Interactions in Black Phosphorus

Orthorhombic black phosphorus (BP) and other layered materials, such as gallium telluride (GaTe) and tin selenide (SnSe), stand out among two-dimensional (2D) materials owing to their anisotropic in-plane structure. This anisotropy adds a new dimension to the properties of 2D materials and stimulates the development of angle-resolved photonics and electronics. However, understanding the effect of anisotropy has remained unsatisfactory to-date, as shown by a number of inconsistencies in the recent literatures. We use angle-resolved absorption and Raman spectroscopies to investigate the role of anisotropy on the electron-photon and electron-phonon interactions in BP. We highlight a non-trivial dependence between anisotropies and flake thickness, photon and phonon energies. We show that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectroscopy without the explicit consideration of excitation wavelength and flake thickness, as commonly used previously.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Real-Time Time-Dependent Density Functional Theory for Simulating Nonequilibrium Electron Dynamics

The explicit real-time propagation approach for time-dependent density functional theory (RT-TDDFT) has increasingly become a popular first-principles computational method for modeling various time-dependent electronic properties of complex chemical systems. Here, in this Perspective, we provide a nontechnical discussion of how this first-principles simulation approach has been used to gain novel physical insights into nonequilibrium electron dynamics phenomena in recent years. Following a concise overview of the RT-TDDFT methodology from a practical standpoint, we discuss our recent studies on the electronic stopping of DNA in water and the Floquet topological phase as examples. Our discussion focuses on how RT-TDDFT simulations played a unique role in deriving new scientific understandings. We then discuss existing challenges and some new advances at the frontier of RT-TDDFT method development for studying increasingly complex dynamic phenomena and systems.

Hamiltonians↗
DOE OSTI2015

Nitric Oxide Reduction to Ammonia by TiO 2 Electrons in Colloid Solution via Consecutive One-Electron Transfer Steps

The reaction mechanism of nitric oxide (NO) reduction by excess electrons on TiO 2 nanoparticles (e TiO2 – ) has been studied under anaerobic conditions. TiO 2 was loaded with 10–130 electrons per particle using γ-irradiation of acidic TiO 2 colloid solutions containing 2-propanol. The study is based on time-resolved kinetics and reactants and products analysis. The reduction of NO by e TiO2 – is interpreted in terms of competition between a reaction path leading to formation of NH 3 and a path leading to N 2 O and N 2 . The proposed mechanism involves consecutive one-electron transfers of NO, and its reduction intermediates HNO, NH 2 O•, and NH 2 OH. The results show that e TiO2 – does not reduce N 2 O and N 2 . Second-order rate constants of e TiO2 – reactions with NO (740 ± 30 M –1 s –1 ) and NH 2 OH (270 ± 30 M –1 s –1 ) have been determined employing the rapid-mixing stopped-flow technique and that with HNO (>1.3 × 10 6 M –1 s –1 ) was derived from fitting the kinetic traces to the suggested reaction mechanism, which is discussed in detail.

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

Effect of the Reconnection Electric Field on Electron Distribution Functions in the Diffusion Region of Magnetotail Reconnection

Abstract Electron distribution functions in the electron diffusion region during symmetric magnetic reconnection are investigated by means of theory and fully kinetic simulations. Crescent‐like striations are formed in distribution functions in the velocity plane perpendicular to the magnetic field. Using an analytical current sheet, we solve the equation of motion for electrons, and derive the shape of a crescent distribution, as a function of the distance from the neutral line, field gradients, and the reconnection electric field. Each crescent is tilted in the velocity plane because of the acceleration by the reconnection electric field, and multiple stripes appear due to multiple meandering bounces. Applying the theory to distribution functions observed in Earth's magnetotail, we deduce the amplitude of the reconnection electric field.

Bessho, N.↗
DOE OSTI2021

Probing electronic structure in berkelium and californium via an electron microscopy nanosampling approach

Due to their rarity and radioactive nature, comparatively little is known about the actinides, particularly those with atomic numbers higher than that of plutonium, and their compounds. In this work, we describe how transmission electron microscopy can provide comprehensive, safe, and cost-effective characterization using only single nanogram amounts of highly-radioactive, solid compounds. Chlorides of the rare elements berkelium and californium are dropcast and then converted in situ to oxides using the electron beam. The f-band occupancies are probed using electron energy loss spectroscopy and an unexpectedly weak spin-orbit-coupling is identified for berkelium. In contrast, californium follows a jj coupling scheme. These results have important implications for the chemistries of these elements and solidify the status of californium as a transitional element in the actinide series.

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

Transition to the Haldane phase driven by electron-electron correlations

One of the most famous quantum systems with topological properties, the spin S = 1 antiferromagnetic Heisenberg chain, is well-known to display exotic S = 1/2 edge states. However, this spin model has not been analyzed from the more general perspective of strongly correlated systems varying the electron electron interaction strength. Here, we report the investigation of the emergence of the Haldane edge in a system of interacting electrons – the two orbital Hubbard model—with increasing repulsion strength U and Hund interaction J H . We show that interactions not only form the magnetic moments but also form a topologically nontrivial fermionic many-body ground-state with zero-energy edge states. Specifically, upon increasing the strength of the Hubbard repulsion and Hund exchange, we identify a sharp transition point separating topologically trivial and nontrivial ground-states. Surprisingly, such a behaviour appears already at rather small values of the interaction, in a regime where the magnetic moments are barely developed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Photoinduced electron transfer from rylenediimide radical anions and dianions to Re(bpy)(CO) 3 using red and near-infrared light

A major goal of artificial photosynthesis research is photosensitizing highly reducing metal centers using as much as possible of the solar spectrum reaching Earth's surface. The radical anions and dianions of rylenediimide (RDI) dyes, which absorb at wavelengths as long as 950 nm, are powerful photoreductants with excited state oxidation potentials that rival or exceed those of organometallic chromophores. These dyes have been previously incorporated into all-organic donor–acceptor systems, but have not yet been shown to reduce organometallic centers. This study describes a set of dyads in which perylenediimide (PDI) or naphthalenediimide (NDI) chromophores are attached to Re(bpy)(CO) 3 through either the bipyridine ligand or more directly to the Re center via a pyridine ligand. The chromophores are reduced with a mild reducing agent, after which excitation with long-wavelength red or near-infrared light leads to reduction of the Re complex. The kinetics of electron transfer from the photoexcited anions to the Re complex are monitored using transient visible/near-IR and mid-IR spectroscopy, complemented by theoretical spectroscopic assignments. The photo-driven charge shift from the reduced PDI or NDI to the complex occurs in picoseconds regardless of whether PDI or NDI is attached to the bipyridine or to the Re center, but back electron transfer is found to be three orders of magnitude slower with the chromophore attached to the Re center. These results will inform the design of future catalytic systems that incorporate RDI anions as chromophores.

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

Two-dimensional electronic-vibrational spectroscopic study of conical intersection dynamics: an experimental and electronic structure study

The relaxation from the lowest singlet excited state of the triphenylmethane dyes, crystal violet and malachite green, is studied via two-dimensional electronic-vibrational (2DEV) spectroscopy. After excitation of the dyes at their respective absorption maxima, the ensuing excited state dynamics are tracked by monitoring the C[double bond, length as m-dash]C aromatic stretch. With the aid of electronic structure calculations, the observed transitions in the 2DEV spectra are assigned to specific geometries and a detailed story of the evolution of the nuclear wavepacket as it diffuses on the excited state potential energy surface (PES) and ultimately passes through the conical intersection is developed. Notably, it is revealed that the relaxation of the lowest singlet excited state involves intramolecular charge transfer while the nuclear wavepacket is on the excited state PES. Finally, through analyzing the center line slopes of the measured peaks, we show how both solvent motions and changes in the molecular dipole moment affect the correlation between electronic and vibrational degrees of freedom. This work clearly demonstrates the usefulness of 2DEV spectroscopy in following the motion of nuclear wavepackets after photoexcitation and in studying the interactions between the molecular dipole moment and surrounding solvent environment.

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

Determinants for proton location and electron coupled proton transfer in hydrogen bonded pentafluorophenol-anion clusters

This work aims to reveal the determinant factors for proton locations and electron coupled proton transfer (ECPT) in biologically relevant hydrogen bonded systems. Pentafluorophenol-anion clusters [C6F5O-•H+•A-]- are chosen to model active sites of biological functions, with the anion A- being systematically varied to ensure the proton affinities (PAs) of the anions well cover the referenced PA of C6F5O- from being appreciably smaller, similar, and to significantly larger. Negative ion photoelectron spectroscopy of these clusters provides spectroscopic evidence showing that proton location in the anionic state is largely following the PA prediction, while ECPT is observed only for the clusters with the anion possessing the electron binding energy (EBE) significantly larger than that of the referenced C6F5O-. Theoretical calculations suggest these clusters are stabilized by forming single strong hydrogen bond between donor and acceptor, and the associated charge and MO analyses fully support the experimental observations. The current holistic cluster model study indicates that PA is the right determinant that can be used to predict the proton location and describe hydrogen bonding structures, while both PA and EBE of anionic groups play important roles in facilitating ECPT process. This work was supported by U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Science, Geosciences, and Biosciences (X.-B.W.), and performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle Memorial Institute for the DOE. Part of this work was supported by the National Natural Science Foundation of China (No-11604046 for JZ; No. 11727810 for ZRS), as well as “Chenguang Program” by Shanghai Education Development Foundation, Shanghai Municipal Education Commission (No. 16CG38) and Natural Science Foundation of Shanghai (No. 16ZR1448100) (JZ).

Zhang, Jian↗