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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

X-ray transient absorption reveals the 1Au (nπ*) state of pyrazine in electronic relaxation

Abstract Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1 B 2u (ππ*) (S 2 ) and 1 B 3u (nπ*) (S 1 ) states, the participation of the optically dark 1 A u (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1 A u (nπ*) and 1 B 3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1 A u (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

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

iSPECTRON: a simulation interface for linear and nonlinear spectra with ab-initio quantum chemistry software

We introduce iSPECTRON, an open source (under the Educational Community License version 2.0) program that parses data from common quantum chemistry software (NWChem, OpenMolcas, Gaussian, Cobramm, etc.), produces the input files for the simulation of linear and nonlinear spectroscopy of molecules with the Spectron code, and analyzes the spectra with a broad range of tools. Vibronic spectra are expressed in term of the electronic eigenstates, obtained through quantum chemistry computations, and vibrational/bath effects are incorporated in the framework of the displaced harmonic oscillator model, where all required quantities are computed at the Franck-Condon point. The code capabilities are illustrated by simulating linear absorption, transient absorption and two dimensional electronic spectra of the pyrene molecule. Two levels of electronic structure theory, TDDFT (with NWChem) and RASSCF/RASPT2 (with OpenMolcas), are compared where possible. Acknowledgements: F.S., A.N., D.R.N., N.G., S.M, M.G. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Award Nos. DE-SC0019484, KC-030103172684. The Spectron code was developed with support from the National Science Foundation (Grant CHE- 1953045). This research benefited from computational resources provided by EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle Memorial Institute for the United States Department of Energy under DOE Contract No. DE-AC05-76RL1830.

Segatta, Francesco↗
NASA NTRS1991

Lightning protection of full authority digital electronic systems

Modern electronic systems are vulnerable to transient and they now provide safety critical functions such as full authority digital electronic control (FADEC) units for fly by wire aircraft. Of the traditional suppression technologies available diodes have gained the wider acceptance, however, they lack the current handling capacity to meet existing threat levels. The development of high speed fold back devices where, at a specified voltage, the off state resistance switches to a very low on state one has provided the equivalent to a semiconductor spark gap. The size of the technology enables it to be integrated into connectors of interconnection cables. To illustrate the performance the technology was developed to meet the Lightning Protection requirements for FADEC units within aeroengines. Work was also carried out to study switching behavior with the waveform 5, the 500 us, 10 kA pulse applied to cable assemblies. This test enabled all the switches in a connector to be fired simultaneously.

Crofts, David↗
NASA NTRS1988

Aircraft jolts from lightning bolts

The trend toward the use of composite materials and digital electronics has renewed the need to quantify the effects of lightning strikes to airplanes, since composite structures do not provide shielding equivalent to that of metal aircraft, and digital systems are potentially more susceptible to upset by electrical transients than are analog electronic systems. A research program, called the Storm Hazards Program, has been run by the National Aeronautics and Space Administration (NASA) for the past eight years and has provided the first statistically significant measurements of the electromagnetic interaction between lightning and aircraft. A NASA-owned F-106B airplane has been flown through thunderstorms about 1500 times at altitudes between 5,000 and 40,000 feet (1,500 to 12,000 meters). The airplane, lightning-hardened and outfitted with special instruments, was hit by lightning 714 times. The types of measurements made and the results are described.

Pitts, Felix L.↗
DOE OSTI2024

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl 2 •– and SO 4 •– )

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for accurate interpretation of their physical and chemical properties. Here, in this work, we explore the transient interactions between trivalent californium ions (Cf 3+ ) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl 2 •- ) and sulfate (SO 4 •- ) radical anions. Chemical kinetics, as measured by integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf 3+ ions. The derived and ionic strength corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf 3+ + Cl 2 •- ) = (8.28 ± 0.61) × 10 5 M -1 s -1 and k(Cf 3+ + SO 4 •- ) = (9.50 ± 0.43) × 10 8 M -1 s -1 under ambient temperature conditions (22 ± 1 °C).

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
NASA NTRS1979

The magnetosphere of Mercury

According to Mariner 10 data on Mercury's magnetosphere, the bow shock and magnetosheath signatures in the magnetic field are entirely consistent with the geometry expected for interaction between a planet-centered magnetic dipole and the solar wind. The geometrically determined distance to the magnetopause stagnation point of solar wind flow was 1.45 plus or minus 0.15 Mercury radii. Comparative scaling of the magnetosphere of Mercury to earth shows that Mercury itself occupies a much larger fraction of the magnetosphere than does the earth. While there is no evidence for the permanent existence of a trapped charged particle radiation belt, intense transient bursts of energetic electrons indicate that a local acceleration process must be active. It is reasonable to assume that this process occurs in the magnetic tail. The interior plasma features compare well with those in the earth's magnetosphere. Characteristic time scales for transient phenomena at Mercury should be reduced by a factor of about 20 in comparison with those on earth - i.e., a few minutes for substorms vs an hour at earth. The origin of the magnetic field is unclear.

Ness, N. F.↗
NASA NTRS2005

Hot-Electron Photon Counters for Detecting Terahertz Photons

A document proposes the development of hot-electron photon counters (HEPCs) for detecting terahertz photons in spaceborne far-infrared astronomical instruments. These would be superconducting- transition-edge devices: they would contain superconducting bridges that would have such low heat capacities that single terahertz photons would cause transient increases in their electron temperatures through the superconducting- transition range, thereby yielding measurable increases in electrical resistance. Single devices or imaging arrays of the devices would be fabricated as submicron-sized bridges made from films of disordered Ti (which has a superconducting- transition temperature of .0.35 K) between Nb contacts on bulk silicon or sapphire substrates. In operation, these devices would be cooled to a temperature of .0.3 K. The proposed devices would cost less to fabricate and operate, relative to integrating bolometers of equal sensitivity, which must be operated at a temperature of approx. = 0.1 K.

Karasik, Boris↗
NASA NTRS1982

Annual Conference on Nuclear and Space Radiation Effects, 19th, Las Vegas, NV, July 20-22, 1982, Proceedings

The results of research concerning the effects of nuclear and space radiation are presented. Topics discussed include the basic mechanisms of nuclear and space radiation effects, radiation effects in devices, and radiation effects in microcircuits, including studies of radiation-induced paramagnetic defects in MOS structures, silicon solar cell damage from electrical overstress, radiation-induced charge dynamics in dielectrics, and the enhanced radiation effects on submicron narrow-channel NMOS. Also examined are topics in SGEMP/IEMP phenomena, hardness assurance and testing, energy deposition, desometry, and radiation transport, and single event phenomena. Among others, studies are presented concerning the limits to hardening electronic boxes to IEMP coupling, transient radiation screening of silicon devices using backside laser irradiation, the damage equivalence of electrons, protons, and gamma rays in MOS devices, and the single event upset sensitivity of low power Schottky devices.

Long, D. M.↗
DOE OSTI2023

Measuring Photoexcited Electron and Hole Dynamics in ZnTe and Modeling Excited State Core-Valence Effects in Transient Extreme Ultraviolet Reflection Spectroscopy

Transient extreme ultraviolet (XUV) spectroscopy is becoming a valuable tool for characterizing solar energy materials because it can separate photoexcited electron and hole dynamics with element specificity. We use surface-sensitive femtosecond XUV reflection spectroscopy to separately measure photoexcited electron, hole, and band gap dynamics of ZnTe, a promising photocathode for CO 2 reduction. We develop an ab initio theoretical framework based on density functional theory and the Bethe-Salpeter equation to robustly assign the complex transient XUV spectra to the material's electronic states. Applying this framework, we identify the relaxation pathways and quantify their time scales in photoexcited ZnTe, including subpicosecond hot electron and hole thermalization, surface carrier diffusion, ultrafast band gap renormalization, and evidence of acoustic phonon oscillations.

14 SOLAR ENERGY↗
NASA NTRS1972

Detailed measurements of the H beta line shape in a transient plasma using a fiber optics slit system

Through the use of fiber optics, a series of very narrow slits have been constructed and placed at the exit plane of a spectrograph. Plasma radiation which is dispersed by the spectrograph is incident on the slits and is transmitted to separate phototubes via the quartz fibers. With this technique, time resolved measurements of the spectral shape of the hydrogen H beta line have been made and used to determine the electron density of a transient plasma. Data obtained in a shock tube indicated that the thermodynamic conditions behind the reflected shock in a mixture of 20 percent H 80 percent He correspond to theoretically predicted conditions.

Stickford, G. H., Jr.↗
NASA NTRS1972

Detailed measurements of the H beta line shape in a transient plasma using a fiber optics slit system.

Through the use of fiber optics, a series of very narrow slits have been constructed and placed at the exit plane of a spectrograph. Plasma radiation which is dispersed by the spectrograph is incident on the slits and is transmitted to separate phototubes via the quartz fibers. With this technique, time resolved measurements of the spectral shape of the hydrogen H beta line have been made and used to determine the electron density of a transient plasma. Data obtained in a shock tube indicated that the thermodynamic conditions behind the reflected shock in a mixture of 20% H2-80% He, at incident shock speeds of 12 to 14 km/sec and pressures of 66.6 to 133.3 N/sq m (0.5 to 1.0 mm Hg), correspond to theoretically predicted conditions. Immediately behind the incident shock at speeds of 17 to 24 km/sec the data indicate that the plasma reached equilibrium then demonstrated a drop in intensity which has been attributed to radiative cooling.

Stickford, G. H., Jr.↗
DOE OSTI2020

An explicit scheme to enforce charge conservation in transient Particle-in-Cell simulations with Maxwell-Boltzmann electrons

The Maxwell-Boltzmann electron model is a very popular approximation of the electronic behavior in electrostatic plasmas, adopted in all cases where the electron dynamics is governed only by a balance between electric and pressure forces. In such a model, the electron dynamics is reduced to solving for a nonlinear Poisson problem together with an additional expression enforcing charge conservation. In this work we derive an expression of charge conservation which can be conveniently applied to explicit schemes to update the reference Boltzmann density, particularly useful when simulating plasma sheaths. The scheme can be equally applied to both steady-state and transient problems. Two examples are shown, a steady-state plasma sheath, and a radio-frequency magnetic presheath. Our proposed scheme allows to enforce global charge conservation locally in time, and can thus be applied to the simulation of transient phenomena.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Two-step charge density wave transition and hidden transient phase in 1 T – TiSe 2

Using variable temperature atomic pair distribution function analysis, we study the emergence of charge density wave (CDW) order in 1T-TiSe 2 and find that it takes place via a two-step transition. First, upon decreasing temperature to about 235(3) K, CDW related lattice distortions emerge in the individual TiSe 2 layers alone. Then, upon further decreasing the temperature, the two-dimensional distortions in the layers couple and the widely recognized three-dimensional 2a o x 2a o x 2c o superstructure emerges at about 205(3) K. Because two different band gaps are known to emerge at the same temperatures, the finding indicates the presence of strong electron-phonon coupling. The transient phase between the two steps lacks inversion symmetry and may serve as a precursor of the debated chiral 1T-TiSe 2 phase. Finally, our findings are important for the understanding of the enigmatic CDW transition in 1T-TiSe 2 and CDW instabilities in van der Waals materials in general.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Light-Induced Renormalization of the Dirac Quasiparticles in the Nodal-Line Semimetal ZrSiSe

In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. In this study we investigate the electronic structure of ZrSiSe, by combining time- and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model ( DFT + U + V ) and by time-dependent DFT + U + V . We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1979

Experimental Investigation of Launch Vehicle Transient Input Simulation in Payload Tests

The technique of electronically simulating the structural dynamics of a launch vehicle in transient tests of payloads using multiple vibration excitation systems was investigated. The development of computer programs to determine transfer functions, synthesize shaker forcing functions, and control vibration exciters is described. A demonstration test using the techniques was described and results are presented. The evaluation of the potential for applying this technique to large Shuttle payloads is discussed.

Rader, P.↗
DOE OSTI2026

How Does Metal Spin State Affect Electronic Communication in Mixed-Valence Dimers? Insights from Ultrafast Near-Infrared and Soft X-ray Transient Absorption Spectroscopy

Recent advancements in photocatalysis, photovoltaics, and quantum information science take advantage of electron spin, and determining how spin multiplicity affects electron transfer is key to understanding these phenomena. Here, in this study, we examine how metal spin state affects electronic communication in an organometallic mixed-valence dimer, ferrocenyl cobaltocenium ([Fe II Cp 2 Co III Cp 2 ] + ). This complex can be photoexcited from its low-spin singlet Fe II ground state to form intermediate-spin triplet Fe II and high-spin quintet Fe II excited states. Using femtosecond optical transient absorption (OTA) spectroscopy with visible (vis), near-infrared (NIR), and short-wave IR (SWIR) probes, supported by time-dependent density functional theory (TD-DFT) calculations, we measure Fe II Co III →Fe III Co II intervalence charge transfer (IVCT) bands in each of the Fe II spin states. Mulliken–Hush analysis of the excited-state IVCT bands was used to compute the electronic coupling between the metal centers in all three spin states, which increased as quintet < triplet < singlet. Meanwhile, the peak energy of the bands, and thus the ΔG of the IVCT transition, increased as triplet < quintet < singlet. Then, to directly probe the electronic structure at each metal center, we employed picosecond soft X-ray transient absorption (XTA) spectroscopy at the Fe and Co L 3 edges. Our results show that the low-spin and high-spin states of [Fe II Cp 2 Co III Cp 2 ] + are valence-localized, while the intermediate-spin state is partially delocalized. The differences in charge delocalization are attributed to differences in orbital occupation and geometry that affect the free energy and superexchange coupling.

Burke, John H. [Univ. of Illinois at Urbana-Champa↗