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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 OSTI2020

Realistic Ion Dynamics through Charge Renormalization in Nonaqueous Electrolytes

While many practically important electrolytes contain lithium ions, interactions of these ions are particularly difficult to probe experimentally because of their small X-ray and neutron scattering cross sections and large neutron absorption cross sections. Molecular dynamics (MD) is a powerful tool for understanding the properties of nonaqueous electrolyte solutions from the atomic level, but the accuracy of this computational method crucially depends on the physics built into the classical force field. In this study, we demonstrate that several force fields for lithium bistriflimide (LiTFSI) in acetonitrile yield a solution structure that is consistent with the neutron scattering experiments, yet these models produce dramatically different ion dynamics in solution. Such glaring discrepancies indicate that inadequate representation of long-range interactions leads to excessive ionic association and ion-pair clustering. We show that reasonable agreement with the experimental observations can be achieved by renormalization of the ion charges using a “titration” method suggested herewith. This simple modification produces realistic concentration dependencies for ionic diffusion and conductivity in <2 M solutions, without loss in quality for simulation of the structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Systematic study of the validity of the eikonal model including uncertainties

Nuclear reactions at intermediate beam energies are often interpreted using the eikonal model. In the analysis of complex reaction probes, where few-body reaction methods are needed, the eikonal method may be used as an efficient way for describing the fragment-target reaction process. In this work, we perform a systematic study to test the validity of the eikonal approximation for nucleon-nucleus reactions. We also quantify uncertainties due to the nucleon optical potential on reaction observables. We inspect the validity of the eikonal model and its semiclassical correction by comparing it to exact solutions (obtained from solving the optical-model equation with a finite-differences method) for a wide range of reactions. We also study the effect of relativistic corrections, both kinematic and dynamic, by effectively incorporating the relativistic effects at intermediate energies. The uncertainties from a Bayesian global optical potential (KDUQ) are propagated to the observables of interest. Our study includes neutron and proton reactions on 27 Al , 40 Ca , 90 Zr , and 208 Pb , for a wide range of energies 𝐸 lab = 0–400 MeV. We calculate neutron-total cross sections (elastic and reactions) as well as proton-absorption cross sections as a function of beam energy, using the eikonal model, the eikonal model with a semiclassical correction, and the exact solution. Here, we also compute angular distributions for the methods above. Our results show that for the proton-absorption cross section, the eikonal model can be used down to around 60 MeV and the semiclassical correction extends its use to 30 MeV. However, the validity of the eikonal model for the neutron-total cross section only goes down to ≈120 MeV, a range extended to ≈ 50 MeV when using the semiclassical correction. We find the semiclassical correction to the eikonal model to be less effective in describing the angular distributions. The 1⁢𝜎 uncertainty intervals on the observables we studied is less than 5% for most of the energies considered, but increases rapidly for higher energies, namely energies outside the range of KDUQ (𝐸 lab > 200MeV).

Cluster models↗
DOE OSTI2020

Vacuum-ultraviolet absorption cross-sections of functionalized cyclic hydrocarbons: Five-membered rings

Cross-sections were measured at 50 °C and 800 Torr using differential absorption spectroscopy in the vacuum-ultraviolet from 5.17 – 9.92 eV for a series of five-membered cyclic hydrocarbons: cyclopentane, cyclopentene, methylcyclopentane, tetrahydrofuran, furan, 2-methyltetrahydrofuran, 2,3-dihydro-5-methylfuran, tetrahydrofuran-3-one, 2-methyl-1,3-dioxolane, cyclopentanol, 2-hydroxytetrahydrofuran, cyclopentanone, 2-cyclopenten-1-one, 3-cyclopenten-1-one, 2-methylcyclopentan-1-one, 2-methylcyclopent-2-en-1-one, 1,2-epoxycyclopentane, 2,3-epoxycyclopentanone, and 3,4-epoxycyclopentanone. The majority of the species are functionalized and are directly related to chain-reactions relevant to combustion, such as substituted cyclic ethers and cycloalkenes. Uncertainty calculations were conducted as a function of photon energy to quantify errors associated with experimental repeatability, signal-to-noise ratio, and gas-phase concentration, yielding an upper limit of 5% in spectral regions where absorption is >2%. Here, the primary objective of the present work is to provide quantitative absorption cross-sections of species commonly produced in combustion reactions. Comparing qualitative differences among the five-membered cyclic species, in order to analyze the effect of functional groups on absorption, is a secondary objective. The majority of the spectra are reported for the first time and enable quantitative isomer-resolved speciation measurements in combustion chemistry.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1989

Precision measurements of photoabsorption cross sections of Ar, Kr, Xe, and selected molecules at 58.4, 73.6, and 74.4 nm

Absolute absorption cross sections have been measured for the rare gases at 58.43, 73.59, and 74.37 nm with an accuracy of + or - 0.8 percent. For the molecules H2, N2, O2, CO, N2O, CO2, and CH4, precision measurements were made at 58.43 nm with an accuracy of + or - 0.8 percent. Molecular absorption cross sections are also reported at 73.59 and 74.37 nm. However, in the vicinity of these wavelengths most molecules exhibit considerable structure, and cross sections measured at these wavelengths may depend on the widths and the amounts of self-reversal of these resonance lines. A detailed discussion is given of the systematic errors encountered with the double-ion chamber used in the cross-sectional measurements. Details are also given of precision pressure measurements.

Samson, James A. R.↗
DOE OSTI2020

Entangled light–matter interactions and spectroscopy

Entangled photons exhibit non-classical light–matter interactions that create new opportunities in materials and molecular science. Notably, in entangled two-photon absorption, the intensity-dependence scales linearly as if only one photon was present. The entangled two-photon absorption cross section approaches but does not match the one-photon absorption cross section. The entangled two-photon cross section also does not follow classical two-photon molecular design motifs. Questions such as these seed the rich but nascent field of entangled light–matter interactions. In this work, we use the experimental developments in entangled photon spectroscopy to outline the current status of the field. Now that the fundamental tools are outlined, it is time to start the exploration of how materials, molecules, and devices can control or utilize interactions with entangled photons.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Application of machine learning approaches in the analysis of mass absorption cross-section of black carbon aerosols: Aerosol composition dependencies and sensitivity analyses

Physics-based models typically require an in-depth understanding of a phenomenon and assumptions of the underlying process(es), which are often hard to obtain in practice, whereas data-driven machine learning models learn the structure and patterns in the training data without any prior theoretical assumptions and then use inference to develop useful predictions. A novel machine learning-based algorithm has been previously developed for the prediction of black carbon mass absorption cross-section (MAC BC ) and applied to a variety of different atmospheric environments. In contrast to light scattering theories which require assumptions about the underlying physics, this algorithm uses time-series data of aerosol properties to estimate the temporally-varying MAC BC at 870 nm. Here, we analyze our algorithm and discuss the influence of aerosol optical properties (such as Ångström exponents and single scattering albedo) and chemical composition on the model outputs and the associated accuracy. Additionally, we conduct sensitivity analyses on our models to understand how the predictions change in response to different sets of input variables. Our support vector machine (SVM) for regression model is the least sensitive to variations in the input variables, although all models tend to exhibit a degradation to their accuracy when scattering Ångström exponents are less than one.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1976

Post sunset behavior of the 6300 A atomic oxygen airglow emission

A theoretical model of the 6300 A OI airglow emission was developed based on the assumptions that both the charged and neutral portions of the Earth's upper atmosphere are in steady state conditions of diffusive equilibrium. Intensities of 6300 A OI emission line were calculated using electron density true height profiles from a standard C-4 ionosonde and exospheric temperatures derived from Fabry-Perot interferometer measurements of the Doppler broadened 6300 A emission line shape as inputs to the model. Reaction rate coefficient values, production mechanism efficiencies, solar radiation fluxes, absorption cross sections, and models of the neutral atmosphere were varied parametrically to establish a set of acceptable inputs which will consistently predict 6300 A emission intensities that closely agree with intensities observed during the post-sunset twilight period by an airglow observatory consisting of a Fabry-Perot interferometer and a turret photometer. Emission intensities that can only result from the dissociative recombination of molecular oxygen ions were observed during the latter portion of the observational period. Theoretical calculations indicate that contamination of the 6300 A OI emission should be on the order of or less than 3 percent; however, these results are very sensitive to the wavelengths of the individual lines and their intensities relative to the 6300 A OI intensity. This combination of a model atmosphere, production mechanism efficiencies, and quenching coefficient values was used when the dissociative photoexcitation and direct impact excitation processes were contributing to the intensity to establish best estimates of solar radiation fluxes in the Schumann--Runge continuum and associated absorption cross sections. Results show that the Jacchia 1971 model of the upper atmosphere combined with the Ackerman recommended solar radiation fluxes and associated absorption cross sections produces theoretically calculated intensities that more closely agree with the observed intensities than all the other combinations.

Smith, R. E.↗
NASA NTRS1992

Martian atmospheric chemistry during the time of low water abundance

The importance of odd hydrogen (or HO(x)) radicals in the catalytic recombination of carbon monoxide and oxygen in the Martian atmosphere is a well known fact. The inclusion of recent chemical kinetics data, specifically temperature-dependent CO2 absorption cross sections, into our one dimensional photochemical model shows that HO(x) is too efficient in this regard. The absorption cross sections of CO2 are smaller than previously assumed; this leads to a reduction in the photolysis rate of CO2 while the photolysis rate of H2O has increased. As a consequence the predicted mixing ratio of CO in our models is substantially less than the observed value of 6.5(10)(exp -4). Simultaneous measurements of water, ozone, and carbon monoxide were obtained in the Martian atmosphere in early Dec. 1990 (L(sub s) for Mars was 344 deg.).

Nair, Hari↗
NASA NTRS1986

Molecular oxygen absorption continua at 195-300 nm and O2 radiative lifetimes

With the aid of new calculations on the transition moments between the six lowest states of O2 (Klotz and Peyerimhoff, 1986), absorption cross sections have been calculated for several oxygen continua. Reasonable agreement is achieved with recent experimental results, the comparison indicating an overestimation of the theoretical transition moment for the A-X system of 20-25 percent. With this adjustment, the calculated radiative lifetime for the O2(A) state is 150 ms, in close agreement with the currently used value. The continua cross sections indicate that absorption by O2(a) cannot be a significant atmospheric process.

Saxon, R. P.↗
NASA NTRS2004

The NDSC ozone and temperature lidar alogorithm intercomparison ubutuatuve (A2I): project overview

In September 2003, the Lidar Working Group (LWG) of the Network for Detection of Stratospheric Change (NDSC) initiated an extensive project to compare the ozone and temperature algorithms used within NDSC. This initiative, referred to later as Algorithm Intercomparison Initiative (A2I), uses simulated lidar signals to test and compare various parts of the ozone and temperature lidar algorithms. In addition to the fact that it meets the requirement of the NDSC protocols, this exercise allows the detailed assessment, by all the participants, of some of the sources and magnitudes of various uncertainties associated with the algorithms, and/or with the theoretical assumptions made in these algorithms. The outcome of the A21 is to try to find common grounds in the way ozone and temperature can be retrieved in order to reduce and possibly eradicate discrepancies due to algorithm issues alone. Specific issues such as homogenizing the choice of Rayleigh extinction cross-sections, ozone absorption cross-sections, a priori information, and the definition of the vertical resolutions are among the primary targets of the A21 outcome.

lidar↗
DOE OSTI2020

Role of water vapour in the absorption of nanosecond 266-nm laser pulses by atmospheric air

The absorption of the Nd : YAG fourth harmonic in air and binary mixtures of water vapour with nitrogen and oxygen at atmospheric pressure has been measured as a function of pulse energy (peak intensity). The mixtures obtained by adding equal amounts of water vapour to dry nitrogen and oxygen have been found to differ significantly in absorption. Preliminary quantitative data have been obtained for two- and three-photon absorption cross sections of water and oxygen molecules: σ{sup (2)}(H{sub 2}O) = (4 ± 1) ± 10{sup –49} cm{sup 4} s and σ{sup (3)}(O{sub 2}) = (5.6 ± 1.4) ± 10{sup –78} cm{sup 6} s{sup 2}. The absorption of 266-nm pulses with peak intensities from 0.05 to 2 GW cm{sup –2} in the near-surface atmosphere has been shown to be determined by two-photon absorption in water vapour and three-photon absorption in oxygen. In moist air containing 1 % water vapour, the absorption coefficient for 266-nm laser pulses exceeds that in dry air by four to five times. There is no absorption in nitrogen. We have developed a technique for photoacoustic measurements of multiphoton absorption cross sections in single-component gases and gas mixtures. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1979

Radiation absorption by the C2 band systems for Jupiter entry conditions

Revised values of the absorption cross sections for seven electronic band systems of C2 have been calculated using recently published experimental data for the electronic transition moments. Using these revised C2 cross section values, computations were made for the radiating flow field over a Jupiter entry probe with coupled ablation injection from a carbon-phenolic heat shield. Results are presented which show that radiation absorption within the ablation layer for the spectral range of 4 to 6 eV is less than that predicted using previous C2 absorption cross section values. The effect of the reduced radiation absorption by the C2 molecule is an increase in the radiative heating rates and ablation mass loss rates for the Jupiter entry conditions considered in the study.

Sutton, K.↗
DOE OSTI2020

Photo response of 164 Dy

Background: Little data is available for the pygmy dipole resonance (PDR) in axially deformed nuclei. Photon-scattering experiments are complicated by high level densities in the PDR region and the small energy difference of transitions to the ground state and to excited states. Purpose: In this work, we report on an experimental study of the low-energy dipole strength distribution of the well-deformed nucleus 164 Dy between 4.0–7.7 MeV. Methods: The low-lying photoresponse of 164 Dy has been investigated using the method of nuclear resonance fluorescence using a quasimonochromatic linearly polarized $\textit{γ}$-ray beam in the energy range of 4.0–7.7 MeV in steps of 0.2 MeV. Results: For excitation energies between 4 MeV and 5 MeV, sufficiently low level densities allow for the identification of individual states, including level energies, reduced transition widths and branching ratios. Energy-averaged mean decay branching ratios, mean population ratios and partial absorption cross sections were determined above 5 MeV up to the neutron-separation threshold at 7.7 MeV. A Lorentzian-shaped enhancement of the partial photo absorption cross section followed by decays back to the ground-state band is found at 6.10(5) MeV with a width of 0.77(23) MeV. A comparison with results from complementary measurements is performed using the framework of the statistical model. Conclusions: The experimental results for the mean population ratios deviate systematically from the statistical model simulation by 30(6)%. However, they are in agreement within one standard deviation of the simulation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1971

Properties of hollow square gravitational wave detectors

Antennas are described with gravitational radiation properties such as polarization, directivity, and scalar and tensor characteristics. These antenna devices are mechanically resonant systems in the shape of a hollow square with fundamental frequencies in the Hz range. An antenna characteristic is that the absorption cross section for quadrupole radiation at the normal frequency of vibration is the same as that of the fundamental if the damping factors are the same. The absorption cross section of quadrupole gravitational radiation is calculated for a class-1 detector of a cylindrical cross section and for a class-2 detector shaped like a hollow square.

Douglass, D. H., Jr.↗