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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 307 records · Page 17
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NASA NTRS2013

Extension of a Kinetic Approach to Chemical Reactions to Electronic Energy Levels and Reactions Involving Charged Species With Application to DSMC Simulations

The ability to compute rarefied, ionized hypersonic flows is becoming more important as missions such as Earth reentry, landing high mass payloads on Mars, and the exploration of the outer planets and their satellites are being considered. Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties are extended in the current work to include electronic energy level transitions and reactions involving charged particles. These extensions are shown to agree favorably with reported transition and reaction rates from the literature for nearequilibrium conditions. Also, the extensions are applied to the second flight of the Project FIRE flight experiment at 1634 seconds with a Knudsen number of 0.001 at an altitude of 76.4 km. In order to accomplish this, NASA's direct simulation Monte Carlo code DAC was rewritten to include the ability to simulate charge-neutral ionized flows, take advantage of the recently introduced chemistry model, and to include the extensions presented in this work. The 1634 second data point was chosen for comparisons to be made in order to include a CFD solution. The Knudsen number at this point in time is such that the DSMC simulations are still tractable and the CFD computations are at the edge of what is considered valid because, although near-transitional, the flow is still considered to be continuum. It is shown that the inclusion of electronic energy levels in the DSMC simulation is necessary for flows of this nature and is required for comparison to the CFD solution. The flow field solutions are also post-processed by the nonequilibrium radiation code HARA to compute the radiative portion of the heating and is then compared to the total heating measured in flight.

Liechty, Derek S.↗
NASA NTRS2014

Extension of a Kinetic Approach to Chemical Reactions to Electronic Energy Levels and Reactions Involving Charged Species with Application to DSMC Simulations

The ability to compute rarefied, ionized hypersonic flows is becoming more important as missions such as Earth reentry, landing high mass payloads on Mars, and the exploration of the outer planets and their satellites are being considered. Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties are extended in the current work to include electronic energy level transitions and reactions involving charged particles. These extensions are shown to agree favorably with reported transition and reaction rates from the literature for near-equilibrium conditions. Also, the extensions are applied to the second flight of the Project FIRE flight experiment at 1634 seconds with a Knudsen number of 0.001 at an altitude of 76.4 km. In order to accomplish this, NASA's direct simulation Monte Carlo code DAC was rewritten to include the ability to simulate charge-neutral ionized flows, take advantage of the recently introduced chemistry model, and to include the extensions presented in this work. The 1634 second data point was chosen for comparisons to be made in order to include a CFD solution. The Knudsen number at this point in time is such that the DSMC simulations are still tractable and the CFD computations are at the edge of what is considered valid because, although near-transitional, the flow is still considered to be continuum. It is shown that the inclusion of electronic energy levels in the DSMC simulation is necessary for flows of this nature and is required for comparison to the CFD solution. The flow field solutions are also post-processed by the nonequilibrium radiation code HARA to compute the radiative portion.

Liechty, Derek S.↗
NASA NTRS1996

Electron Excitation Cross Sections for the C II Transitions 2s(exp 2)2p P-2(exp 0) yields 2s2p(exp 2) P-4, D-2, and S-2

Experimental and theoretical excitation cross sections are reported for the transitions 2s(exp 2) 2p-2(exp 0) yield 2s2p(exp 2)P-4, D-2, and S-2 in C II. The transition wavelengths (energies) are 2324 A (5.34 eV), 1335 x (9.29 eV), and 1036 A (11.97 eV), respectively. Use is made of electron energy-loss and merged beams methods. The energy range covered is from below each threshold (4 - 11 eV) to 15-24 eV. As in previous work with O II, care was taken to assess and minimize the metastable fraction in the C II beam, to account for contributions from nearby energy-loss features, and to collect the full angular range of inelastically scattered electrons. A comparison is made for each transition between experiment and new 8-state R-matrix calculations. Subject headings: atomic data ultraviolet: general

Smith, Steven J.↗
NASA NTRS1983

The low-lying 2-sigma-minus states of OH

The configuration-interaction method is used to determine the electronic wave functions of the two lowest 2-sigma-minus states of OH using four different atomic orbital basis sets. Potential energy curves, transition moments, oscillator strengths, and photodissociation cross sections are obtained. Electronic transition dipole moments connecting the excited 1 2-sigma-minus and 2(D)2-sigma-minus states with each other and with the ground chi-2-pi state are presented as functions of internuclear distance. The theoretical absorption oscillator strengths for the D-2-sigma-minus(v prime = 0) from chi-2-pi(v double prime = 0) transition are in good agreement with the empirical value derived from astronomical measurement. The photodissociation cross sections for absorption from the v prime = 0, 1, and 2 levels of the ground state into the continuum of the 1 2-sigma-minus state are calculated, and the interstellar and cometary photodissociation rates are derived.

Van Dishoeck, E. F.↗
NASA NTRS1986

Differential electron scattering cross sections for the first optically forbidden and resonance transitions in Mg II, Zn II and Cd II

Differential electron scattering cross sections have been measured for dipole-forbidden and resonance transitions in Mg II, Zn II and Cd II in the angular range theta = 4-17 deg at 50 eV. These provide the first recorded angular distributions for an optically forbidden transition. It is found that while the cross section for excitation of the 4s (2)S-3d(9)4s(2) (2)D transition in Zn II is small, those for the 3s (2)S-3d (2)D, 4s (2)S (unresolved lines) in Mg II, and the 5s (2)S-4d(9)5s(2) D in Cd II are comparable in magnitude with the cross sections for resonance excitation. In addition, for Cd II it is found that the allowed and forbidden transitions have very similar angular distributions, and it is proposed that excitation to the 2D state may be dominated by a virtual 'double-dipole' transition via the 2P state. Also, the total excitation cross section of the resonance 2P state in Cd II is a factor of four higher than that predicted by the Gaunt factor approximation, suggesting that the accepted value for the oscillator strength may be too low.

Williams, I. D.↗
NASA NTRS1980

Electron impact cross sections for v = 0-1 vibrational excitation in CO at electron energies of 3 to 100 eV

The paper discusses the measurements of normalized, absolute differential cross-sections for electron-impact vibrational transition in CO. Integral and momentum-transfer cross-sections were obtained from these differential results; incident electron energies were in the 3 to 100 eV range, and the scattering angles were 10 to 135 deg. A comparison of these results with measurements made with pure vibrationally elastic cross-sections in N2, and with the data based on a two-potential theory for electron scattering from diatomic molecules showed that the present CO cross-sections are similar to those in N2. This indicates that the long-range effect of the small CO dipole moment is negligible in promoting vibrational excitation; moreover, a significant peak in the integral and momentum-transfer cross-sections is observed at 20 eV. Finally, the low-energy integral cross-sections are compared with a recent set of swarm data for vibrational excitation.

Chutjian, A.↗
NASA NTRS1991

Effect of isotopic substitution on the visible absorption spectrum of ozone

Gas-phase spectra for (O-16)3 and (O-18)3 are examined to consider the electronic states which cause the Chappuis bands. Specifications of the spectrometer and spectra calibrations are given. The results of both a conventional description of the vibrational structure and an examination of the autocorrelation functions are discussed. Transitions to two electronically excited states are the significant features of the isotope shifts. The transitions are characterized by adiabatic energies about 750/cm apart, and the higher energy approaches 16,400/cm. The vibrational structure of the higher state accompanies a relatively simple progression, and the lower state accompanies one isolated band. The autocorrelation analysis shows that for (O-16)3, major recurrences near 31 and 41 fs are found, and longer equivalent times are found for (O-18)3. At longer times, minor recurrences are also found. The results are considered significant for the understanding of the electronic and vibrational structures of the Chappuis bands. Suggestions for future work based on these results are discussed.

Anderson, Stuart M.↗
NASA NTRS1979

Two-potential approach for electron-molecular collisions at intermediate and high energies - Application to e-N2 scatterings

A general theoretical approach is proposed for the calculation of elastic, vibrational, and rotational transitions for electron-molecule scattering at intermediate and high-electron-impact energies. In this formulation, contributions to the scattering process come from the incoherent sum of two dominant potentials: a short-range shielded nuclear Coulomb potential from individual atomic centers, and a permanent/induced long-range potential. Application to e-N2 scattering from 50-500 eV incident electron energies has yielded good agreement with absolutely calibrated experiments. Comparisons with other theoretical approaches are made. The physical picture as well as the general features of electron-molecule scattering process are discussed within the framework of the two-potential approach.

Choi, B. H.↗
NASA NTRS1985

The relation between hard X-ray and transition-region line emission in solar flares

Observational evidence suggests that both the hard X-ray and ultraviolet emission from the impulsive phase of flares result from an electron beam. The results of model calculations that are consistent with this theory are presented. The impulsive phase is envisioned as occurring in many small magnetically confined loops, each of which maintains an electron beam for only a few seconds. This model successfully matches several observed aspects of the impulsive phase. The corona is heated to less than 2 million K, maximum enhanced emission occurs in lines formed near 100,000 K, and there is only slight enhancement between 100,000 and 2 million K. The slope of the observed relationship between hard X-ray and Ov 1371 A emission is also matched, but the relative emission is not. The calculations indicate that UV emission lines formed below a temperature of about 100,000 K will arise predominantly from the chromospheric region heated by the electron beam to transition region temperatures. Emission lines formed at higher temperatures will be produced in the transition region. This should be detectable in density-sensitive line ratios. To account successfully for the impulsive UV emission, the peak temperature in the impulsively heated loops must remain below about 2 million K. Thus the model implies that the impulsive heating takes place in different loops from the hotter gradual phase emission.

Mariska, J. T.↗
NASA NTRS1984

Surface and allied studies in silicon solar cells

Measuring small-signal admittance versus frequency and forward bias voltage together with a new transient measurement apparently provides the most reliable and flexible method available for determining back surface recombination velocity and low-injection lifetime of the quasineutral base region of silicon solar cells. The new transient measurement reported here is called short-circuit-current decay (SCCD). In this method, forward voltage equal to about the open-circuit or the maximum power voltage establishes excess holes and electrons in the junction transition region and in the quasineutral regions. The sudden application of a short circuit causes an exiting of the excess holes and electrons in the transition region within about ten picoseconds. From observing the slope and intercept of the subsequent current decay, the base lifetime and surface recombination velocity can be determined. The admittance measurement previously mentioned then enters to increase accuracy particularly for devices for which the diffusion length exceeds the base thickness.

Lindholm, F. A.↗
NASA NTRS1983

Angular distribution for electron excitation of the 4(2)S yields 4(2)P transition in Zn II - Comparison of experiment and theory

Differential electron-scattering cross sections for inelastic excitation of an ion have been measured for the first time. Experiments were carried out in a cross electron-ion beam geometry for the 4(2)S yields 4(2)P transition in Zn II at 75 eV. In addition, differential cross sections were calculated at energies between 15 and 100 eV in a five-state close-coupling approximation in which 4s, 4p, 3d(9)4s(2), 5s, and 4 d states were included. Agreement in shape between theory and experiment at 75 eV is excellent.

Chutjian, A.↗
NASA NTRS1976

Orbital electron capture by the nucleus

The theory of nuclear electron capture is reviewed in the light of current understanding of weak interactions. Experimental methods and results regarding capture probabilities, capture ratios, and EC/Beta(+) ratios are summarized. Radiative electron capture is discussed, including both theory and experiment. Atomic wave function overlap and electron exchange effects are covered, as are atomic transitions that accompany nuclear electron capture. Tables are provided to assist the reader in determining quantities of interest for specific cases.

Bambynek, W.↗
NASA NTRS1997

Electron Excitation Cross Sections for the S II Transitions: 3s(exp 2)3p(exp 3) 4S(exp o) approaches 3s(exp 2)3p(exp 3) 2D(exp o), 2P(exp o), and 3s3p(exp 4) 4P

Experimental and theoretical collisional excitation cross sections are reported for the transitions 3s(exp 2)3p(exp 3)4S(exp o) approaches 3s(exp 2)3p(exp 3) 2D(exp o), 2P(exp o), and 3s3P(exp 4) 4P in S II. The transition wavelengths (energies) are 6716 A (1.85 eV), 4069 A (3.05 eV), and 1256 A (9.87 eV), respectively. In the experiments, use is made of the energy-loss merged-beams method. The metastable fraction of the S II beam was assessed and minimized. The contribution of elastically scattered electrons was reduced by the use of a lowered solenoidal magnetic field and a modulated radio-frequency voltage on the analyzing plates and by retarding grids to reject the elastically scattered electrons with larger Larmor radii. For each transition, comparisons are made among experiments, the new 19 state R-matrix calculation, and three other close-coupling calculations.

Liao, C.↗