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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
NASA NTRS1978

On dielectronic recombination and resonances in excitation cross sections

It is found that the contribution to the dielectronic recombination rate of stabilizing transitions of the outer electron is within the 30% accuracy of the Burgess rates calculated by neglecting this process even for Delta n = 0 transitions for astrophysically important ions. Autoionization to excited levels, ignored in the Burgess procedure and shown by Jacobs et al. (1977) to be important for some ions in the calculation of the dielectronic recombination rate, is discussed with reference to the resonance excitation of some astrophysically important X-ray lines. In particular, the contribution of resonances to the excitation of several lines in the helium and neon isosequences is estimated.

Raymond, J. C.↗
NASA NTRS1978

Laser excited fluorescence in the cesium-xenon excimer and the cesium dimer

Argon ion laser lines are used to excite fluorescence in a mixture of cesium and xenon. Excimer band fluorescence is observed at higher pressures (about 1 atm) while at lower pressures (several torr) a diffuse fluorescence due to the cesium dimer is observed whose character changes with exciting wavelength. The excimer fluorescence is shown to be directly related to the location of the exciting wavelength within previously measured Cs/Xe line shapes. This fact suggests that the excimer systems may be efficiently pumped through these line shapes. Qualitative energy-level schemes are proposed to explain the observations in both the excimer and dimer systems.

Exton, R. J.↗
NASA NTRS1978

Excitation of instability waves in a two-dimensional shear layer by sound

The excitation of instability waves in a plane compressible shear layer by sound waves is studied. The problem is formulated mathematically as an inhomogeneous boundary-value problem. A general solution for abitrary incident sound wave is found by first constructing the Green's function of the problem. Numerical values of the coupling constants between incident sound waves and excited instability waves for a range of flow Mach number are calculated. The effect of the angle of incidence in the case of a beam of acoustic waves is analyzed. It is found that for moderate subsonic Mach numbers a narrow beam aiming at an angle between 50 to 80 deg to the flow direction is most effective in exciting instability waves.

Tam, C. K. W.↗
NASA NTRS1979

Calculation of resonant effects in electron-impact excitation of positive ions Application to oxygen VII

The general reaction theory of Feshbach is applied to the calculation of resonant effects in near-threshold electron-positive-ion excitation. The theory divides configuration space into open- and closed-channel parts, resonance effects being described by the closed-channel part. The open-channel part is handled in a distorted-wave approximation to the set of open-channel coupled equations. Various methods are suggested for handling the closed-channel part. However, an 'attached-excited-target approximation' is used explicitly, which is further approximated by a set of uncoupled closed-channel equations. As an example, the 1 1S-2 1P excitation cross section of O VII below the 3 3S threshold is calculated. Various distorted-wave approximations are investigated and results from most of them are quite similar. Resonant effects arising from the attachment of the colliding electron with the 3 3S state are found to be small, but other close-lying n = 3 states have not yet been included.

Pindzola, M. S.↗
NASA NTRS1979

Atomic electron excitation probabilities during orbital electron capture by the nucleus

Approximate probabilities of electron excitation (shakeup/shakeoff) from various atomic states during nuclear ns electron capture have been calculated in the sudden approximation, using Hartree-Fock wave functions. Total excitation probabilities are much lower than during inner-shell ionization by photons or electrons, and ns states are more likely to be excited than np states. This latter result is borne out by K-alpha X-ray satellite spectra.

Crasemann, B.↗
NASA NTRS1980

Broadband jet noise amplification by a pure tone excitation

A model is proposed for the change in turbulent structure of a round jet in the presence of an acoustic excitation. The excitation is initial amplitude at the jet exit. As these waves propagate downstream they extract energy from the mean flow and transfer it to the random turbulence. This results in an increase in the levels of the turbulence and a resulting increase in the radiated broadband noise. An examination is made of the effect of excitation level and frequency on the jet flow. The numerical procedure allows for radial as well as axial variations in the averaged properties of jet to be calculated. The results indicate that the presence of a finite amplitude instability wave increases the spreading of the jet.

Morris, P. J.↗
NASA NTRS1979

The excitation of density waves at the Lindblad and corotation resonances by an external potential

The linear response of a differentially rotating two-dimensional gas disk, both with and without self-gravity, to a rigidly rotating external potential is calculated on the assumptions that the speed of sound is much smaller that the orbital velocity and that the external potential varies on the scale of the disk radius. The results show that: (1) the external potential exerts torques on the disk only at the Lindblad and corotation resonances; (2) the torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; (3) the torque at corotation has the sign of the radial vorticity gradient; and (4) the torques are of the same order of magnitude at both types of resonance and independent of the speed of sound in the disk. It is found that the external potential also excites density waves in the vicinity of the Lindblad and corotation resonances, that the long trailing wave is excited at a Lindblad resonance, and that short trailing waves are excited at the corotation resonance. The behavior of particle disks is briefly discussed, and the external torques on particle disks are proven to be identical to those on gas disks

Goldreich, P.↗
NASA NTRS1980

MCSCF wave functions for excited states of polar molecules - Application to BeO

A previously reported multi-configuration self-consistent field (MCSCF) algorithm based on the generalized Brillouin theorem is extended in order to treat the excited states of polar molecules. In particular, the algorithm takes into account the proper treatment of nonorthogonality in the space of single excitations and invokes, when necessary, a constrained optimization procedure to prevent the variational collapse of excited states. In addition, a configuration selection scheme (suitable for use in conjunction with extended configuration interaction methods) is proposed for the MCSCF procedure. The algorithm is used to study the low-lying singlet states of BeO, a system which has not previously been studied using an MCSCF procedure. MCSCF wave functions are obtained for three 1 Sigma + and two 1 Pi states. The 1 Sigma + results are juxtaposed with comparable results for MgO in order to assess the generality of the description presented here.

Bauschlicher, C. W., Jr.↗
NASA NTRS1980

Electron-impact excitation of the singlet states of N2. I - The Birge-Hopfield system /b 1 pi u - X 1 Sigma g +/

Results of a study of the electron-impact excitation of the b 1 pi u state of N2, one of the singlet states thought to be excited by precipitating electrons in the auroral zones, and of its predissociation and radiative relaxation through the emission of the Birge-Hopfield band system (b 1 pi u - X 1 Sigma g +) are presented. A collimated electron beam was passed through N2 gas producing a variety of atomic states through dissociative excitation, and the radiation resulting from relaxation of these states was observed by VUV and visible-IR monochromators. Absolute emission cross sections for 11 Birge-Hopfield bands are obtained for energies from threshold to 500 eV, and used to calculate the absolute transition probabilities for BH(1, v-prime) bands and the variation of the electric dipole moment with internuclear distance. With the exception of the v-prime equals 1, 5 and 6 vibrational levels, all b 1 pi u levels are found to predissociate with a specific predissociation branching ratio greater than 0.99, representing a major source of nitrogen atoms.

Zipf, E. C.↗
NASA NTRS1980

A laboratory study on the dissociative recombination of vibrationally excited O2/+/ions

The dissociative recombination of vibrationally excited O2(+) ions is studied in light of the possible importance of this reaction in upper atmospheric chemistry. A plasma spectroscopy experiment was performed in a microwave cavity filled by an argon-oxygen mixture, with O(1S) production monitored by measurements of the 5577-A afterglow, the O2(+) density and the electron concentration. Plasma and optical data reveal the predominant afterglow ions to be Ar2(+) and O2(+), with an effective O(1S) dissociative recombination coefficient of 2.1 x 10 to the -8th cu cm/sec, corresponding to a quantum yield of 10%. Experiments with an argon-krypton-oxygen mixture reveal that vibrationally excited O2(+) ions are the chief source of the O(1S) atoms, with a specific recombination coefficient for the dissociation of O2(+)(2 pi g) into O(1S) and O(1D) of 4.2 x 10 to the -9th cu cm/sec. A comparison of the laboratory results with Atmospheric Explorer data on the 5577-A airglow implies that O2(+) ions in the sunlit ionosphere are vibrationally excited to the same degree as in the laboratory, with the vibrational relaxation of these ions much slower than dissociative recombination. Results also predict a dawn-twilight asymmetry in the effective O(1S) yield due to the normal variation of electron content.

Zipf, E. C.↗
NASA NTRS1980

Emission line shapes produced by dissociative excitation of atmospheric gases

The spectral line shapes of the radiation emitted from O atoms produced by the dissociative excitation of O2, CO, CO2 and NO are investigated. Doppler line shapes are derived from time-of-flight spectra of O (5S0) atoms decaying by the emission of 1356-A radiation after being produced in electron impact experiments at incident electron energies from 25 to 300 eV. It is shown that the effective line width of the radiation is large compared with the Doppler absorption widths of ambient O atoms in both photoelectron and auroral excitation, and thus the dissociatively excited component of the O I 1304-A airglow will behave as though it were optically thin, exhibiting pronounced limb brightening effects and a scale height characteristic of the initial, local source function. It is found that the average kinetic energy of the dissociation fragments inferred from O I (5S) time-of-flight spectra is in good agreement with that of O I (3S) atoms in the electron impact dissociation of CO2, although not for O2. Finally, it is suggested that although electron impact dissociation of CO and CO2 contributes to the 1304-A emission in the upper atmosphere of Venus, it cannot be the dominant source of this radiation since the absolute cross sections for the reaction are too small.

Zipf, E. C.↗
NASA NTRS1979

The mechanics of vortex pairing in a circular jet under controlled excitation

The vortex pairing phenomenon in the near field circular jet under controlled excitation is monitored by the method of phase-averaging. Hot-wire measurements were made in a 7.62 cm diameter air at Re(D) = 32,000, excited at St(D) = 0.85 with an excitation amplitude of 3%. The trajectories of the vortex centers and their longitudinal and transverse convection velocities during the pairing program are measured. Spatial distributions of the average velocity, vorticity, coherent Reynolds numbers, streamlines and pseudo-streamfunction are presented at a particular phase of the pairing. These represent field measurements at one particular time and do not involve the use of Taylor hypothesis.

Zaman, K. B. M. Q.↗
NASA NTRS1981

Turbulence in sound excited jets - Measurements and theory

The mechanisms by which the turbulent structure of an axisymmetric jet is modified by the presence of an acoustic excitation are examined. A model is described in which the excitation triggers instability waves at the jet exit. As these waves propagate downstream they extract energy from the mean flow and transfer it to the random turbulence. This results in an increase of the random turbulence levels and a more rapid mixing and spreading for the jet. Models are introduced for the Reynolds stress and the 'wave-induced stress'. It is shown that at high frequencies the presence of the instability wave may reduce the random turbulence levels. Numerical calculations are presented for both the radial and axial variation in the time-averaged properties of the flow as a function of excitation conditions. The calculations are compared with measurements of fluctuating velocity and pressure in a round jet with a Reynolds number of 375,000, based on jet diameter and exit velocity.

Morris, P. J.↗
NASA NTRS1980

Vortex pairing in a circular jet under controlled excitation. I - General jet response. II - Coherent structure dynamics

Hot-wire and flow visualization studies were performed in three air jets subjected to pure-tone excitation. The instability, vortex roll-up, and transition to the controlled excitation were investigated. The conditions most favorable to vortex parting were determined as a function of the excitation Strouhal number, the Reynolds number, and the initial shear-layer state; it was shown that the rolled-up vortex rings undergo pairing under 'the shear layer mode', and the 'jet-column mode' when the Strouhal numbers based on the initial shear-layer momentum thickness are 0.012 and 0.85, respectively. Coherent ring-like vortical structures could be educed to the end of the potential core; however, the paired vortex becomes weaker with increasing downstream distances. The transverse transport of 'u' momentum by the coherent structures was much larger during the pairing process than in regions where a single vortex is studied.

Zaman, K. B. M. Q.↗
NASA NTRS1981

Excited ozone is a possible source of atmospheric N2O

Consideration is given to the possibility that internally excited ozone formed in the three-body recombination reaction between oxygen atoms and molecules may be a potential source of atmospheric N2O through a gas-phase reaction with nitrogen molecules. Determinations of the wavelength dependence of the quantum yield for N2O formation from the photolysis of dilute solutions of O3 in liquid N2 and of the O(1D) quantum yield in the gas-phase photolysis of O3 are interpreted as suggesting the possibility of the excited ozone reaction, and a new expression for N2O quantum yield is derived. An expression for the production rate of N2O through the proposed reaction is then obtained and used to calculate atmospheric concentrations and fluxes of N2O. The theoretical profile obtained is found to agree better with experimental data than that obtained without the excited ozone reaction taken into account and to demonstrate a wide variability in N2O mixing ratios. In addition, the existence of the new N2O source is noted to imply a significantly smaller flux of N2O from surface microbiological activities, and provide a possible physical basis for latitudinal and temporal N2O variations and an additional stability for the ozone layer.

Prasad, S. S.↗
NASA NTRS1981

Plasma waves excited in the upstream region by density gradient of energetic-particle streams associated with the earth's bow shock

The present study is concerned with the excitation of plasma waves at the boundaries of the streams of energetic particles which are associated with the earth's bow shock. The analysis was motivated by observational results acquired with the ISEE spacecraft (Parks et al., 1981). It is shown theoretically that the density gradient of energetic ions can only excite ion waves, whereas that of energetic electrons can excite both ion waves and Langmuir waves. This conclusion appears to be consistent with observations.

Wu, C. S.↗
NASA NTRS1981

Direct nuclear excitation of a He-3 - CO2 gas mixture

Probe measurements using a continuous wave CO2 laser have been carried out on a He-3 - CO2 gas mixture directly excited by the He-3(n,p)H-3 reaction. At a gas pressure of 1 atm and thermal neutron flux up to 10 to the 17th n/sq cm sec, total extinction of the probe beam was observed. Addition of N2 to the gas mixture (up to 20%) produced a media such as is used in electric discharge CO2 lasers. However, this mixture produced the same results as the He-3 - CO2 mixture. Absorption of the laser beam indicates that with direct nuclear excitation, it is the lower laser level which is excited. Therefore direct nuclear pumping of a CO2 laser does not appear feasible.

Jalufka, N. W.↗
NASA NTRS1981

Measurements and predictions of turbulence in sound excited jets

The modifications to the structure of turbulence in a round jet excited by acoustic tones upstream of the jet exit are measured and compared with predictions. The prediction scheme assumes that the acoustic waves excite instability waves or large turbulent structures in the jet. The presence of these waves at finite amplitude causes an increase in the levels of the random turbulence. Models are presented for the interaction between the instability waves and the turbulence. The numerical scheme permits the enhanced turbulence and large turbulent structures to modify the shape of the mean velocity profile. The changes in the turbulence spectra with excitation are measured. The jet has a Reynolds number of 3.75 x 10 to the 5th and the initial boundary layer is turbulent.

Morris, P. J.↗