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At least 253 records · Page 14

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NASA NTRS1987

Collisional excitation of an asymmetric rotor, silicon dicarbide

Rotational excitation rates have been computed for the asymmetric top molecule SiC2 in collisions with low-energy He atoms. The intermolecular forces were obtained from an electron gas model, and collision dynamics were treated within the infinite-order sudden approximation. Total excitation rates, i.e., summed over final levels, are expected to be accurate to about 50 percent, and the larger state-to-state rates are likely to be within a factor of about 2 of the correct values, although some of the smaller (and less important) rates may be less accurate. These rates are also thought to reflect, within this level of accuracy, rates for excitation by collisions with H2 molecules.

Palma, Amedeo↗

NASA NTRS1987

Collisional excitation of interstellar sulfur dioxide

State-to-state rotational excitation rates for the asymmetric top molecule SO2 in collisions with low-energy He atoms have been computed. The intermolecular forces were obtained from an electron gas model, and collision dynamics were treated with the finite-order sudden approximation. The total excitation rate is probably accurate to better than 50 percent; however, individual state-to-state rates may be in error by factors of 2 or 3, and some smaller rates may be accurate only to an order of magnitude. Present results are expected to reflect within the same level of accuracy rates for excitation by collisions with H2 molecules.

Palma, Amedeo↗

NASA NTRS1987

Collisional excitation of interstellar cyclopropenylidene

Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation.

Green, Sheldon↗

NASA NTRS1987

Collisional interchange instability. II - A comparison of the numerical simulations with the in situ experimental data

A two-dimensional model applicable to F region ionosphere plasma instabilities has been developed and described in a companion paper. Here the model is applied to equatorial F region irregularities and is tested against rocket and satellite data. As a diagnostic, simulated data sets are created which are similar to the one-dimensional measurements of plasma density performed by space probes and the Fourier transform of these data is taken in the same manner and used by the space experimentors. Unlike previous simulations of this phenomenon, an inherent anisotropy is found in the instability development which is mirrored in the in situ data. Evidence that the shallow spectral slopes which often characterize spread F rocket spectra near the F peak may be due to a change in the angle between the rocket velocity vector and the characteristic directions in the medium is presented.

Kelley, M. C.↗

NASA NTRS1988

A partially collisional model of the Titan hydrogen torus

Occasional collision effects are encompassed by the present numerical model of atomic hydrogen densities in Titan's hydrogen torus, which is noted to be azimuthally symmetric; the torus density peaks sharply at Titan's orbit, and rapidly decreases in the both radially outward and perpendicular directions. Collisions of Saturnian atoms with the torus population produce a corona. Without this coronal population, the torus model would not reproduce the Voyager 2 Lyman-alpha intensities. These observations may nevertheless be modelled by a torus-plus-corona model that also reconciles the Pioneer 11 torus observations with those of the two Voyager spacecraft.

Hilton, Douglas A.↗

NASA NTRS1988

Collisional-radiative switching - A powerful technique for converging non-LTE calculations

A very simple technique has been developed to converge statistical equilibrium and model atmospheric calculations in extreme non-LTE conditions when the usual iterative methods fail to converge from an LTE starting model. The proposed technique is based on a smooth transition from a collision-dominated LTE situation to the desired non-LTE conditions in which radiation dominates, at least in the most important transitions. The proposed approach was used to successfully compute stellar models with He abundances of 0.20, 0.30, and 0.50; Teff = 30,000 K, and log g = 2.9.

Hummer, D. G.↗

NASA NTRS1988

Collisional properties of ice spheres at low impact velocities

The results of experimental studies on the impact properties of water ice are discussed. Stable temperatures of 85 K and pressures as low as 10 to the -5th Torr were achieved using a new apparatus consisting of a compound disk pendulum and a stainless steel, temperature-controlled cryostat. The coefficient of restitution as a function of velocity was obtained for ice spheres with four different radii of curvature and with a variety of surface conditions. These data can be well fitted by an exponetial law epsilon of given form for most measurements. Surface conditions can drastically alter the resulting value of epsilon, however. In particular, the presence of frost or a roughened contact surface can lower epsilon at a given velocity by 10-30 percent from that of a smooth sphere. It is shown how the presence of frost can change the velocity behavior of epsilon from an exponential to a power-law form. The applications of the results to the dynamics of Saturn's rings are discussed.

Hatzes, Artie P.↗

NASA NTRS1988

Collisional stripping of Mercury's mantle

A three-dimensional smoothed-particle hydrocode is the basis of the present numerical simulations of conditions under which a giant collision between a proto-Mercury and a planet one-sixth its size would lead to the loss of most of the silicate mantle of Mercury and thereby account for its anomalously high density. A head-on collision at 20 km/sec, and an off-axis impact parameter of half the radius of the proto-Mercury at 35 km/sec, are approximately equal in damage yielded; both will yield a remnant whose characteristics are those of the present Mercury.

Benz, Willy↗

NASA NTRS1988

Gravity domains and assembly of the North American continent by collisional tectonics

A gravity trend map of North America, based on a horizontal Bouguer gravity gradient map produced from gravity data for Canada and the conterminous United States, is presented and used to define a continental mosaic of gravity trend domains akin to structural domains. Contrasting trend characteristics at gravity domain boundaries support the concept of outward growth of the continent primarily by accretionary tectonics. Gravity patterns, however, indicate a different style of tectonics dominated in the development of now-buried Proterozoic orogenic belts in the south-central United States, supporting a view that these belts formed along the leading edge of a southward-migrating Proterozoic continental margin.

Thomas, M. D.↗

NASA NTRS1988

Analytic limits on the forms of spectra possible from optically thin collisional bremsstrahlung source models

The constraints on hard X-ray bremsstrahlung spectral forms required in order for them to correspond to physically acceptable (nonnegative) electron distributions in thin-target, thick-target, and thermal source models are discussed. The extent to which various spectra can be attributed to the different models is examined, showing that many possible spectra cannot be described by all, or in some cases, any of the models. It is shown that for any bremsstrahlung cross section, the thick-target and thermal models require that successively higher derivatives of the thin-target constraint have the appropriate sign. It is found that thermal models are the most restrictive, and that thin-target models are the least restrictive. Explicit analytic constraint expressions are derived for all three cases for the Kramers cross section and examples of acceptable and unacceptable spectra are given. Application of these criteria to the testing and exclusion of models is discussed.

Brown, John C.↗

NASA NTRS1988

Collisional quenching of atoms and molecules on spacecraft thermal protection surfaces

Preliminary results of a research program to determine energy partitioning in spacecraft thermal protection materials due to atom recombination at the gas-surface interface are presented. The primary focus of the research is to understand the catalytic processes which determine heat loading on Shuttle, Aeroassisted OTV, and NASP thermal protection surfaces in nonequilibrium flight regimes. Highly sensitive laser diagnostics based on laser-induced fluorescence and resonantly-enhanced multiphoton ionization spectroscopy are used to detect atoms and metastable molecules. At low temperatures, a discharge flow reactor is employed to measure deactivation/recombination coefficients for O-atoms, N-atoms, and O2. Detection methods are presented for measuring O-atoms, O2 and N2, and results for deactivation of O2 and O-atoms on reaction-cured glass and Ni surfaces. Both atom recombination and metastable product formation are examined. Radio-frequency discharges are used to produce highly dissociated beams of atomic species at energies characteristic of the surface temperature. Auger electron spectroscopy is employed as a diagnostic of surface composition in order to accurately define and control measurement conditions.

Marinelli, W. J.↗

NASA NTRS1988

Collisional excitation of interstellar water

Rates for rotational excitation of water molecules in collisions with He atoms have been obtained from a new, accurate theoretical interaction potential. Rates among the lowest 40 ortho levels are given for kinetic temperatures to 1400 K and among the lowest 29 para levels for kinetic temperatures to 800 K.

Palma, Amedeo↗

NASA NTRS1989

Simulation of collisional transport processes and the stability of planetary rings

The utility of the phase-space fluid method for the study of planetary ring dynamics is presently demonstrated through the numerical solution of a model kinetic equation for a flattened Keplerian disk. Attention is given to ringlets composed of single-sized particles, as well as to ringlets composed of two different-sized particles; in the latter case, the ringlets evolve in such a way that the lighter particles are confined by the heavier ones. The results obtained indicate that some natural process may sharpen the optical depth profile of edges even without an external forcing mechanism, and that intermediate optical depths are dynamically preferred in some cases.

Brophy, Thomas G.↗

NASA NTRS1989

Oscillator strengths and electron collisional excitation cross sections for atomic oxygen

The electron-impact excitations of 12 states of O I are analyzed using a close-coupling approximation. The absorption oscillator strengths for dipole-allowed transitions among the states are calculated. Cross sections for excitation of the states by electron impact in the energy region 13.87 to 100 eV are compared with the data of Smith (1976) and Doering and Gulcicek. It is observed that there is a correlation between the data at energies 13.87, 20, and 30 eV for the (3)P-3p(5)P transition and at energies 13.87 and 16.5 eV for the (3)P-3s(5)S(0) transitions; however, for the dipole-allowed (3)P-3d(3)D(0) and (3)P-3s(prime)(3)D(0) transitions there are discrepancies in magnitude.

Tayal, S. S.↗

NASA NTRS1989

Collisional excitation of interstellar molecules - Deuterated water, HDO

Using an accurate theoretical interacting potential and coupled-channel molecular scattering calculations, excitation rates for deuterated water (HDO) in collisions with He atoms were computed. Data are presented for rates among the 34 rotational levels with energy below 400/cm for temperatures between 50 and 500 K. Excitation rate constants were determined using data from fixed energy cross sections averaged over thermal (Boltzmann) distributions of collision energies. Based on the agreement (for H2O) of rates from two different theoretical potentials and on agreement with the limited experimental data, the larger rates for HDO were estimated to be generally accurate to better than 50 percent.

Green, Sheldon↗

NASA NTRS1990

Electron beam injection during active experiments. II - Collisional effects

During active beam experiments, the presence of high neutral densities at low altitudes and/or during thruster firings has been observed to modify the spacecraft charging and the properties of the beam. Two-dimensional (three-velocity) electromagnetic particle simulations with ionizing collisions incorporated are used to investigate the modification of the beam-plasma interaction as the neutral density is increased. It is shown that when the spacecraft is uniformly immersed in a neutral cloud, most of the ionization is produced by direct ionization by the beam and its secondaries, rather than via vehicle-induced or wave-induced ionization for the neutral densities considered.

Winglee, R. M.↗

NASA NTRS1990

Collisional quenching of O(1D) by O(3P)

Metastable O(1D) atoms may be quenched in collisions with ground state O(3P) atoms by transitions in the avoided crossing regions of the three lowest 3Pi(g) states of O2 of which the lowest separates to O(3P) + O(3P) and the two upper to O(3P) + O(1D). Quantal calculations of the adiabatic potential energy curves of the 3Pi(g) states are carried out with particular attention to an avoided crossing region in the lowest two states around a nuclear separation of 3.2a(0). Diabatic potential matrix elements are constructed from the adiabatic curves by imposing the requirement that they be smooth everywhere. A multi-state diabatic formulation is used to describe the scattering and the cross-sections for the collision-induced quenching of O(1D) atoms are calculated.

Yee, J.-H.↗

NASA NTRS1991

Cross sections for production of H(2p, 2s, 1s) by electron collisional dissociation of H2

The excitation function of H Ly-alpha from the astrophysically important dissociation of electron-excited H2 over the range 10-700 eV has been measured. The analysis predicts the cross section to energies higher than the present experimental limit, and it is found that the predicted shape is in close agreement with measured results. At 6 eV the cross section is dominated by the electric dipole first Born component, while at 100 eV the electric dipole component constitutes 73 percent of the total H(2p) cross section. The cross sections of the H(2s) and H(1s) components are calculated.

Ajello, J. M.↗