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At least 415 records · Page 23

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NASA NTRS1988

Calculations of rate constants for the three-body recombination of H2 in the presence of H2

A new global potential energy hypersurface for H2 + H2 is constructed and quasiclassical trajectory calculations performed using the resonance complex theory and energy transfer mechanism to estimate the rate of three body recombination over the temperature range 100 to 5000 K. The new potential is a faithful representation of ab initio electron structure calculations, is unchanged under the operation of exchanging H atoms, and reproduces the accurate H3 potential as one H atom is pulled away. Included in the fitting procedure are geometries expected to be important when one H2 is near or above the dissociation limit. The dynamics calculations explicitly include the motion of all four atoms and are performed efficiently using a vectorized variable-stepsize integrator. The predicted rate constants are approximately a factor of two smaller than experimental estimates over a broad temperature range.

Schwenke, David W.↗

NASA NTRS1989

Introduction to dissociative recombination

Dissociative recombination (DR) of molecular ions with electrons has important consequences in many areas of physical science. Ab-initio calculations coupled with resonant scattering theory and multichannel quantum defect studies have produced detailed results illuminating the role of ion vibrational excitation, the quantum yields of the DR products, and the role of Rydberg states. The theoretical and experimental results are discussed.

Guberman, Steven L.↗

NASA NTRS1989

Observations of a shock and a recombination layer at the contact surface of Comet Halley

Results are presented on observations in the vicinity of the contact surface of the Comet Halley, obtained by the Giotto ion mass spectrometer, with emphasis placed on two specific events observed in this region on the inbound pass. One was a burst of energized ions (about 20 eV) of 2-sec duration observed two seconds before the contact surface was encountered, which coincided with a pulse in magnetic field strength interpreted by Neubauer (1988) as a fast-mode shock traveling away from the contact surface. The second was a sharp spike in ion densities observed at the contact surface by the mass analyzer, centered approximately at the inner edge of the contact surface. This ion-density spike is interpreted as a boundary layer into which the radial ionospheric flow enters and piles up; the density increase is limited by recombination.

Goldstein, B. E.↗

NASA NTRS1989

'Design of CO-O2 recombination catalysts for closed-cycle CO2 lasers'

Pulsed CO2 lasers have many applications in aeronautics, space research, weather monitoring and other areas. Full exploitation of the potential of these lasers is hampered by the dissociation of CO2 that occurs during laser operation. The development of closed-cycle CO2 lasers requires active CO-O2 recombination (CO oxidation) catalysts and design methods for implementation of catalysts inside lasers. This paper will discuss the performance criteria and constraints involved in the design of monolith catalyst configurations for use in a closed-cycle laser and will present a design study performed with a computerized design program that had been written. Trade-offs between catalyst activity and dimensions, flow channel dimensions, pressure drop, O2 conversion and other variables will be discussed.

Guinn, K.↗

NASA NTRS1989

Ab initio studies of dissociative recombination

Quantum chemical calculations of the dissociative recombination of O2(+) and N2(+) are reported. An approach for calculating autoionization widths from high-principal-quantum-number Rydberg states is summarized, and an example is presented for the lowest dissociative state of O2. For O2(+), the 1Sigma(+)u state is the sole source of O(1S) from the lowest 10 vibrational levels of the ion. Rate coefficients for generating O(1S) and O(1D) at ionospheric temperatures are reported.

Guberman, Steven L.↗

NASA NTRS1989

Dissociative recombination in interstellar clouds

Molecular ions play a significant role in the chemistry and evolution of interstellar molecular clouds, even though these regions are overwhelmingly neutral. The dissociative recombination (DR) process governs the abundances of many of these ions and of related neutral species. The gas-phase ion-molecule chemistry of the simplest species is summarized, with emphasis on those problems which are most sensitive to uncertain rates or product branching ratios of DR processes. Examples of the kinds of information needed about DR processes are presented. The importance of the H3(+) ion and prospects for its direct observation are discussed.

Black, John H.↗

NASA NTRS1990

A theoretical prediction of hydrogen molecule dissociation-recombination rates including an accurate treatment of internal state nonequilibrium effects

The dissociation and recombination of H2 over the temperature range 1000-5000 K are calculated in a nonempirical manner. The computation procedure involves the calculation of the state-to-state energy transfer rate coefficients, the solution of the 349 coupled equations which form the master equation, and the determination of the phenomenological rate coefficients. The nonempirical results presented here are in good agreement with experimental data at 1000 and 3000 K.

Schwenke, David W.↗

NASA NTRS1990

Observations of columnal recombination in the ionization tracks of energetic heavy nuclei in an argon-methane gas mixture

Measurements of ionization signals resulting from the passage of energetic heavy nuclei through a gas mixture consisting of 95 mol percent Ar plus 5 mol percent CH4, at an absolute pressure of 3 atm are presented. The measurements take place under a uniform electric field perpendicular to the ionization track. The signals were compared to the calculated energy losses, with an assumption of proportionality between energy loss rate and ionization rate. Significant deviations from proportionality are found for energy loss rate grater than about 3000 MeV sq cm/g, while fractional deviations are found to be proportional to the energy loss rate (dE/dx) exp m, where m is equal to about two. These results are attributed to the columnal recombination.

Wiedenbeck, Mark E.↗

NASA NTRS1991

Did the universe recombine?

The Zel'dovich-Sunyaev model-independent arguments for the existence of a neutral hydrogen phase is reviewed in light of new limits on the Compton y parameter from COBE. It is concluded that with baryon densities compatible with standard cosmological nucleosynthesis, the universe could have remained fully ionized throughout its history without producing a detectable spectral distortion. It is argued that it is unlikely that spectral observations of the cosmic microwave background will ever require the universe to have recombined for flat cosmologies.

Bartlett, James G.↗

NASA NTRS1991

Dissociative recombination of the ground state of N2(+)

Large-scale calculations of the dissociative recombination cross sections and rates for the v = 0 level of the N2(+) ground state are reported, and the important role played by vibrationally excited Rydberg states lying both below and above the v = 0 level of the ion is demonstrated. The large-scale electronic wave function calculations were done using triple zeta plus polarization nuclear-centered-valence Gaussian basis sets. The electronic widths were obtained using smaller wave functions, and the cross sections were calculated on the basis of the multichannel quantum defect theory. The DR rate is calculated at 1.6 x 10 to the -7th x (Te/300) to the -0.37 cu cm/sec for Te in the range of 100 to 1000 K, and is found to be in excellent agreement with prior microwave afterglow experiments but in disagreement with recent merged beam results. It is inferred that the dominant mechanism for DR imparts sufficient energy to the product atoms to allow for escape from the Martian atmosphere.

Guberman, Steven L.↗

NASA NTRS1990

Measurement of surface recombination velocity on heavily doped indium phosphide

Surface recombination velocity (SRV) on heavily doped n-type and p-type InP was measured as a function of surface treatment. For the limited range of substrates and surface treatments studied, SRV and surface stability depend strongly on the surface treatment. SRVs of 100,000 cm/sec in both p-type and n-type InP are obtainable, but in n-type the low-SRV surfaces were unstable, and the only stable surfaces on n-type had SRVs of more than 10to the 6th cm/sec.

Jenkins, Phillip↗

NASA NTRS1991

Infrared recombination lines of hydrogen from young objects in the southern Galactic plane

Near infrared recombination lines of hydrogen are observed in twelve young objects in the southern Galactic plane. The sample includes Herbig-Haro objects and IRAS dark-cloud point sources from the 1987 catalog of Persson and Campbell. In four of the IRAS sources two or three infrared lines are measured, and their intensity ratios are consistent with models of optically thick ionized winds. The intrinsic line shapes, retrieved from maximum-entropy deconvolutions, indicate gas velocities of 100 km/s or more as expected from ionized winds. These sources are apparently embedded pre-main-sequence objects with outflows. They include some of the brightest known YSOs.

Beck, Sara C.↗

NASA NTRS1992

Recombination of 5-eV O(3P) atoms with surface-adsorbed NO - Spectra and their dependence on surface material and temperature

Measurements have been conducted of the 300-850 nm recombination spectra associated with 5-eV collisions of O(3P) atoms with NO adsorbed on surfaces of MgF2, Ni, and Ti. Attention is given to the dependence of chemiluminescence intensity on surface temperature over the 240-340 K range. While all three materials tend to emit at the lower temperatures, MgF2 exhibits the greatest tendency to chemiluminescence. Both results are reflective of the greater packing density of surface-adsorbed NO at the lower temperatures for each surface. The activation energy for each surface is independent of emission wavelength, so that the same species is emitting throughout the wavelength range.

Orient, O. J.↗

NASA NTRS1991

Evaluation of the minority carrier diffusion length and surface-recombination velocity in GaAs p/n solar cells

The minority carrier diffusion length (Lp) and the surface recombination velocity (Vs) were measured as a function of distance (x) from the p-n junction in GaAs p/n concentrator solar cells. The measured Vs values were used in a theoretical expression for the normalized electron-beam-induced current. A fitting procedure was then used to fit this expression with experimental values to obtain Lp. The results show that both Vs and Lp vary with x. Lp measured in irradiated cells showed a marked reduction. These values were compared to those measured previously which did not account for Vs.

Hakimzadeh, Roshanak↗

NASA NTRS1993

Effect Of Auger Recombination In An Ion Track

Report presents theoretical calculations of contribution of Auger recombination to depletion of charge carriers from ionization track left by passage of energetic heavy ion through silicon-based electronic device.

Edmonds, Larry D.↗

NASA NTRS1993

Minority carrier diffusion length and edge surface-recombination velocity in InP

A scanning electron microscope was used to obtain the electron-beam-induced current (EBIC) profiles in InP specimens containing a Schottky barrier perpendicular to the scanned (edge) surface. An independent technique was used to measure the edge surface-recombination velocity. These values were used in a fit of the experimental EBIC data with a theoretical expression for normalized EBIC (Donolato, 1982) to obtain the electron (minority carrier) diffusion length.

Hakimzadeh, Roshanak↗

NASA NTRS1993

Measurement of the minority carrier diffusion length and edge surface-recombination velocity in InP

A scanning electron microscope (SEM) was used to measure the electron (minority carrier) diffusion length (L(sub n)) and the edge surface-recombination velocity (V(sub s)) in zinc-doped Czochralski-grown InP wafers. Electron-beam-induced current (EBIC) profiles were obtained in specimens containing a Schottky barrier perpendicular to the scanned (edge) surface. An independent technique was used to measure V(sub s), and these values were used in a theoretical expression for normalized EBIC. A fit of the experimental data with this expression enabled us to determine L(sub n).

Bailey, Sheila G.↗

NASA NTRS1992

Wall catalytic recombination and boundary conditions in nonequilibrium hypersonic flows - With applications

The meaning of catalysis and its relation to aerodynamic heating in nonequilibrium hypersonic flows are discussed. The species equations are described and boundary conditions for them are derived for a multicomponent gas and for a binary gas. Slip effects are included for application of continuum methods to low-density flows. Measurement techniques for determining catalytic wall recombination rates are discussed. Among them are experiments carried out in arc jets as well as flow reactors. Diagnostic methods for determining the atom or molecule concentrations in the flow are included. Results are given for a number of materials of interest to the aerospace community, including glassy coatings such as the RCG coating of the Space Shuttle and for high temperature refractory metals such as coated niobium. Methods of calculating the heat flux to space vehicles in nonequilibrium flows are described. These methods are applied to the Space Shuttle, the planned Aeroassist Flight Experiment, and a hypersonic slender vehicle such as a transatmospheric vehicle.

Scott, Carl D.↗