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At least 271 records · Page 15

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NASA NTRS1972

Linear momentum transfer effects in molecular dissociation produced by electron impact.

In this study of molecular dissociation produced by electron impact, diatomic systems and polyatomic molecules are considered, and attention is given to the effects of thermal motion and of momentum transfer in the collision process. A procedure is described which makes it possible to 'construct' both the laboratory angular distribution and velocity distribution of the atomic fragments (or, alternatively, the time-of-flight distribution). The calculation assumes that s-wave electron scattering predominates, i.e., that excitation occurs near threshold. The computational procedure may also be reversed to allow construction of possible molecular models to fit given experimental angular and velocity distribution data.

Misakian, M.↗

NASA NTRS1972

Dissociation energies of some high temperature molecules containing aluminum

The Knudsen cell mass spectrometric method has been used to investigate the gaseous molecules Al2, AlSi,AlSiO, AlC2, Al2C2, and AlAuC2. Special attention was given to the experimental considerations and techniques needed to identify and to measure ion intensities for very low abundance molecular species. Second- and third-law procedures were used to obtain reaction enthalpies for pressure calibration independent and isomolecular exchange reactions. Dissociation energies for the molecules were derived from the measured ion intensities, free-energy functions obtained from estimated molecular constants, and auxiliary thermodynamic data. The bonding and stability of these aluminum containing molecules are compared with other similar species.

Stearns, C. A.↗

NASA NTRS1973

Dissociation rate of bromine diatomics in an argon heat bath

The evolution of a collection of 300 K bromine diatomics embedded in a heat bath of argon atoms at 1800 K was studied by computer, and a dissociation-rate constant for the reaction Br2 + BR + Ar yields Br + Ar was determined. Previously published probability distributions for energy and angular momentum transfers in classical three-dimensional Br2-Ar collisions were used in conjunction with a newly developed Monte Carlo scheme for this purpose. Results are compared with experimental shock-tube data and the predictions of several other theoretical models. A departure from equilibrium is obtained which is significantly greater than that predicted by any of these other theories.

Razner, R.↗

NASA NTRS1972

Dissociative excitation of molecular hydrogen by electron impact.

A pulsed electron beam was employed in the experiment to excite a diffuse gas of hydrogen molecules. The energy resolution of the electron gun permitted careful measurements of the thresholds for the production of slow and fast H(2s) atoms. The experiment was conducted in a vacuum system that facilitated a systematic study of the angular distribution of the fragments in the dissociative excitation process. The results permit the identification of the several excited states that are involved in the production of the H(2s) fragments.

Misakian, M.↗

NASA NTRS1973

A shock-tube determination of the CN ground state dissociation energy and electronic transition moments for the CN violet and red band systems

The CN ground state dissociation energy and the sum of squares of the electronic transition moments of the CN violet bands have been simultaneously determined from spectral emission measurements behind incident shock waves. The unshocked test gases were composed of various CO2-CO-N2-Ar mixtures, and the temperatures behind the incident shocks ranged from 3500 to 8000 K. The variation of the electronic transition moment with internuclear separation was found to be small for both the CN violet and red band systems.

Arnold, J. O.↗

NASA NTRS1974

VUV dissociative excitation cross sections of H2O, NH3, and CH4 by electron impact

Absolute excitation functions for excited fragments resulting from electron bombardment of H2O, NH3, and CH4 by low-energy electrons (0 to 300 eV) have been measured in the vacuum ultraviolet (1100 to 1950 A). The predominant emission for each molecule was the H Lyman-alpha line, while the O I, N I, C I, and C II emissions were at least an order of magnitude weaker. Absolute cross sections at 100 eV are given along with the appearance potential of the various processes and the possible dissociative-excitation channels through which such processes proceed.

Morgan, H. D.↗

NASA NTRS1974

Shock-tube study of carbon monoxide dissociation kinetics

Carbon monoxide dissociation-rate data were obtained over the temperature range 5600-12,000 K. The experiments were conducted with undiluted CO to emphasize rate constants applicable to molecular gas systems. Data were obtained as time-resolved pressure measurements on the end wall of a shock tube and, in some cases, as emission histories of the C2 Swan system (0-0 band) behind incident shock waves. Results confirm the presence of C2 as an intermediate species in CO decomposition.

Hanson, R. K.↗

NASA NTRS1975

Shock wave structure in an ideal dissociating gas

Composition changes within the shock layer due to chemical reactions are considered. The Lighthill ideal dissociating gas model was used in an effort to describe the oxygen type molecule. First, the two limiting cases, when the chemical reaction rates are very slow and very fast in comparison to local convective rates, are investigated. Then, the problem is solved for arbitrary chemical reaction rates.

Liu, K. H.↗

NASA NTRS1975

Dissociative recombination source for O I /1D/ atoms

A study of the nighttime dissociative recombination production of O(1D) is reported. The data were gathered by a rocket payload carrying an ion mass spectrometer, Langmuir probe, retarding potential analyzer, and 6300-A photometer. The specific recombination rate to produce O(1D) atoms is deduced to be (2.8 + or - 1.0) times 10 to the minus 8th cu cm per sec and is 30% of the total laboratory rate. The quenching rate at 250 km is 0.0044 + or - 0.0015 per sec.

Sharp, W. E.↗

NASA NTRS1977

The yield of N/2D/ atoms in the dissociative recombination of NO/+/

The quantum yield or branching ratio of N(2D) atoms formed in the reaction e + NO(+) yields N + O was measured to be 76% plus or minus 6%. Photoionization of buffered nitric oxide by a flash lamp was studied using time-resolved atomic absorption. Atoms were produced both by direct photodissociation and by dissociative recombination, and these two effects were separated by means of SF6 as an electron scavenger.

Kley, D.↗

NASA NTRS1977

The vibrational energy levels and dissociation energy of O2/+/ /X 2 Pi g/

The absolute vibrational energy levels of the X 2 Pi g state of O2(+) have been determined by the method of photoelectron spectroscopy in the range nu = 0-25. The precision of the method is comparable to that of conventional high resolution optical spectroscopy. The dissociation energy obtained by a linear extrapolation of the vibrational separations is found to be 6.67 plus or minus 0.18 eV.

Samson, J. A. R.↗

NASA NTRS1978

H2 cooling, dissociation, and infrared emission in shocked molecular clouds

Models are presented of interstellar shocks in molecular clouds over ranges of ambient molecular density from 1000 to 10 million per cu cm and shock velocity from 6 to 14 km/s. Estimates of H2-H2 collisional-excitation rates are used to derive the H2 radiative cooling rates from vibrational-rotational quadrupole transitions as a function of n(H2) and temperature. The emissivities integrated through the shock of the strongest infrared lines in the v = 1-0, 2-0, and 2-1 bands of H2. The effectiveness of H2 dissociative cooling is considered for the highest-velocity shocks. The H2 line intensities from such shocks are compared with those produced by the 'competitive' mechanism of UV pumping for two likely driving mechanisms of shocks - wind-driven shells and expanding H II regions.

Shull, J. M.↗

NASA NTRS1980

Dislocation dissociation in some f.c.c. metals

The dissociation of a perfect screw dislocation into a stacking fault in an f.c.c. lattice is modeled by the modified lattice statics. The interatomic potentials are obtained from the work of Esterling and Swaroop and differ substantially from those empirical potentials usually employed in defect simulations. The calculated stacking fault widths for aluminum, copper, and silver are in good agreement with weak beam microscopy results.

Esterling, D. M.↗

NASA NTRS1980

Dissociative charge transfer of H/+/ ions with H2 and D2 molecules from 78 to 330 K

The dissociative charge transfer of He(+) ions with H2 and D2 molecules has been studied using a temperature-variable drift-tube mass-spectrometer apparatus over the temperature range 78 to 330 K. The binary rate coefficients are small at 300 K, approximately 10 to the -13th to 10 to the -14th cu cm/sec, and only slightly larger at 78 K. Termolecular contributions to the binary rate coefficients are found to be small at 330 K but increase substantially with decreasing temperature. Two-body charge transfer with D2 is found to be slower than with H2 by a factor of 10, in good agreement with recent theoretical predictions, although the measured values of the rate coefficients are larger by a factor of about 4 than the predicted values.

Johnsen, R.↗

NASA NTRS1981

On the role of vibrational excitation in dissociative recombination

An improved physical model of dissociative recombination is presented and applied to experimental data on the temperature dependence of rate coefficients for the rare-gas and atmospheric-gas ions. It is shown that in the charge neutralisation of the rare-gas dimer ions, autoionisation plays an important role (at least in comparison with the atmospheric-gas ions) and contributes to the fast fall-off in the rate coefficient with vibrational excitation observed in shock tube studies. Numerical estimates of the observed fall-off in rate coefficient with increasing vibrational excitation are also presented.

Cunningham, A. J.↗

NASA NTRS1981

The dissociative recombination of O2/+/ in the ionosphere

Aeronomical determinations of the dissociative recombination reaction rate coefficient for O2(+) and alpha depend directly on a knowledge of the rate coefficient for the charge exchange of O(+) with O2 and k. The aeronomical determination of alpha is reevaluated using Atmosphere Explorer satellite data in light of a subsequent laboratory measurement of k (Chen et al., 1978). The results are found to be in good agreement with laboratory determinations of the coefficient for night-time conditions. For data obtained under sunlit conditions, however, the results differed significantly with those of the laboratory measurements. These results imply that the state of the O2(+) molecule major thermospheric processes needs to be examined in greater detail.

Torr, M. R.↗

NASA NTRS1983

Dissociated Airflow Effects During Plasma-Arc Testing

Program computes heating rates and surface friction effects. COLDARC predicts heating rate and surface friction on test article during plasmaarc testing. Uses simplified frozen-flow model to represent dissociated airflow and predict heat flux and surface friction, including effects of retarded atomic recombination from test facility data. COLDARC written in FORTRAN IV.

Miller, B. A.↗

NASA NTRS1983

Production of negative ions by dissociative electron attachment to SO2

Dissociative electron attachment cross section measurements for the production of O(-), S(-), and SO(-) have been performed utilizing a crossed target SO2 molecule beam-electron beam geometry. The relative flow technique is employed to determine the absolute values of cross sections. The attachment energies corresponding to various cross section maxima are: 4.30 and 7.1 eV for O(-)/SO2; 4.0, 7.5, and 8.9 eV for S(-)/SO2, and 4.7 and 7.5 eV for SO(-)/SO2.

Orient, O. J.↗