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At least 235 records · Page 13

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NASA NTRS1986

Dissociative excitation of the N(+)(5S) state by electron impact on N2 - Excitation function and quenching

Metastable N(+)(5S) ions were produced in the laboratory by dissociative excitation of N2 with energetic electrons. The resulting radiative decay of the N(+)(5S) state was observed with sufficient resolution to completely resolve the doublet from the nearby N2 molecular radiation. The excitation function was measured from threshold to 500 eV. The cross section peaks at a high electron energy and also exhibits a high threshold energy both of which are typical of dissociative excitation-ionization processes. This finding complicates the explanation of electron impact on N2 as the mechanism for the source of the 2145 A 'auroral mystery feature' by further increasing the required peak cross section. It is suggested that the apparent N(+)(5S) quenching in auroras may be an artifact due to the softening of the electron energy spectrum in the auroral E region.

Erdman, P. W.↗

NASA NTRS1986

Molecular dissociation and shock-induced cooling in fluid nitrogen at high densities and temperatures

Radiative temperatures and electrical conductivities were measured for fluid nitrogen compressed dynamically to pressures of 18-90 GPa, temperatures of 4000-14,000 K, and densities of 2-3 g/cu cm. The data show a continuous phase transition above 30 GPa shock pressure and confirm that (delta-P/delta-T)v is less than 0, as indicated previously by Hugoniot equation-of-state experiments. The first observation of shock-induced cooling is also reported. The data are interpreted in terms of molecular dissociation, and the concentration of dissociated molecules is calculated as a function of density and temperature.

Radousky, H. B.↗

NASA NTRS1987

The production of O(1S) from dissociative recombination of O2(+)

The first theoretical calculations of the rate coefficient alpha for dissociative recombination of O2(+) leading to O(1S) are reported for a wide range of temperatures. The findings are discussed in terms of the potential energy curves for the ground state of O2(+) and for the dissociative 1Sigma(u) state calculated here. Values of alpha for the equilibrium case in which the electron and vibrational temperatures are identical are shown.

Guberman, Steven L.↗

NASA NTRS1988

The production of O(1D) from dissociative recombination of O2(+)

The results of large scale ab initio calculations of the rates for production of O(1D) by dissociative combination of O2(+) are presented for electron temperatures in the range 100 to 3000 K. A 1-delta-u state is the dominant dissociative route from v = 0 and a 3-sigma-u(-) state is the most important route from v = 1 and v = 2. The calculated total rate for O(1D) production from v = 0 is 2.21(+0.21, -0.24) x 10(-7) x (T sub e/300) exp -.46 near room temperature. The v = 1 and v = 2 rates are about 17 percent and 47 percent smaller respectively, than the v = 0 rate at 300 K.

Guberman, Steven L.↗

NASA NTRS1988

Parameterized cross sections for Coulomb dissociation in heavy-ion collisions

Simple parameterizations of Coulomb dissociation cross sections for use in heavy-ion transport calculations are presented and compared to available experimental dissociation data. The agreement between calculation and experiment is satisfactory considering the simplicity of the calculations.

Norbury, John W.↗

NASA NTRS1989

The dissociation energy of N2

The requirements for very accurate ab initio quantum chemical prediction of dissociation energies are examined using a detailed investigation of the nitrogen molecule. Although agreement with experiment to within 1 kcal/mol is not achieved even with the most elaborate multireference CI (configuration interaction) wave functions and largest basis sets currently feasible, it is possible to obtain agreement to within about 2 kcal/mol, or 1 percent of the dissociation energy. At this level it is necessary to account for core-valence correlation effects and to include up to h-type functions in the basis. The effect of i-type functions, the use of different reference configuration spaces, and basis set superposition error were also investigated. After discussing these results, the remaining sources of error in our best calculations are examined.

Almloef, Jan↗

NASA NTRS1990

Dissociation phenomena in electron-beam sustained carbon dioxide lasers

A number of applications are emerging requiring efficient, long pulse, long-life sealed CO2 lasers. Examples include the proposed NASA and ESA wind lidars. Electron-beam sustained discharge devices are strong contenders. Unlike self-sustained discharges, e-beam sustenance readily provides efficient performance from large volume discharges and offers pulse lengths well in excess of the microsecond or so generally associated with self-sustained devices. In the case of the e-beam sustained laser, since the plasma is externally maintained and operated at electric field strengths less than that associated with the glow to arc transition, the discharges can be run even in the presence of strongly attacking species such as O2. Build up of large levels of attacking contaminants is nevertheless undesirable as their presence reduces the current drawn by the plasma and thus the pumping rate to the upper laser level. The impedance rise leads to a mismatch of the pulse forming network with a consequent loss of control over energy deposition, operating E/N, and gain. Clearly CO2 dissociation rates, the influence of dissociation products on the discharge and gain, and tolerance of the discharge to these products need to be determined. This information can then be used to assess co-oxidation catalyst requirements for sealed operation.

Harris, Michael R.↗

NASA NTRS1990

Dissociative recombination of molecular ions with electrons

An overview is presented for the present state of the art of laboratory measurements of the dissociative recombination of molecular ions with electrons. Most work has focussed on obtaining rates and their temperature dependence, as these are of primary interest for model calculations of ionospheres. A comparison of data obtained using the microwave afterglow method, the flowing afterglow technique, and the merged beam technique shows that generally the agreement is quite good, but there are some serious discrepancies, especially in the case of H(3+) recombination, that need to be resolved. Results of some earlier experimental work need to be reexamined in the light of more recent developments. Such cases are pointed out and a compilation of rate coefficients that have withstood scrutiny is presented. Recent advances in experimental methods, such as the use of laser-in-duced fluorescence, make it possible to identify some neutral products of dissociative recombination. What has been done so far and what results one might expect from future work are briefly reviewed.

Johnsen, Rainer↗

NASA NTRS1989

The dissociation energy of N2

Almlof, Jan↗

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 NTRS1990

Dissociation and ionization of molecular gas in the spiral arms of M51

Researchers derive the star formation rate and efficiency in the arm and interarm regions of M51 from observations of the molecular (Lo et al. 1987) and ionized (van der Hulst et al. 1988) phases of the interstellar medium, and show that the HI observations of Tilanus and Allen (1989) are consistent with dissociation of molecular gas by these young, massive stars if n sub H greater than or equal to 200 cm (-2). However, these stars are not able to dissociate or ionize all the gas, and at least 60 percent must remain molecular in the interarm regions. The efficiency of star formation in M51 seems to be similar to that in the Galaxy, and does not appear to be enhanced in the spiral arms. Therefore, the effect of the strong density wave may be only to concentrate the gas, and hence the young stars, to the arm regions.

Lees, J. F.↗

NASA NTRS1991

Vibration-dissociation coupling in nonequilibrium flows

The purpose of the study is to evaluate all important physical phenomena contributing to the vibration-dissociation problem by developing a numerical technique for the simulation of the vibrational relaxation and dissociation/recombination of N(2) in an isothermal box. A set of coupled master equations representing each vibrational level of N(2) is solved, and both V-V and V-T exchanges between molecules and atoms are included, along with the effects of dissociation and recombustion from/to each level. It is observed that the inverse range parameter has a strong impact on the vibrational exchange rates and relaxation. In a heating case, the lower vibrational levels relax through a series of equilibrium distributions, while in a cooling case, a population inversion is formed in the upper levels. Other cases of heating and cooling will be simulated, and together with this work, used to develop a matrix of nonequilibrium relaxation runs.

Landrum, D. Brian↗

NASA NTRS1991

Theoretical study of the C-H bond dissociation energies of CH4, C2H2, C2H4, and H2C2O

The successive C-H bond dissociation energies of CH4, C2H2, C2H4, and H2C2O (ketene) are determined using large-basis sets and a high level of correlation treatment. For CH4, C2H2, and C2H4 the computed values are in excellent agreement with experiment. Using these results, the values 107.9 + or - 2.0 and 96.7 + or - 2.0 kcal/mol are recommended for the C-H bond dissociation energies of H2C2O and HC2O, respectively.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1991

Vibrational relaxation and dissociation in nitrogen

Calculations of the vibrational and dissociation transition probabilities are made for N2-N2 and N2-N collisions by means of a semiclassical N-state approximation. The flaws in previous techniques are reviewed, with special attention given to the prediction of overtones. The method presented ignores the effects of molecular rotation and employs a revised extended Rydberg intermolecular potential to describe diatom-diatom and diatom-atom collisions. The collision velocities investigated exhibit probabilities of less than unity by means of the N-state method. The continuum is quantized to treat dissociation, and the collision results demonstrate probability enhancements for V-V-T transitions in both bound-bound and bound-free transitions. The technique is of particular interest for the theoretical modeling of reentry flows such as those encountered in aerobraking maneuvers.

Varghese, Philip L.↗

NASA NTRS1991

The computation of C-C and N-N bond dissociation energies for singly, doubly, and triply bonded systems

The bond dissociation energies (De) of C2H2, C2H4, C2H6, N2, N2H2, and N2H4 are studied at various levels of correlation treatment. The convergence of De with respect to the one-particle basis is studied at the single-reference modified coupled-pair-functional (MCPF)level. At all levels of correlation treatment, the errors in the bond dissociation energies increase with the degree of multiple bond character. The multireference configuration-interaction (MRCI) De values, corrected for an estimate of higher excitations, are in excellent agreement with those determined using the size-extensive averaged-coupled-pair-functional (ACPF) method. The full-valence complete-active-space self-consistent-field (CASSCF)/MRCI calculations are reproduced very well by MRCI calculations based on a CASSCF calculation that includes in the active space only those electrons involved in the C-C or N-N bonds.

Langhoff, Stephen R.↗

NASA NTRS1991

Electron impact excitation and dissociation of N2 via the b 1Pi(u) state

Electron impact excitation of the b 1Pi(u) state in N2 plays a prominent role in the dissociation of the molecule and thus in the production of atomic nitrogen in planetary atmospheres. Electron impact excitation cross sections combined with electron-impact-induced fluorescence measurements can yield the corresponding dissociation cross sections. Serious discrepancies exist among excitation cross sections reported in the literature. To clarify the situation, these cross sections were measured at two impact energies using electron energy loss spectroscopy. The new results are in agreement with recent values deduced from optical measurements and fall midway between previous results which are too high or low by factors of 2.

Ratliff, J. M.↗

NASA NTRS1989

The computation of C-C and N-N bond dissociation energies for singly, doubly, and triply bonded systems

The bond dissociation energies (D sub e) of C2H2, C2H4, C2H6, N2, N2H2, and N2H4 are studied at various levels of correlation treatment. The convergence of D sub e with respect to the one particle basis is studied at the single reference modified coupled-pair functional (MCPF) level. At all levels of correlation treatment, the errors in the bond dissociation energies increase with the degree of multiple bond character. The multireference configuration interaction (MRCI) D sub e values, corrected for an estimate of higher excitations, are in excellent agreement with those determined using the size extensive averaged coupled pair functional (ACPF) method. It was found that the full valence complete active space self consistent field (CASSCF)/MRCI calculations are reproduced very well by MRCI calculations based on a CASSCF calculation that includes in the active space only those electrons involved in the C-C or N-N bonds. To achieve chemical accuracy (1 kcal/mole) for the D sub e values of the doubly bonded species C2H4 and N2H2 requires one particle basis sets including up through h angular momentum functions (l = 5) and a multireference treatment of electron correlation: still higher levels of calculation are required to achieve chemical accuracy for the triply bonded species C2H2 and N2.

Langhoff, Stephen R.↗

NASA NTRS1991

Fluid hydrogen at high density - Pressure dissociation

A model for the Helmholtz free energy of fluid hydrogen at high density and high temperature is developed. This model aims at describing both pressure and temperature dissociation and ionization and bears directly on equations of state of partially ionized plasmas, as encountered in astrophysical situations and high-pressure experiments. This paper focuses on a mixture of hydrogen atoms and molecules and is devoted to the study of the phenomenon of pressure dissociation at finite temperatures. In the present model, the strong interactions are described with realistic potentials and are computed with a modified Weeks-Chandler-Andersen fluid perturbation theory that reproduces Monte Carlo simulations to better than 3 percent. Theoretical Hugoniot curves derived from the model are in excellent agreement with experimental data.

Saumon, Didier↗