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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Collisionless dissociation and isotopic enrichment of SF6 using high-powered CO2 laser radiation

Dissociation of S-32F6 and the resultant isotopic enrichment of S-34F6 using high-powered CO2 laser radiation has been studied with higher experimental sensitivity than previously reported. Enrichment factors have been measured as a function of laser pulse number, wavelength, energy and time duration. A geometry independent dissociation cross section is introduced and measured values are presented. Threshold energy densities, below which no dissociation was observed, were also determined.

Gower, M. C.↗
NASA NTRS1978

The room-temperature dissociation of the compound Au3Si

Three gold-18.6 at.% silicon specimens which were solidified by quenching from above the liquidus temperature have been reexamined after being stored at room temperature for about 8 years. It was found that the 'metastable' compound Au3Si formed on quenching had begun to dissociate by a surface nucleated, solid-state reaction. The reaction front had advanced about 0.2 mm from the surface, leaving a rim of dissociation products. These dissociation products were studied using optical and scanning electron microscopy, electron beam microprobe analysis and an X-ray micro-diffraction technique. It was concluded that the reaction produces elemental gold and silicon, as at higher reaction temperatures, but that the scale of dispersion is so extremely fine that the resulting microstructure cannot be resolved by scanning electron microscopy.

Bhattacharya, D.↗
NASA NTRS1980

A study of the processes in the RF hydrogen gas dissociator

The role of the RF gas dissociator in the hydrogen maser is examined. Based on collisional and plasma transport processes, the performance of the source is investigated. It is found that while the complexity of the collisional processes in the RF dissociator prohibits an easily obtained quantitative expression for the performance of the source, it is nevertheless possible to make general inferences concerning the qualitative performance based on collisional effects. An analytical expression for the efficiency of the source in atom production is obtained based on plasma transport processes. On the basis of this study some recommendations are made for the development of more efficient RF hydrogen gas dissociators for use in masers.

Maleki, L.↗
NASA NTRS1981

Study of the dissociation of molecular hydrogen

Dissociators used to obtain an RF plasma discharge for hydrogen masers and the test system used for operation and evaluation of the dissociators are described. A compact sorption cartridge using a graphite matrix is tested as part of a hydrogen scavenging system. Testing of a vacuum enclosed hydrogen dissociator suitable for long term operation in space is described.

Vessot, R. F. C.↗
NASA NTRS1981

Electron-temperature dependence of dissociative recombination of electrons with N2/+/.N2 dimer ions

The variation with electron temperature of the dissociative recombination of electrons with N2(+).N2 dimer ions is investigated in light of the importance of such ions in the lower ionosphere and in laser plasmas. Dissociative recombination coefficients were determined by means of a microwave afterglow mass spectrometer technique for electron temperatures from 300-5600 K and an ion and neutral temperature of 300 K. The recombination coefficient is found to be proportional to the -0.41 power of the electron temperature in this range, similar to that observed for the CO(+).CO dimer ion and consistent with the expected energy dependence for a fast dissociative process.

Whitaker, M.↗
NASA NTRS1983

The Delta band dissociation of nitric oxide - A potential mechanism for coupling thermospheric variations to the mesosphere and stratosphere

Depletion of solar radiation at discrete wavelengths by nitric oxide has a significant impact on the dissociation rate of this gas in the delta(0, 0) and delta(1, 0) bands. Inclusion of the opacity provided by a typical NO profile reduces the dissociation rate in the upper stratosphere to 50-75 pct of that predicted when the optical depth calculation omits this contribution. A substantial fraction of the NO column abundance as measured near the stratopause resides in the lower thermosphere where correlations of NO with solar and magnetic activity are well documented. Variations in the thermospheric NO abundance therefore modulate the radiation field at the precise wavelengths absorbed by this molecule in the mesosphere and upper stratosphere. Predicted changes in the dissociation rate arising from a varying thermospheric opacity exceed 8 pct throughout the mesosphere and reach 15 pct between 65 km and 95 km.

Frederick, J. E.↗
NASA NTRS1983

The dissociation between subjective workload and performance A multiple resource approach

A theory of the dissociation between subjective measures of mental workload and performance is described. The theory proposes that subjective measures are heavily driven by the number of tasks or task elements that a subject must perform concurrently. However they are relatively less sensitive to whether these tasks compete for common or separate resources, and to the difficulty of a single task, particularly if this difficulty is related to response factors. Performance, on the other hand, is particularly influenced by single task difficulty of both a perceptual and response nature and by resource competition between tasks. A set of three experiments are described to examine the dissociation between subjective difficulty measures and performance. These experiments employ different combinations of three tasks: tracking, memory search, and a simulated air traffic control problem. The results supported all forms of dissociation predicted by the theory and the implications of results to workload measurement are discussed.

Wickens, C. D.↗
NASA NTRS1984

The dissociation of H2 on the Ni(100) surface

The dissociation of H2 on the (100) surface of Ni is investigated using a cluster model. The mechanism for dissocation of H2 directly above a Ni atom has little to no barrier and involves the Ni 3d electrons; elimination of the Ni 3d interaction with the H2 increases the barrier to more than 50 kcal/mol. The dissociation at the bridge site, treated without the Ni 3d interaction, leads to a barrier of about 30 kcal/mol, leading to the conclusion that the dissociation of H2 at any site on a Ni(100) surface requires strong 3d participation. The results are quantitatively different if the Ni 4p orbitals are not included. The effects of cluster size on the results are also discussed.

Siegbahn, P. E. M.↗
NASA NTRS1985

Dissociation of metastable O2 as a potential source of atmospheric odd oxygen

An analysis of the possible dissociation of metastable oxygen molecules subject ot constraints imposed by selection rules for molecular transitions, airglow observations, and atmospheric chemistry leads to the following conclusions. Dissociation of O2(b1Sigma g +) must produce a negligible number of oxygen atoms at all altitudes in the earth's atmosphere. However, if the dissociation cross section of O2(a1Delta g) has a maximum value in the range 10 to the -20th to 10 to the -19th/sq cm, then the process O2(a1Delta g) + h(nu) yields O2(C3Delta u) yields O(3P) + O(3P) will constitute a significant, and potentially the major, source of odd oxygen in the uppermost stratosphere and mesosphere.

Frederick, J. E.↗
NASA NTRS1985

Causes of dissociation between subjective workload measures and performance - Caveats for the use of subjective assessments

Dissociations between subjective workload assessments and performance were investigated. The difficulty of a Sternberg memory search task was manipulated by varying stimulus presentation rate, stimulus discernibility, value of good performance, and automaticity of performance. All Sternberg task conditions were performed both alone and concurrently with a tracking task. Bipolar subjective workload assessments were collected. Dissociations between workload and performance were found related to automaticity, presentation rate, and motivation level. The results were interpreted as supporting the hypothesis that the specific cognitive processes responsible for subjective assessments can differ from those responsible for performance. The potential contamination these dissociations could inflict on operational workload assessments is discussed.

Vidulich, M. A.↗
NASA NTRS1986

The production of CO(+) (B2Sigma +) from dissociative photoionization excitation of CO2

The dissociative photoionization excitation process in CO2 is studied. In contrast to previous studies, attention is focused on the vibrational and rotational levels produced in fragment ions, partial cross-section measurements for producing such fragment ions in a specific quantum state, and the mechanisms that govern the dissociative ionization excitation processes. The partial fluorescence cross section for the production of CO(+) (B2Sigma +) from CO2 over a wide wavelength range was measured. It is concluded that the production of the CO(+) (B2Sigma +) fragment near the threshold is through a direct dissociative photoionization process.

Wu, C. Y. R.↗
NASA NTRS1988

Two-temperature interpretation of dissociation rate data for N2 and O2

The existing experimental data on dissociation of nitrogen and oxygen obtained using shock-tubes during the 1960's and 1970's are reinterpreted using the two-temperature thermo-chemical model developed recently in order to determine the rate coefficients consistent with the model. In this model, the vibrational-electronic temperature is calculated by integrating a separate conservation equation accounting for the suppression of vibrational energy during dissociation due to preferential removal of high vibrational states. The rate coefficient is assumed to be a function of the geometrically-averaged temperature between the translational-rotational temperature and the vibrational-electronic temperature. By comparing the computed overall and species densities with the experimental data, the rate coefficient values most consistent with the model, and their ranges of uncertainty, are deduced for dissociation of N2 through collisions with N2 or N, and for O2 through collisions with O2, O or N2. It is seen that a single set of such rate coefficients fit all existing experimental data closely. According to the two-temperature model, density and species density are insensitive to the rate coefficients, and so the rate coefficients so determined are uncertain to within a factor of at least 1.5.

Park, Chul↗
NASA NTRS1988

Dissociation of performance and subjective measures of workload

A theory is presented to identify sources that produce dissociations between performance and subjective measures of workload. The theory states that performance is determined by (1) amount of resources invested, (2) resource efficiency, and (3) degree of competition for common resources in a multidimensional space described in the multiple-resources model. Subjective perception of workload, multidimensional in nature, increases with greater amounts of resource investment and with greater demands on working memory. Performance and subjective workload measures dissociate when greater resources are invested to improve performance of a resource-limited task; when demands on working memory are increased by time-sharing between concurrent tasks or between display elements; and when performance is sensitive to resource competition and subjective measures are more sensitive to total investment. These dissociation findings and their implications are discussed and directions for future research are suggested.

Yeh, Yei-Yu↗
NASA NTRS1988

Comparison of vibration dissociation coupling and radiative heat transfer models for AOTV/AFE flowfields

A series of detailed studies comparing various vibration dissociation coupling models, reaction systems and rates, and radiative heating models has been conducted for the nonequilibrium stagnation region of an AFE/AOTV vehicle. Atomic and molecular nonequilibrium radiation correction factors have been developed and applied to various absorption coefficient step models, and a modified vibration dissociation coupling model has been shown to yield good vibration/electronic temperature and concentration profiles. While results indicate sensitivity to the choice of vibration dissociation coupling model and to the nitrogen electron impact ionization rate, by proper combinations accurate flowfield and radiative heating results can be obtained. These results indicate that nonequilibrium effects significantly affect the flowfield and the radiative heat transfer. However, additional work is needed in ionization chemistry and absorption coefficient modeling.

Carlson, Leland A.↗
NASA NTRS1989

Translation-vibration-dissociation coupling in nonequilibrium hypersonic flows

A new simple and computationally efficient model was developed, describing the evolution of vibrational states during relaxation and dissociation. The model is based on dividing the nitrogen molecules into two types, those in the vibrational states at a lower level, whose vibrational energy is below a cutoff energy, and those in an upper level, with vibrational energy above the cutoff. Dissociation occurs at the upper level, and recombination returns molecules to the lower level. The model was applied to two flows of engineering interest, the flow through a normal Mach 15 shock wave at 60 km, and a supersonic quasi-one-dimensional flow in a nozzle. Results are compared to those obtained by existing translation-vibration-dissociation coupling models, with results indicating significant differences between the models.

Candler, Graham↗
NASA NTRS1989

Fast molecular shocks. II - Emission from fast dissociative shocks

The line radiations emitted in the cooling gas behind a fast dissociative shock are studied. The intensities emitted in high rotational transitions of the neutral molecules CO, SiO, HCN, CN, NO, and SO are estimated, as well as in rovibrational transitions of the molecular ions HeH(+) and OH(+) in radio recombination lines of atomic hydrogen and in fine-structure transitions of C, C(+), O, and Si(+). The predictions are compared with the observed intensities of line emission from the Orion-KL region. For Orion-KL the observations do not exclude, but probably do not require, the presence of a fast dissociative shock. Emission from SiO in high-J rotational states and from vibrationally excited OH(+), HeH(+), HeH(+), and SO(+) may be detectable from dissociative shocks under suitable conditions of preshock density and shock velocity; such emission may prove to be a useful diagnostic probe of fast shock activity.

Neufeld, David A.↗
NASA NTRS1989

Dissociative recombination in aeronomy

The importance of dissociative recombination in planetary aeronomy is summarized, and two examples are discussed. The first is the role of dissociative recombination of N2(+) in the escape of nitrogen from Mars. A previous model is updated to reflect new experimental data on the electronic states of N produced in this process. Second, the intensity of the atomic oxygen green line on the nightside of Venus is modeled. Use is made of theoretical rate coefficients for production of O (1S) in dissociative recombination from different vibrational levels of O2(+).

Fox, J. L.↗
NASA NTRS1992

Development of a new model for vibration-dissociation coupling in nitrogen

A new model for vibration-dissociation coupling is derived using knowledge obtained from a simulation of the vibrational relaxation of heated nitrogen by collisional processes. This bilevel model is based on the assumption that nitrogen can be represented by two molecular species. The vibrational energy relaxation is represented by a modified form of the Landau-Teller equation. The effects of the bilevel model on a typical hypervelocity flowfield computation are examined and compared to the Park TTv model. Specifically, it is found that the bilevel model exhibits a decreased dissociation rate. This is believed to be due to the inhibition of dissociation by nonequilibrium vibrational relaxation.

Landrum, D. B.↗