The growth of deactivated layers on CsI/Na/ scintillating crystals
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Goodman, N. B.↗
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The excitation and collisional deactivation of metastable N/2P/ atoms in auroras
The concentration and altitude distribution of metastable N(2P) atoms was measured in a diffuse IBC II(+) auroral arc. The dominant N(2P) source is shown to be the dissociative excitation of N2 by electron impact with a minor contribution from the dissociative recombination of N2(+) ions. The possibility that an ion-molecule process involving atomic oxygen and vibrationally excited N2(+) ions is a significant N(2P) source is examined. Values for the proportional yield of N(+), N(2P), N(2D), and N(4S) atoms from electron-impact dissociation of N2 under optically thick conditions are given.
Zipf, E. C.↗
Rapid deactivation of N(2D) by O - Impact on thermospheric and mesospheric odd nitrogen
One- and two-dimensional models of thermospheric odd nitrogen are used to explore the consequences of the recently measured fast quenching of N(2D) by O. A large rate coefficient for this reaction profoundly affects the odd-nitrogen chemistry by removing N(2D) as a source of NO and increasing the concentration of N(4S), which destroys NO. The model calculations show that, as the quenching rate increases, the NO and N(2D) densities decrease, while N(4S) densities increase. Comparisons with Atmosphere Explorer and Solar Mesosphere Explorer satellite observations are made. Use of the fast quenching rate in the models causes the NO peak altitude, typically observed near 110 km, to rise to 140 km.
Fesen, C. G.↗
Chlorine Deactivation in the Lower Stratospheric Polar Regions during late Winter: Results from UARS
Recovery from enhanced chlorine conditions in the lower stratospheric polar regions of both hemispheres is investigated using data from the Upper Atmosphere Research Satellite (UARS). Microwave Limb Sounder (MLS) measurements of ClO within the polar vortices are used to infer ClO(sub x) (ClO + 2Cl202) abundances that are then correlated with simultaneous Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of ClON02 and Halogen Occultation Experiment (HALOE) measurements of HCl obtained starting within 5 days of the end of the MLS and CLAES high-latitude observing periods in each hemisphere. Time series of vortex-averaged mixing ratios are calculated on two potential temperature surfaces (585 K and 465 K) in the lower stratosphere for approximately month-long intervals during late winter: August 17 - September 17, 1992, in the southern hemisphere and February 12 - March 16, 1993, in the northern hemisphere. The observed mixing ratios are adjusted for the effects of vertical transport using diabatic vertical velocities estimated from CLAES tracer data. In the northern hemisphere, the decrease in ClO, is balanced on both surfaces by an increase in ClON02- In the southern hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In contrast to the northern hemisphere, there is no significant chemical change in vortex-averaged ClON02 at 465 K, and there is an apparent decrease in ClON02 at 585 K, even after the enhanced ClO abundances have started to recede. Results from the SLIMCAT chemical transport model initialized with UARS data and run with OH + ClO yields HCl + 02 as an 8% channel suggest that the primary recovery product in the south during this time period is not ClON02, but HCl. HALOE HCl mixing ratios are extrapolated back to the time of the MLS and CLAES data. At 585 K, the chlorine budget can be made to balance by extrapolating HCl back to a value of 0.6 parts per billion by volume (ppbv) at the beginning of the study period; at 465 K, the contribution from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlorine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complications caused by ongoing activation, incomplete photochemical assumptions, and/or inadequate data quality.
Santee, M. L.↗
Fitting Vibrational Energy Parameters to Falloff Experiments: CH3 + CH3 + He (0.6-2 Torr 200-298 K)
There are many reported experiments and theoretical calculations for the combination of methyl free radicals diluted in a non-reactive 'deactivator. At high pressure the ethane (C2H6) combination product is the sole product; with decreasing pressure the chemically activated C2H6 will decompose. The falloff in the observed rate coefficient is the result of the competition between collisional stabilization of the chemically activated CA and unimolecular decomposition. The dependence of the rate coefficient on pressure, temperature and collision properties is complex and can not be calculated from first principles. The understanding of this system is not only of fundamental importance but is relevant to the recent detection of methyl free radicals in the atmospheres of Saturn - and Neptune. The temperatures of these outer planet atmospheres are in the 140-200 K region with total pressures (predominately H2 and He) less than 0.2 Torr. Experimentally determined rate coefficients have been reported for this reaction at T = 296-906 K and T = 200-408 K mostly with argon as the deactivator. At T = 200 K only the high pressure rate coefficient has been determined. Complete falloff curves over a wide temperature range (200-1600 K) with a variety of weak collider models used to simulate argon as the deactivator have also been reported by Klippenstein and Harding (KH). More recently we have reported the experimental rate coefficients in the falloff region with helium as the deactivator at 200 and 298 K. In this paper we have used the calculated falloff curves reported by KH for argon to determine the average energy transferred per collision for helium in our recently reported experiments. Collision rates were converted using Lennard Jones parameters; the temperature dependence of this conversion factor is noted. The helium experiments were consistent with a down of approximately 100 cm (exp-1); the temperature dependence was slight. The magnitude of down and its temperature dependence will be compared to other systems with similar substrate complexity and reaction energetics.
Tardy, Dwight C.↗
Effect of pH on subunit association and heat protection of soybean alpha-galactosidase
Soybeans contain the enzyme alpha-galactosidase, which hydrolyzes alpha-1, 6 linkages in stachyose and raffinose to give sucrose and galactose. We have found that galactose, a competitive product inhibitor of alpha-galactosidase, strongly promotes the heat stability of the tetrameric form of the enzyme at pH 4.0 and at temperatures of up to 70 degrees C for 60 min. Stachyose and raffinose also protect alpha-galactosidase from denaturation at pH 4.0 although to a lesser extent. Glucose and mannose have little effect. At pH 7.0 the enzyme is a monomer, and galactose has no effect on the heat stability of the enzyme. In the absence of heat protection of the enzyme by added sugars, a series deactivation mechanism was found to describe the deactivation data. In comparison, a unimolecular, non-first order deactivation model applies at pH 4.0, where heat protection effects were observed. At a temperature above 60 degrees C, simple deactivation is a suitable model. The results suggest that alpha-galactosidase conformation and heat stability are directly related.
Non-NASA Center↗
New insights into differential baroreflex control of heart rate in humans
Recent data indicate that bilateral carotid sinus denervation in patients results in a chronic impairment in the rapid reflex control of blood pressure during orthostasis. These findings are inconsistent with previous human experimental investigations indicating a minimal role for the carotid baroreceptor-cardiac reflex in blood pressure control. Therefore, we reexamined arterial baroreflex [carotid (CBR) and aortic baroreflex (ABR)] control of heart rate (HR) using newly developed methodologies. In 10 healthy men, 27 +/- 1 yr old, an abrupt decrease in mean arterial pressure (MAP) was induced nonpharmacologically by releasing a unilateral arterial thigh cuff (300 Torr) after 9 min of resting leg ischemia under two conditions: 1) ABR and CBR deactivation (control) and 2) ABR deactivation. Under control conditions, cuff release decreased MAP by 13 +/- 1 mmHg, whereas HR increased 11 +/- 2 beats/min. During ABR deactivation, neck suction was gradually applied to maintain carotid sinus transmural pressure during the initial 20 s after cuff release (suction). This attenuated the increase in HR (6 +/- 1 beats/min) and caused a greater decrease in MAP (18 +/- 2 mmHg, P < 0.05). Furthermore, estimated cardiac baroreflex responsiveness (DeltaHR/DeltaMAP) was significantly reduced during suction compared with control conditions. These findings suggest that the carotid baroreceptors contribute more importantly to the reflex control of HR than previously reported in healthy individuals.
Non-NASA Center↗
Ion-atom association reactions in the rare gases.
A simple resonance theory of three-body ion-atom association reactions is presented. The reaction is considered as proceeding through the formation of a long lived orbiting resonance complex between the atom and the ion. The population of these quasi-bound states is estimated assuming thermal equilibrium. A stable molecular ion may then be formed upon deactivation of the complex by collision with a third body. Various simplifying approximations to the potential curves and surfaces are employed. Furthermore, the deactivation cross sections for the relevant complexes are estimated from the corresponding atomic 'sizes.' A simple analytical formula for the three-body rate constant is thus derived. Reasonable agreement with experiment is obtained for He(+) in He and fair agreement for other light systems.
Dickinson, A. S.↗
Ruby laser induced emission from NO2
Two different types of emission from excited NO2 were observed using pulsed ruby laser light at 6943 A. The first type of fluorescence was seen in the near IR and results from the single photon excitation of NO2 from the ground 2-A1 state. By observing the emission as a function of time an unexpected behavior was observed in the near IR and could be explained by a consecutive deactivation mechanism, wherein a secondary species is preferentially detected. A second type of emission recently observed in the blue spectral region is weaker and is due to a multiphoton process. The intensity of the blue emission is a function of the cube of the laser intensity at low pressures and approaches the square at high pressures. This variation is attributed to simultaneous deactivation of the excited NO2 intermediate by collision (square) and by anti-Stokes Raman scattering off of the excited NO2 (cube).
Hakala, D. F.↗
Double-quantum vibrational energy transfer in the O3-CO and O3-OCS systems
Using the CO2 laser induced fluorescence technique at 298 K for excitation of the P(30) transition in O3 or the P(22) transition in OCS, double-quantum vibrational energy transfer was studied in the O3-CO and O3-OCS binary gas systems. Rapid rates were observed for energy transfer from the CO (nu = 1) to the coupled O3(200), O3(101), and O3(002) levels, as well as for the transfer from the coupled O3(001) and O3(100) levels to the OCS(020) level. Deactivation of OCS by N2, Ar, and OCS was studied and deactivation rate constants were measured. The experimental results are compared with those calculated using the first order perturbation theory of dipole transition-moment interactions (Sharma and Brau, 1969) as modified by Tam (1972). An attempt is made to interpret the obtained results.
Hui, K.-K.↗
Pulsed radiolysis of model aromatic polymers and epoxy based matrix materials
Models of primary processes leading to deactivation of energy deposited by a pulse of high energy electrons were derived for epoxy matrix materials and polyl-vinyl naphthalene. The basic conclusion is that recombination of initially formed charged states is complete within 1 nanosecond, and subsequent degradation chemistry is controlled by the reactivity of these excited states. Excited states in both systems form complexes with ground state molecules. These excimers or exciplexes have their characteristics emissive and absorptive properties and may decay to form separated pairs of ground state molecules, cross over to the triplet manifold or emit fluorescence. ESR studies and chemical analyses subsequent to pulse radiolysis were performed in order to estimate bond cleavage probabilities and net reaction rates. The energy deactivation models which were proposed to interpret these data have led to the development of radiation stabilization criteria for these systems.
Gupta, A.↗
Pulse radiolysis of epoxy-based matrix materials
Electron beam pulse radiolysis (600 KeV, 3 nsec) experiments have been carried out on epoxy-based matrix materials. Time and wavelength resolved emission and transient absorption techniques as well as ESR studies have been utilized to identify and monitor short lived reactive intermediates resulting from energetic electron impact. An energy deactivation model based on the results of this work has been developed. Recombination of primary charged species is found to be fast, resulting in the formation of longer lived excited electronic states and radicals which control the subsequent energy deactivation.
Coulter, D. R.↗
The undercooling of liquids
The formation by melt quenching of such metastable structures as glassy or microcrystalline solids and highly supersaturated solutions is made possible by the extreme resistance of most melts to homophase crystal nucleation at deep undercooling. This nucleation resistance contrasts sharply with the very low kinetic resistance to the movement of crystal-melt interfaces, once formed, in metals and other fluid systems at even minute undercooling. The methods of nucleation study which have proven especially effective in bypassing nucleation by heterophase impurities thereby exposing the high resistance of melts to homophase nucleation may be summarized as follows: observation of the crystallization behavior of dispersed small droplets; drop tube experiments in which liquid drops solidify, under containerless conditions, during their fall in the tube; and observation of the crystallization of bulk specimens immersed in fluxes chosen to dissolve or otherwise deactivate (e.g., by wetting) heterophase nucleants. This method has proven to be remarkably effective in deactivating such nucleants in certain pure metals.
Turnbull, D.↗
Metal-support interactions during the adsorption of CO on thin layers and islands of epitaxial palladium
Islands and continuous layers of palladium were grown in an ultrahigh vacuum on substrates of Mo(110)c(14 x 7)-O, designated MoO(x), and of clean Mo(110). It was found that as-deposited islands and layers exhibited bulk palladium adsorption properties for CO when deposited at room temperature and for palladium thicknesses in excess of about 3 monolayers. CO adsorption was drastically reduced, however, on annealing. For islands, annealing temperatures of as low as 400 K led to some reduction in CO adsorption whereas more severe reductions were found to occur at 600 K for islands and at 800 K for continuous multilayers. The deactivation depended on the palladium thickness, the substrate species and the extent of thermal treatments. Auger electron spectroscopy, temperature-programmed desorption and Delta-Phi measurements were combined to interpret the deactivation behavior in terms of substrate-support interactions involving the diffusion of substrate species towards the palladium surface.
Park, C.↗
Reactivation study for NASA Lewis Research Center's hypersonic tunnel facility
The Hypersonic Tunnel Facility (HTF) at NASA Lewis Research Center's Plum Brook Station is a blowdown, free-jet, nonvitiated propulsion facility capable of Mach 5, 6, and 7 with true temperature, altitude, and air composition simulation. The facility has been in a deactivated status for 13 years. Discussed are the capabilities of HTF, and the results of a deactivation study recently conducted to determine the cost, schedule, and technical effort required to restore HTF to its original design operating capabilities are summarized.
Haas, Jeffrey E.↗
Reactivation study for NASA Lewis Research Center's Hypersonic Tunnel Facility
The Hypersonic Tunnel Facility (HTF) at NASA Lewis Research Center's Plum Brook Station is a blowdown, free-jet, nonvitiated propulsion facility capable of Mach 5, 6, and 7 with true temperature, altitude, and air composition simulation. The facility has been in a deactivated status for 13 years. Discussed are the capabilities of HTF, and the results of a deactivation study recently conducted to determine the cost, schedule, and technical effort required to restore HTF to its original design operating capabilities are summarized.
Haas, Jeffrey E.↗
Solutions to time variant problems of real-time expert systems
Real-time expert systems for monitoring and control are driven by input data which changes with time. One of the subtle problems of this field is the propagation of time variant problems from rule to rule. This propagation problem is even complicated under a multiprogramming environment where the expert system may issue test commands to the system to get data and to access time consuming devices to retrieve data for concurrent reasoning. Two approaches are used to handle the flood of input data. Snapshots can be taken to freeze the system from time to time. The expert system treats the system as a stationary one and traces changes by comparing consecutive snapshots. In the other approach, when an input is available, the rules associated with it are evaluated. For both approaches, if the premise condition of a fired rule is changed to being false, the downstream rules should be deactivated. If the status change is due to disappearance of a transient problem, actions taken by the fired downstream rules which are no longer true may need to be undone. If a downstream rule is being evaluated, it should not be fired. Three mechanisms for solving this problem are discussed: tracing, backward checking, and censor setting. In the forward tracing mechanism, when the premise conditions of a fired rule become false, the premise conditions of downstream rules which have been fired or are being evaluated due to the firing of that rule are reevaluated. A tree with its root at the rule being deactivated is traversed. In the backward checking mechanism, when a rule is being fired, the expert system checks back on the premise conditions of the upstream rules that result in evaluation of the rule to see whether it should be fired. The root of the tree being traversed is the rule being fired. In the censor setting mechanism, when a rule is to be evaluated, a censor is constructed based on the premise conditions of the upstream rules and the censor is evaluated just before the rule is fired. Unlike the backward checking mechanism, this one does not search the upstream rules. This paper explores the details of implementation of the three mechanisms.
Yeh, Show-Way↗
Heterogeneous photocatalytic oxidation of atmospheric trace contaminants
The following subject areas are covered: (1) design and construction of continuous flow photoreactor for study of oxidation of trace atmospheric contaminants; (2) establishment of kinetics of acetone oxidation including adsorption equilibration, variation of oxidation rate with acetone concentration and water (inhibitor), and variation of rate and apparent quantum yield with light intensity; (3) exploration of kinetics of butanol oxidation, including rate variation with concentration of butanol, and lack of inhibition by water; and (4) exploration of kinetics of catalyst deactivation during oxidation of butanol, including deactivation rate, influence of dark conditions, and establishment of photocatalytic regeneration of activity in alcohol-free air.
Ollis, David F.↗