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At least 541 records · Page 30
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NASA NTRS1987

The use of atmospheric measurements to constrain model predictions of ozone change from chlorine perturbations

Atmospheric photochemistry models have been used to predict the sensitivity of the ozone layer to various perturbations. These same models also predict concentrations of chemical species in the present day atmosphere which can be compared to observations. Model results for both present day values and sensitivity to perturbation depend upon input data for reaction rates, photodissociation rates, and boundary conditions. A method of combining the results of a Monte Carlo uncertainty analysis with the existing set of present atmospheric species measurements is developed. The method is used to examine the range of values for the sensitivity of ozone to chlorine perturbations that is possible within the currently accepted ranges for input data. It is found that model runs which predict ozone column losses much greater than 10 percent as a result of present fluorocarbon fluxes produce concentrations and column amounts in the present atmosphere which are inconsistent with the measurements for ClO, HCl, NO, NO2, and HNO3.

Douglass, Anne R.↗
NASA NTRS1987

Tunable ultraviolet laser studies of photon-molecular interactions of cometary interest

Most of the cometary radicals are thought to be formed in the coma by photodissociation processes. Successful modeling of the coma requires a detailed knowledge of the products that are formed from the various parent molecules and the energy partitioning among these products. This information has to be obtained as a function of wavelength, because the sun is not a monochromatic source. In this review, the status of the experimental knowledge of some key molecules is discussed, along with the prospects of some new laboratory techniques that can fill in the gaps in the present knowledge.

Jackson, William M.↗
NASA NTRS1987

Observations of molecules in comets

Ultraviolet and visible spectroscopy of comets has identified a large number of species in the coma, most of which appear to be the photodissociation and photoionization products of the 'parent' molecules evaporated directly from the cometary nucleus. Analyses of cometary spectra support the icy conglomerate model of the nucleus with H2O as the dominant ice species. Two molecules detected in the ultraviolet, CO and S2, are of particular interest to the study of the cosmogonic evolution of cometary grains. CO appears to be a highly variable constituent from comet to comet, while S2, first observed in comet IRAS-Araki-Alcock in 1983, is found in no other celestial source. Both of these molecules appear to be parent molecules.

Feldman, P. D.↗
NASA NTRS1988

Photoabsorption and phototdissociation of molecules important in the interstellar medium

The photoabsorption and photodissociation cross sections of interstellar molecules and radicals were measured in the 90 to 200 nm region using synchrotron radiation, F sub 2 laser, excimer lasers, and discharge lamps as light sources. These data are currently needed for determining the formation and destruction rates of molecules and radicals by the interstellar radiation field.

Lee, Long C.↗
NASA NTRS1987

The photochemistry of some possible cometary CN parent species

Laboratory work on the photochemistry of HC3N, C4N2, and CH3CN in relation to their possible role as CN parent molecules in comets is discussed. Photodissociation of HC3N, photolysis of C4N2, and quantum yields of excited CN(B) and CN(A) radicals from the VUV photolysis of CH3CN are considered.

Halpern, Joshua B.↗
NASA NTRS1987

Submillimeter and far infrared line observations of M17 SW: A clumpy molecular cloud penetrated by UV radiation

Millimeter, submillimeter, and far infrared spectroscopic observations of the M17 SW star formation region are discussed. The results require the molecular cloud near the interface to be clumpy or filamentary. As a consequence, far ultraviolet radiation from the central OB stellar cluster can penetrate into the dense molecular cloud to a depth of several pc, thus creating bright and extended (CII) emission from the photodissociated surfaces of dense atomic and molecular clumps or sheets. The extended (CII) emission throughout the molecular cloud SW of the M17 complex has a level 20 times higher than expected from a single molecular cloud interface exposed to an ultraviolet radiation field typical of the solar neighborhood. This suggests that the molecular cloud as a whole is penetrated by ultraviolet radiation and has a clumpy or filamentary structure. The number of B stars expected to be embedded in the M17 molecular cloud probably can provide the UV radiation necessary for the extended (CII) emission. Alternatively, the UV radiation could be external, if the interstellar radiation in the vicinity of M17 is higher than in the solar neighborhood.

Stutzki, J.↗
NASA NTRS1987

Ab initio studies of low-lying 3Sigma(-), 3Pi, and 5Sigma(-) states of NH. I - Potential curves and dipole moment functions

Configuration interaction wave functions, potential energy curves, and dipole moment functions have been calculated for the four lowest 3Sigma(-) and the three lowest 3Pi states and 5Sigma(-) states of NH. The electronic wave functions were constructed to give a balanced description of valence-Rydberg interactions. Two repulsive states have been identified as important photodissociation pathways. Spectroscopic constants are presented for the bound states, and results are compared to other theoretical and experimental work. The possible predissociation of the A 3Pi state by the 1 5Sigma(-) state is discussed.

Goldfield, Evelyn M.↗
NASA NTRS1988

The ozone hole - The role of polar stratospheric cloud particles

The role of polar stratospheric clouds in the formation of the Antarctic ozone hole is considered. Several researchers have suggested that the decrease in ozone over Antarctica is related to the polar stratospheric clouds (PSCs) which had been observed in the antarctic winter stratosphere. Some of the pertinent characteristics of polar stratospheric clouds are discussed, and it is shown how these clouds may participate in the ozone destruction process. The satellite data for PSCs is analyzed, and statistical information regarding the number and maximum extinctions of these clouds is presented. Evidence that the polar stratospheric clouds are composed of frozen nitric acid is considered. It is suggested that the evaporation of the clouds, in late August and September, will release HOCl and HNO3 to the environment. This could be followed by the photodissociation of HOCl to OH and Cl, which would very effectively destroy ozone. However, the ozone destruction mechanism could be halted when enough of the evaporated nitric acid is photolized.

Hamill, Patrick↗
NASA NTRS1987

Activity of comet P/Halley 23-25 March, 1986 - IUE observations

A large but gradual increase in the brightness of the emission bands of comet Halley and, thus, in the coma abundance of OH, CS, CO2(+), and dust was observed with the International Ultraviolet Explorer (IUE) satellite from Mar. 23 to 25, 1986. This brightness change was also monitored by the Fine Error Sensor (FES) tracking and acquisition camera at a higher temporal resolution than that of the spectrophotometric measurements. The amplitude of the increase varied among the species, depending on the lifetime of the parent molecule. By comparing the FES light curve with the optical light curve measured in the light of C2 by Millis and Schleicher (1986), it can be shown that this increase in brightness corresponds to part of the periodic light curve of the comet. None of the IUE observations show evidence of the large increase in H2O reported to have occurred on March 24.7 by Weaver et al. (1986). It is concluded that the activity reported here is controlled primarily by photodissociation and ionization of molecules ejected form an active area of the nucleus that rotates into the field of view of the sun approximately every 7.4 days.

Mcfadden, L. A.↗
NASA NTRS1987

A rotational-state population analysis of the high-resolution IUE observation of CS emission in comet P/Halley

The high resolution spectrum of comet P/Halley has been measured in the region of the CS emission. CS in comet Halley is produced by photodissociation of CS2 and subsequently reexcited by broad band solar radiation. An inversion procedure is introduced to extract the rotational population distribution from the dispersed fluorescence spectrum. Intensity anomalies in the wavelength region corresponding to P and Q branch emission are considered.

Prisant, M. G.↗
NASA NTRS1988

A comprehensive model of ion diffusion and charge exchange in the cold Io torus

A comprehensive analytic model of radial diffusion in the cold Io torus is developed. The model involves a generalized molecular cloud theory of SO2 and its dissociation fragments SO, O2, S, and O, which are formed at a relatively large rate by solar UV photodissociation of SO2. The key component of the new theory is SO, which can react with S(+) through a near-resonant charge exchange process that is exothermic. This provides a mechanism for the rapid depletion of singly ionized sulfur in the cold torus and can account for the large decrease in the total flux tube content inward of Io's orbit. The model is used to demonstrate quantitatively the effects of radial diffusion in a charge exchange environment that acts as a combined source and sink for ions in various charge states. A detailed quantitative explanation for the O(2+) component of the cold torus is given, and insight is derived into the workings of the so-called plasma 'ribbon'.

Barbosa, D. D.↗
NASA NTRS1987

The abundance of interstellar CO

Data from recent observations of interstellar CO column densities are compiled in tables and graphs and compared with the predictions of theoretical models. The energy-level structure of CO and the excitation of interstellar CO are discussed; UV-absorption and IR line observations are characterized; and techniques used to estimate the CO column density from mm-wavelength observations (based on calibration of the CO line intensity with the column density of H2) are described in detail. Consideration is then given to CO chemistry under interstellar-cloud conditions (isotope fractionation and photodissociation) and models for diffuse, translucent, and dense clouds. A number of unsolved problems requiring additional observations or laboratory investigations are identified.

Van Dishoeck, Ewine F.↗
NASA NTRS1988

Heating efficiencies in the thermosphere of Venus reconsidered

Heating efficiencies less than 10 percent are required by recent models of the low neutral temperatures in the daytime thermosphere of Venus. It is considered here whether such values are justifiable from a molecular point of view. The primary uncertainty in a calculation of the heating efficiency is the fraction of energy, f sub v, that appears as vibrational excitation of product molecules in quenching, photodissociation, and exothermic chemical reactions. The current state of knowledge of energy partitioning in chemical reactions is discussed and a range of likely values for f sub v is deduced. The calculated heating efficiencies fall in the range 16-25 percent over the altitude range from 115 to 200 km. It is suggested that the heating efficiency should not be taken as a free parameter and that the cold Venus thermosphere has not yet been satisfactorily explained.

Fox, J. L.↗
NASA NTRS1987

Perfluorobutyl iodides as gain media for a solar-pumped iodine laser amplifier

The effectiveness of t-C4F9I and n-C4F9I as gain media for a space-based solar-pumped iodine photodissociation laser is investigated experimentally. The optically coupled flashlamp/amplifier-tube apparatus described by Hwang et al. (1986) is employed, driving the amplifier with 4-microsec 4.2-mJ TEM(00) pulses from a flashlamp-pumped laser oscillator; the variation of amplification with energy-extraction time was monitored by inserting a delay of up to 1 msec between the firing of the two flashlamps and measuring the delay between the optical pulses. The results are presented graphically and briefly characterized. The performance of t-C4F9I is found to be better than that of n-C4F9I (or n-C3F7I), with an absorption band shifted toward the visible (for improved utilization of solar radiation), higher gain (by a factor of 2) at all delay times, and better chemical reversibility. The gain did not depend significantly on temperature in any of the iodides.

Tabibi, Bagher M.↗
NASA NTRS1987

High power CW iodine laser pumped by solar simulator

An iodine photodissociation laser was pumped by a long Ar arc as the solar simulator to produce a 10-W CW output. Continuous lasing for 1 h was achieved with a flow of the laser material n-C3F7I. The 10-W CW output is the highest produced to date and establishes the feasibility of developing a solar-pumped laser for space power transmission.

Lee, Ja H.↗
NASA NTRS1988

Temperature-dependent absorption cross sections for hydrogen peroxide vapor

Relative absorption cross sections for hydrogen peroxide vapor were measured over the temperature ranges 285-381 K for lambda = 230 nm-295 nm and 300-381 K for lambda = 193 nm-350 nm. The well established 298 K cross sections at 202.6 and 228.8 nm were used as an absolute calibration. A significant temperature dependence was observed at the important tropospheric photolysis wavelengths lambda over 300 nm. Measured cross sections were extrapolated to lower temperatures, using a simple model which attributes the observed temperature dependence to enhanced absorption by molecules possessing one quantum of O-O stretch vibrational excitation. Upper tropospheric photodissociation rates calculated using the extrapolated cross sections are about 25 percent lower than those calculated using currently recommended 298 K cross sections.

Nicovich, J. M.↗
NASA NTRS1988

Monte Carlo particle-trajectory models for neutral cometary gases. I - Models and equations. II - The spatial morphology of the Lyman-alpha coma

The mathematical derivations of various methods employed in the Monte Carlo particle-trajectory model (MCPTM) are presented, and the application of the MCPTM to the calculation of the photochemical heating of the inner coma through the partial thermalization of cometary hydrogen atoms produced by the photodissociation of water is discussed. This model is then used to explain the observed morphology of the spatially extended Ly-alpha comas of comets. The rocket and Skylab images of the Ly-alpha coma of Comet Kohoutek are examined.

Combi, Michael R.↗
NASA NTRS1988

Abundances in Comet Halley at the time of the spacecraft encounters

Spectrophotometric observations of Comet Halley obtained in March 1986 at the time of the spacecraft encounters with the comet are described, and the column densities derived from the integrated emission band fluxes are presented. The scale lengths are determined from the spatial profiles of the observed species. Production rates are derived and the parent identities and abundance ratios of the studied species are discussed. The data indicate that the abundance of the primordial condensate NH3 comprises about 0.2 percent of the total volatile fraction of the comet nucleus. The ratio of production rates Q(NH2)/Q(H2O) = 0.003 indicates that Q(NH3)/Q(H2O) is roughly 0.3 percent, which is nearly 10 times lower than the value determined from Giotto ion mass spectrometer data. The production rate Q(CH)/Q(H2O) of 0.007 and the CH spatial profile indicate that CH cannot derive entirely from direct photodissociation of CH4.

Wyckoff, Susan↗