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The detection of polymeric organic compounds in the mass spectrum of Comet Halley obtained with the Positive Ion Cluster Composition analyzer on Giotto are examined. It is found that, in addition to polyoxymethylene, other polymers and complex molecules may exist in the comet. It is suggested that polymerized hydrogen cyanide may be a source for the observed CN and NH2 jets.
The structure and emission spectrum of J shocks in molecular gas are studied over a broad range of conditions. It is found that at high densities chemistry has a profound effect on the emission spectrum: the density behind the shock is sufficiently high that some of the internal energy of the newly formed H2 molecules is transformed to the gas as heat by collisional deexcitations, producing the H2 formation plateau. In this temperature plateau, endothermal reactions and neutral-neutral chemical reactions with activation energies can proceed efficiently, producing significant quantities of warm H2, CO, OH, and H2O and enhanced columns of warm atoms and ions. The heat generated by the H2 formation is radiated in collisionally excited atomic fine-structure lines.
Continuous-flow plasma discharge techniques show production rates of hydrocarbons and nitriles in N2 + CH4 atmospheres appropriate to the stratosphere of Titan, and indicate that a simple eddy diffusion model together with the observed electron flux quantitatively matches the Voyager IRIS observations for all the hydrocarbons, except for the simplest ones. Charged particle chemistry is very important in Triton's stratosphere. In the more CH4-rich case of Titan, many hydrocarbons and nitriles are produced in high yield. If N2 is present, the CH4 fraction is low, but hydrocarbons and nitriles are produced in fair yield, abundances of HCN and C2H2 in Triton's stratosphere exceed 10 to the 19th molecules/sq cm per sec, and NCCN, C3H4, and other species are predicted to be present. These molecules may be detected by IRIS if the stratosphere is as warm as expected. Both organic haze and condensed gases will provide a substantial UV and visible opacity in Triton's atmosphere.
The adaptation of the molecular R-matrix method, originally developed for electron-molecule collision studies, to positron scattering is discussed. Ab initio R-matrix calculations are presented for collisions of low energy positrons with a number of diatomic systems including H2, HF and N2. Differential elastic cross sections for positron-H2 show a minimum at about 45 deg for collision energies between 0.3 and 0.5 Ryd. The calculations predict a bound state of positronHF. Calculations on inelastic processes in N2 and O2 are also discussed.
UV and VUV absorption and emission spectroscopy is used to probe jet cooled molecules, free radicals, and clusters in the gas phase. Due to efficient cooling inhomogeneous effects on spectral line widths are eliminated. Therefore from these spectra, both structural and dynamical information is obtained. The photoproducts of these reactions are probed by resonance enhanced multiphoton ionization.
It has long been speculated that earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10 to the 9th to 3.8 x 10 to the 9th years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, it is found that 4.5 x 10 to the 9th years ago earth was accreting intact cometary organics at a rate of at least about 10 to the 6th to 10 to the 7th kilograms per year, a flux that thereafter declined with a half-life of about 10 to the 8th years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, about 3 x 10 to the 12th kilograms and about 6 x 10 to the 14th kilograms, respectively.
The volume mixing ratios of five minor gases (CH4, N2O, CO, H2O, and O3) have been retrieved through the middle atmosphere from the analysis of 0.01/cm resolution infrared solar occultation spectra recorded near 28 deg N and 48 deg S latitudes with the Atmospheric Trace Molecule Spectroscopy instrument, flow on board Spacelab 3 (April 30 through May 6, 1985). The results are in general agreement with reported measurements from ground-, balloon-, and satellite-based instruments for the same seasons. In detail, the vertical profiles of these gases show the effects of the upper and middle atmospheric transport patterns dominant during the season of these observations. The profiles inferred at different longitudes around 28 deg N suggest a near-uniform zonal distribution of these gases, for conditions corresponding to late spring. The sunrise occultation measurements point to a larger longitudinal variability in the vertical distribution of these gases at 48 deg S.
The Spacelab 3 Atmospheric Trace Molecule Spectroscopy measurements in 1985 of stratospheric NO/NO2 abundance ratios were compared with abundance ratios calculated using a time-dependent photochemical model. A good agreement between calculated and observed values of the NO/NO2 abundance ratio was found through much of the stratosphere, indicating that the temperature-dependent rate constant K3 (NO + O3) used in the model is properly parameterized.
NASA technical memorandum discusses recent advances at NASA in calculation of electronic spectra of diatomic molecules.
Highly conjugated organic polymers typically have large non-resonant electronic susceptibilities, which give the molecules unusual optical properties. To enhance these properties, defects are introduced into the polymer chain. Examples include light doping of the conjugated polymer and synthesis, conjugated polymers which incorporate either electron donating or accepting groups, and conjugated polymers which contain a photoexcitable species capable of reversibly transferring its electron to an acceptor. Such defects in the chain permit enhancement of the second hyperpolarizability by at least an order of magnitude.
A review is presented of the objectives, instrumentation, performance and results of the ATMOS program developed by NASA-JPL as part of the Spacelab 3 shuttle payload. ATMOS was developed to obtain high-resolution spectroscopic information of the middle atmosphere, from which the vertical distribution of the most possible trace and minor molecules could be retrieved. A complete occultation included not only data recorded when the optical path traversed the earth's atmosphere, but also many spectra with tangent heights big enough for no more telluric absorptions to be detected. The averaging of such 'high sun' observations has provided high quality solar spectra totally free of atmospheric absorption features.
Results are presented on a reanalysis and reinterpretation of solar IR absorption spectra obtained during April-May 1985 observations by the Atmospheric Trace Molecule Spectroscopy instrument aboard Spacelab 3. Results yield updated volume mixing ratio profiles of ClONO2, HCl, and HF. It is shown that the updated HCl and HF results reflect changes below 30 km due to the use of smaller pressure-broadened half-widths, and that the changes in updated ClONO2 profiles are a consequence of adopting improved sets of line parameters. The new ClONO2 have an accuracy of + or - 20 percent.
Following a suggestion of Blake et al. (1987), an attempt was made to account for the unusually large abundances of selected oxygen-containing organic molecules in the so-called 'compact ridge' source directed toward Orion KL by a gas-phase chemical model in which large amounts of water are injected into the source from the IRc2 outflow. Although quantitative model results show that the calculated abundances of methanol, methyl formate, and dimethyl ether can be enhanced relative to their values in the absence of water injection, the enhancements fall far short of explaining the very large observed abundances of these species. Models in which methanol is injected rather than water are more successful, although the source of the methanol is unclear.
The vertical distributions of the important highly reactive stratospheric species, ClO and HO2, have been inferred from Spacelab 3 (May 1985) Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements of more detectable radical and reservoir species. A simple steady state algebraic expression for ClO, utilizing the observed ClONO2/NO2 abundance ratio, approximates the ClO results of a time-dependent photochemical model at sunset (30 deg N). Balloon measurements of ClO and comparisons of time-dependent photochemical model calculations of ClONO2 and HCl with the corresponding ATMOS profiles suggest that the actual ClO values are less than the time-dependent model profiles for ClO at sunset and sunrise (47 deg S). Errors in the current model simulation of the partitioning among the principal free chlorine species (HCl, ClONO2, and ClO) are indicated. The accuracy of the time-dependent model calculations of HO2 is suggested by the agreement between model results and ATMOS observations for HO2NO2 and balloon measurements of HO2. This confirms for the first time the procedure suggested previously by a number of authors of deriving HO(x) abundances from observed fields of O3 and H2O.
Conjugated organic molecules with electron-donating and -accepting moieties can exhibit large electronic second-order nonlinearities, or first hyperpolarizabilities, beta. The present two-state, four-orbital independent-electron analysis of beta leads to the prediction that its absolute value will be maximized at a combination of donor and acceptor strengths for a given conjugated bridge. Molecular design strategies for beta optimization are proposed which give attention to the energetic manipulations of the bridge states. Experimental results have been obtained which support the validity of this approach.
Elastic electron scattering cross sections from 5-30 eV are reported for the molecules C2H4, C2H6, C3H8, Si2H6, and GeH4, obtained using an implementation of the Schwinger multichannel method for distributed-memory parallel computer architectures. These results, obtained within the static-exchange approximation, are in generally good agreement with the available experimental data. These calculations demonstrate the potential of highly parallel computation in the study of collisions between low-energy electrons and polyatomic gases. The computational methodology discussed is also directly applicable to the calculation of elastic cross sections at higher levels of approximation (target polarization) and of electronic excitation cross sections.
The rate coefficient for the ion-molecule reaction NH3(+) + H2 yields NH4(+) + H has been calculated as a function of temperature with the use of the statistical phase space approach. The potential surface and reaction complex and transition state parameters used in the calculation have been taken from ab initio quantum chemical calculations. The calculated rate coefficient has been found to mimic the unusual temperature dependence measured in the laboratory, in which the rate coefficient decreases with decreasing temperature until 50-100 K and then increases at still lower temperatures. Quantitative agreement between experimental and theoretical rate coefficients is satisfactory given the uncertainties in the ab initio results and in the dynamics calculations. The rate coefficient for the unusual three-body process NH3(+) + H2 + He yields NH4(+) + H + He has also been calculated as a function of temperature and the result found to agree well with a previous laboratory determination.
The ozone profile calculated using a 1D time-dependent photochemical model, with recommended absorption cross sections and reaction rate constants, is systematically less than Spacelab 3 (May 1985) atmospheric trace molecule spectroscopy (ATMOS) observations at sunset, 30 deg N latitude, throughout the upper stratosphere and mesosphere. Model results are about 9 percent smaller than the nominal ATMOS O3 value at 40 km and 50 percent less at 76 km. Given the uncertainties associated with the ATMOS profile, the model 'deficit' may be as large as 15 percent at 36 km, increasing to about 70 percent at 76 km. A similar model O3 deficit exists at sunrise, 47 deg S latitude, in the stratosphere: nominal differences of not more than 25 percent, increasing to not more than 45 percent when the ATMOS uncertainties are considered. Model results closely approximate ATMOS observations in both the upper stratosphere and mesosphere upon enhancing odd-oxygen production by increasing the O2 Schumann-Runge band and Herzberg-continuum cross section values used in the calculations.