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At least 343 records · Page 19

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NASA NTRS1979

Metastable isomers - A new class of interstellar molecules

The abundances of a variety of metastable isomers of small organic molecules, analogous to HNC/HCN, in dense interstellar clouds are considered. These metastable species, some of which are thought to exist as intermediates in laboratory organic chemical reactions, are of considerable interest to chemists. Current ideas of gas-phase, ion-molecule chemistry are utilized to demonstrate that such metastable species should often be present in dense clouds in sufficient abundance to be observed. Unfortunately, the spectral constants of metastable isomers have rarely been determined in the laboratory, and quantum chemical calculations of a varying degree of accuracy must be utilized; results are included of some new quantum chemical calculations. The interstellar chemistry and expected microwave spectra of a representative sample of possibly important interstellar metastable isomers are discussed.

Green, S.↗

NASA NTRS1979

Ultraviolet observations of interstellar molecules and grains from Spacelab

The paper presents observational results obtained on interstellar grains and molecules and suggests promising areas for Spacelab research. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions, on the nature of UV extinction at short wavelengths, the presence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands, and the polarization properties of grains in UV wavelengths. The advances in observations of molecules will include the ability to prove more heavily-obscured regions, coverage of wavelength regions inadequately studied with previous instruments, and the capability of observing in optical absorption species detected in the same line of sight in radio emission.

Snow, T. P., Jr.↗

NASA NTRS1980

Collisional excitation of interstellar molecules - Water

Rate constants for excitation of water molecules by collisions with He atoms have been obtained from extensive theoretical calculations. For ortho-H2O, rates are presented for all spectroscopic levels below 1000/cm and for kinetic temperatures to 500 K; for para-H2O, levels below 300/cm and kinetic temperatures to 250 K are included. It is suggested that rates of excitation by collisions with hydrogen molecules are similar to those for He atoms.

Green, S.↗

NASA NTRS1980

Association reactions of C/+/ ions with H2 and D2 molecules at 78 and 300 K

The paper deals with an investigation of associated reactions of C(+) ions with H2 and D2 molecules, using drift-tube-mass spectrometer techniques. It is found that at H2 and D2 densities of 1 x 10 to the 16th per cu cm, the reactions proceed by three-body association, the rate coefficients at 300 and 78 K being somewhat higher for D2 than for H2. The measurements place an upper limit on the rate coefficients for radiative association, a process thought to constitute an important step in the formation of molecules in interstellar gas clouds

Johnsen, R.↗

NASA NTRS1981

Elastic scattering and rotational excitation of a polyatomic molecule by electron impact - Acetylene

Differential, integral, momentum transfer, and partial cross sections have been calculated for elastic scattering and rotational excitation of C2H2 by 10-eV electrons. The effective potential includes static, exchange, and polarization interactions calculated by the INDOX/1s method and the semiclassical exchange approximation with adiabatic polarization at large electron-molecule distances. The scattering is treated by well converged rotational close coupling using the centrifugal dominant scheme to select the channels included and including up to 32 coupled channels for a given total angular momentum. The calculated integral cross sections for pure elastic scattering and rotation excitation are 54.5 and 41.4 a(0)squared, respectively. These are much larger than the values (34.4 and 18.6 a(0)squared) previously (Onda and Truhlar, 1979) calculated for the isoelectronic molecule N2, at this energy. This illustrates how the greater spatial extent of C2H2 greatly increases the cross sections for pure elastic and rotationally inelastic scattering.

Thirumalai, D.↗

NASA NTRS1980

Laboratory studies of some of the major ion-molecule reactions occurring in cometary comae

Laboratory results have been obtained for some of the key ion-molecule reactions which should occur in the inner coma of comets. A complete listing is given of laboratory data presently available for ion-molecule reactions occurring in a mixture of H2O, CO2, CO, CH4, N2, and NH3 cometary volatiles. From these data, the most important reactions occurring in the coma of a water-dominated comet are extracted for use in models of the physics and chemistry of cometary comae.

Huntress, W. T., Jr.↗

NASA NTRS1980

Semiclassical study of the quenching of excited-state fluorine atom by hydrogen molecule - Comparison between reactive and nonreactive processes

Semiclassical calculations are carried out for the quenching of excited-state fluorine atom by collinear collisions with hydrogen molecule. The overall quenching probability is the sum of two contributions: the reactive quenching probability associated with the formation of hydrogen fluoride and the nonreactive quenching probability leading to ground-state fluorine atom and hydrogen molecule. The reactive probability is greater in the threshold region of the collision energy, whereas the nonreactive probability dominates for energies above the threshold region.

Yuan, J.-M.↗

NASA NTRS1981

Investigations of electron helicity in optically active molecules using polarized beams of electrons and positrons

A positronium-formation experiment with a high sensitivity to a possible relation between the helicity of beta particles emitted in nuclear beta decay and the optical asymmetry of biological molecules is presented. The experiment is based on a mechanism in which the electrons in optically active molecules possess a helicity of less than 0.001, too weak to detect in radiolysis experiments, the sign of which depends on the chirality of the isomer. A helicity-dependent asymmetry is sought in the formation of the triplet ground state of positronium when a low-energy beam of polarized positrons of reversible helicity interacts with an optically active substance coating a channel electron multiplier. Asymmetries between positronium decays observed at positive and negative helicities for the same substance can thus be determined with a sensitivity of 0.0001, which represents a factor of 100 improvement over previous positronium experiments.

Gidley, D. W.↗

NASA NTRS1981

An ICR investigation of ion-molecule reactions of HCN

Laboratory reaction rates and product distributions measured by the ion cyclotron resonance technique are reported for a series of reactions of HCN(+) ions with the neutral molecules HCN, CH4, NH3, H2O, CO, CO2, N2 and O2. Rate coefficients are also reported for reactions of positive ions derived from these molecules with HCN. A comparison of the results with a survey of literature values for rate coefficients of HCN(+) reactions shows some exothermic proton transfers from HCN(+) to be inefficient.

Mcewan, M. J.↗

NASA NTRS1981

Molecules in interstellar clouds

The physical conditions and chemical compositions of the gas in interstellar clouds are reviewed in light of the importance of interstellar clouds for star formation and the origin of life. The Orion A region is discussed as an example of a giant molecular cloud where massive stars are being formed, and it is pointed out that conditions in the core of the cloud, with a kinetic temperature of about 75 K and a density of 100,000-1,000,000 molecules/cu cm, may support gas phase ion-molecule chemistry. The Taurus Molecular Clouds are then considered as examples of cold, dark, relatively dense interstellar clouds which may be the birthplaces of solar-type stars and which have been found to contain the heaviest interstellar molecules yet discovered. The molecular species identified in each of these regions are tabulated, including such building blocks of biological monomers as H2O, NH3, H2CO, CO, H2S, CH3CN and H2, and more complex species such as HCOOCH3 and CH3CH2CN.

Irvine, W. M.↗

NASA NTRS1981

Nature and origin of organic molecules in comets

The organic molecules sublimating from the cometary nucleus either represent the 'frost' of interstellar molecules that condensed onto those interstellar grains that have later accreted into comets, or they represent the 'snows' that condensed onto silicate grains during the cooling phase of the presolar nebula. Even if one does not accept the speculations of Hoyle and Wickramashinghe (1977), the fact that prebiotic chemistry could have been brought from the interstellar space or from the solar nebula to the Earth by comets is an intriguing possibility that cannot be ruled out at this stage.

Delsemme, A. H.↗

NASA NTRS1983

Diatomic molecules and metallic adhesion, cohesion, and chemisorption - A single binding-energy relation

Potential-energy relations involving a few parameters in simple analytic forms have been found to represent well the energetics of a wide variety of diatomic molecules. However, such two-atom potential functions are not appropriate for metals. It is well known that, in the case of metals, there exist strong volume-dependent forces which can never be expressed as pairwise interactions. The present investigation has the objective to show that, in spite of the observation concerning metals, a single binding-energy relation can be found which accurately describes diatomic molecules as well as adhesion, cohesion, and chemisorption on metals. This universality reveals a commonality between the molecular and metallic bond.

Ferrante, J.↗

NASA NTRS1983

Plasma ion-induced molecular ejection on the Galilean satellites - Energies of ejected molecules

First measurements of the energy of ejection of molecules from icy surfaces by fast incident ions are presented. Such results are needed in discussions of the Jovian and Saturnian plasma interactions with the icy satellites. In this letter parameters describing the ion-induced ejection and redistribution of molecules on the Galilean satellites are recalculated in light of the new laboratory data.

Johnson, R. E.↗

NASA NTRS1983

An ICR study of ion-molecule reactions of PH(n)+ ions

The reactions of PH(n)+ ions (n = 0-3) were examined with a number of neutrals using ion-cyclotron-resonance techniques. The reactions examined have significance for the distribution of phosphorus in interstellar molecules. The results indicate that interstellar molecules containing the P-O bond are likely to be more abundant than those containing the P-H bond.

Thorne, L. R.↗

NASA NTRS1984

Entropy bottlenecks in ion-molecule reactions

The significance of entropy bottlenecks in dissociation and recombination pathways in the prototype ionic system CH3 + CH3(+) has been investigated. Ion-molecule systems are shown to react through an entirely different dynamics than neutral systems, due to intrinsic differences in the shapes of the relevant potential surfaces. Consequences with regard to the interpretation of experimental rate parameters in the ion-molecule area are discussed.

Dodd, J. A.↗

NASA NTRS1984

Measured ion-molecule reaction rates for modelling Titan's atmosphere

Rate coefficients for several two- and three-body ion-molecule reactions involving hydrocarbons have been determined at thermal energies and above using drift tube-mass spectrometer techniques. The measured rates for clustering and breakup reactions involving CH5(+) and C2H5(+) ions in methane are found to be strongly temperature dependent in the range from 80 to 240 K. The equilibrium constants determined for these reactions differ somewhat from those of Hiraoka and Kebarle (1975). Rate coefficients for two-body reactions of CH5(+), C2H5(+), N(+), H(+), and D(+) ions with methane and/or ethane have been measured. The results indicate that the product yields of several of the fast ion-molecule reactions depend strongly on ion energy (temperature), and therefore previous room-temperature results may be of limited value for model calculations of Titan's atmosphere.

Dheandhanoo, S.↗

NASA NTRS1986

Pregraphitic molecules and the red-rectangle emission

In light of the recent identification of an infrared emission band at 3.3, 6.2, 7.7, 8.6, and 11 microns as emissions from pregraphitic molecules, the possibility that a large broad emission bump in the red part of the spectrum of the Red-Rectangle (AFGL 918) nebula is luminescence from the same molecules was investigated. The abundance, luminescence yield of the carrier, and the phosphorescence is discussed.

Dhendecourt, L.↗

NASA NTRS1986

Photoabsorption and photodissociation of molecules important in the interstellar medium

In the period from May 15, 1985 to May 14, 1986, the photoabsorption and photodissociation cross sections of the interstellar radical of SO and the interstellar molecules of HCl, H2CO, and CH3CN were measured and the results were reported in scientific papers. In the meantime, a windowless apparatus is used to measure the photoabsorption and photodissociation cross sections of CO in the 90-105 nm region. The optical data obtained in this research program are needed for the determination of the formation and destruction rates of molecules and radicals in the interstellar medium. Accomplishments in this research period are summarized below.

Lee, L. C.↗