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At least 685 records · Page 38

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NASA NTRS1989

Observations of some oxygen-containing and sulfur-containing organic molecules in cold dark clouds

Observations of nine oxygen- and sulfur-containing organic molecules have been made toward the cold dark clouds TMC-1 and L134N. The presence of paraketene (H2C2O) in TMC-1 is confirmed for orthoketene, and has been observed for the first time and a total ketene column density of about 10 to the 13th/sq cm is found. Thioformaldehyde (H2CS) is easily detectable in both TMC-1 and L134N, with a column density about five times larger in the former source. The fractional abundance of ketene is comparable to the predictions of ion-molecule chemistry, while that of thioformaldehyde in TMC-1 is one to two orders of magnitude greater than that expected from such models at steady state. Interstellar sulfur chemistry thus continues to be poorly understood. Upper limits are set for the column densities of formic acid (HCOOH), vinyl alcohol (CH2CHOH), methyl formate (HCO2CH3), formamide (NH2CHO), methyl mercaptan (CH3SH), isothiocyanic acid (HNCS), and thioketene (H2C2S) in both sources.

Irvine, W. M.↗

NASA NTRS1989

Principles and procedures for determining absolute differential electron-molecule (atom) scattering cross sections

Procedures and calibration techniques for measuring the absolute elastic and inelastic differential cross sections (DCS) for electron impact on molecular (atomic) species are described and illustrated by examples. The elastic DCS for the molecule under study is first determined by calibration against helium using the relative flow technique. The second step involves the production of energy-loss spectra for the instrument response function, the unfolding of overlapping inelastic structures and the normalization of inelastic intensities to the elastic cross sections. It is concluded that this method of determining absolute differential electron-molecule (atom) scattering cross sections is generally applicable and provides reliable results.

Nickel, J. C.↗

NASA NTRS1990

Interstellar grain chemistry and organic molecules

The detection of prominant infrared absorption bands at 3250, 2170, 2138, 1670 and 1470 cm(-1) (3.08, 4.61, 4.677, 5.99 and 6.80 micron m) associated with molecular clouds show that mixed molecular (icy) grain mantles are an important component of the interstellar dust in the dense interstellar medium. These ices, which contain many organic molecules, may also be the production site of the more complex organic grain mantles detected in the diffuse interstellar medium. Theoretical calculations employing gas phase as well as grain surface reactions predict that the ices should be dominated only by the simple molecules H2O, H2CO, N2, CO, O2, NH3, CH4, possibly CH3OH, and their deuterated counterparts. However, spectroscopic observations in the 2500 to 1250 cm(-1)(4 to 8 micron m) range show substantial variation from source reactions alone. By comparing these astronomical spectra with the spectra of laboratory-produced analogs of interstellar ices, one can determine the composition and abundance of the materials frozen on the grains in dense clouds. Experiments are described in which the chemical evolution of an interstellar ice analog is determined during irradiation and subsequent warm-up. Particular attention is paid to the types of moderately complex organic materials produced during these experiments which are likely to be present in interstellar grains and cometary ices.

Allamandola, L. J.↗

NASA NTRS1990

Laboratory simulation of interstellar grain chemistry and the production of complex organic molecules

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared (5000 to 500 cm(-1), 2 to 20 microns m), the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources, some deeply embedded in dark molecular clouds, are now available. These spectra provide a powerful probe, not only for the identification of interstellar molecules in both the gas solid phases, but also of the physical and chemical conditions which prevail in these two very different domains. By comparing these astronomical spectra with the spectra of laboratory ices one can determine the composition and abundance of the icy materials frozen on the cold (10K) dust grains present in the interior of molecular clouds. These grains and their ice mantles may well be the building blocks from which comets are made. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, researchers present the infrared spectra of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds. The last component was incorporated into this particular experiment as a tracer of the behavior of a non-aromatic hydrocarbon. The change in the composition that results from ultraviolet photolysis of this ice mixture using a UV lamp to simulate the interstellar radiation field is shown. Photolysis produces CO, CO2, CH4, HCO, H2CO, as well as a family of moderately volatile hydrocarbons. Less volatile carbonaceous materials are also produced. The evolution of the infrared spectrum of the ice as the sample is warmed up to room temperature is illustrated. Researchers believe that the changes are similar to those which occur as ice is ejected from a comet and warmed up by solar radiation. The warm-up sequence shows that the nitrile or iso-nitrile bearing compound produced during photolysis evaporates between 200 and 250K, suggesting that it is carried by a small molecular species. These molecules could be similar to the source material in Comet Halley that is ejected in grains into the coma, freed by sublimation, and photolyzed by solar radiation to produce the observed jets.

Allamandola, L. J.↗

NASA NTRS1989

Detection of a new circumstellar carbon chain molecule, C4Si

A new interstellar carbon chain molecule, C4Si, has been detected in the envelope of the evolved star IRC + 10216. This molecule is the carrier of six unidentified lines which had been detected during the molecular line survey at Nobeyama Radio Observatory. The identification was made through astronomical detections followed by quantum chemical calculations and laboratory spectroscopic experiments. The rotational constant and the centrifugal distortion constant were B(0) = 1533.77206(146) MHz and D(0) = 0.00005827(35) MHz, respectively, where the numbers in parentheses represent one standard deviation in units of the last significant digits. The rotation temperature and the column density were 15 + or - 2 K and (7 + or - 1) x 10 to the 12th/sq cm, respectively, assuming a source size of 25 arcsec.

Ohishi, Masatoshi↗

NASA NTRS1989

Precision measurements of photoabsorption cross sections of Ar, Kr, Xe, and selected molecules at 58.4, 73.6, and 74.4 nm

Absolute absorption cross sections have been measured for the rare gases at 58.43, 73.59, and 74.37 nm with an accuracy of + or - 0.8 percent. For the molecules H2, N2, O2, CO, N2O, CO2, and CH4, precision measurements were made at 58.43 nm with an accuracy of + or - 0.8 percent. Molecular absorption cross sections are also reported at 73.59 and 74.37 nm. However, in the vicinity of these wavelengths most molecules exhibit considerable structure, and cross sections measured at these wavelengths may depend on the widths and the amounts of self-reversal of these resonance lines. A detailed discussion is given of the systematic errors encountered with the double-ion chamber used in the cross-sectional measurements. Details are also given of precision pressure measurements.

Samson, James A. R.↗

NASA NTRS1990

A comparison of internal energy calculation methods for diatomic molecules

Various methods of calculating the internal energy of diatomic molecules are studied. An accurate and efficient method for computing the eigenvalues of the vibrational Schroedinger equation for an arbitrary potential is developed. The method is based on a finite-element discretization using the cubic Lobatto element. A combination of spectrum slicing and the Laguerre algorithm is used to solve for the eigenvalues. A simple method to compute the quasi-bound states is presented. For N2 molecules, all vibrational-rotational states of eleven available electronic potentials are computed, and summed to obtain the exact internal energy function with temperature. The total computation required 314 seconds of CPU-time on NASA's Cray 2 computer. Various approximate models are discussed and compared with the exact numerical simulation. It is shown that the splitting of the macroscopic internal energy into separate electronic, rotational, and vibrational energies is not justified at high temperatures.

Liu, Yen↗

NASA NTRS1990

Central-force potentials for interaction of rotationally and vibrationally excited molecules

A general and computationally efficient approach evaluating the central-force potentials for interaction of rotationally and vibrationally excited molecules and molecular ions is presented. Examples of numerical calculations are given for interactions of diatomic systems (H2, N2 and O2) in ground electronic states. The approach can be applied in a straightforward way to interactions of atoms and atomic ions with diatomic molecules in arbitrary rotational-vibrational states.

Kunc, Joseph A.↗

NASA NTRS1989

Influence of temperature on the infrared spectrum of the coronene molecule

Laboratory experiments were performed to study the temperature dependence of the absorption spectrum of the coronene molecule (C24H12), believed to be representative of the polycyclic aromatic hydrocarbon (PAH) molecules present in the interstellar medium. The main results is that both line positions and total intensities are almost unchanged when varying the temperature so that the spectrum is mostly temperature independent in the explored range, supporting the modeling of the IR emission originally made by Leger and Puget where room temperature was used in the calculations. In the structure of the band shape, small temperature correlated changes are observed. Qualitative arguments are given to interpret them.

Bernard, Jean-Phillipe↗

NASA NTRS1988

Second and third order nonlinear optical properties of conjugated molecules and polymers

Second- and third-order nonlinear optical properties of some newly synthesized organic molecules and polymers are reported. Powder second-harmonic-generation efficiencies of up to 200 times urea have been realized for asymmetric donor-acceptor acetylenes. Third harmonic generation chi(3)s have been determined for a series of small conjugated molecules in solution. THG chi(3)s have also been determined for a series of soluble conjugated copolymers prepared using ring-opening metathesis polymerization. The results are discussed in terms of relevant molecular and/or macroscopic structural features of these conjugated organic materials.

Perry, Joseph W.↗

NASA NTRS1991

Theoretical description of spectral line profiles of parent molecules in cometary comae

The present overview of cometary spectral-line profiles obtainable through advancements in high-resolution spectroscopic studies, which allow the retrieval of coma kinematic properties from velocity-resolved spectral-line profiles, incorporates the most important gas dynamic processes into an outflow model which is tailored to the interpretation of spectroscopic observations of parent molecules. The model is then used to study the influence on parent-molecule spectral line profile formation of the field-of-view, the expansion velocity, the kinetic temperature, and the anisotropic outflow distributions.

Hu, Hong-Yao↗

NASA NTRS1991

Detection of a new carbon-chain molecule, CCO

A new carbon-chain molecule, CCO 3Sigma(-), has been detected in the cold dark molecular cloud TMC-1. The excitation temperature and the column density of CCO are, respectively, about 6 K and about 6 x 10 to the 11th/sq cm. This column density corresponds to a fractional abundance relative to H2 of about 6 x 10 to the -11th. This value is two orders of magnitude less than the abundance of the related carbon-chain molecule CCS, and about half that of C3O. The formation mechanism for CCO is discussed.

Ohishi, Masatoshi↗

NASA NTRS1991

Laboratory simulation of the photoprocessing and warm-up of cometary and pre-cometary ices: Production of complex organic molecules

The recent missions to Comet Halley detected large quantities of organic material on grains as well as organic molecules in the gas phase. A possible origin of these materials is the energetic processing of ice mantles on the grains prior to comet formation, either in the pre-solar nebula or the interstellar medium. This process was simulated in the laboratory by depositing interstellar ice analogs (H2O/CH3OH/CO/NH3) on a cold (10 K) substrate with simultaneous UV irradiation. The material evaporating during warm-up of the photolyzed ice as well as the residue remaining at room temperature was analyzed by a number of techniques. It was found that a large number of organic molecules of various complexity are synthesized during the simulation process, stressing the possible significance of UV photolysis for producing the organic Comet material.

Schutte, W. A.↗

NASA NTRS1992

Laboratory simulation of the photoprocessing and warm-up of cometary and pre-cometary ices - Production and analysis of complex organic molecules

Schutte, W. A.↗

NASA NTRS1992

Radiative shocks and hydrogen molecules in pregalactic gas - The effects of postshock radiation

Previous calculations of radiative shocks in pregalactic gas are generalized and substantially improved. The hydrodynamical conservations are solved, along with the rate equations for nonequilibrium ionization, recombination, and molecule formation, and the equation of radiative transfer, for steady-state shocks in a gas of primordial compositions. This calculation self-consistently includes the effects of the diffuse postshock emission as well as a possible external radiation flux on the postshock flow and the preshock ionization levels. A previous result is confirmed that the shocked gas cools faster than it can recombine and, as a result, is able to form an H2 concentration as high as 0.003 via the formation of H(-) and H2(+) intermediaries due to the enhanced nonequilibrium ionization fraction at 10,000 K. It is shown that, as the level of the external radiation flux is increased, the formation of and cooling by H2 molecules can be inhibited and delayed. The detailed radiation spectra and ionizing photon number fluxes emergent from these shocks are presented.

Kang, Hyesung↗

NASA NTRS1992

Hydrogen molecules and chains in a superstrong magnetic field

The electronic structures of hydrogen polymolecules H(n) (n = 2,3,4,...) is studied in a superstrong magnetic field (B greater than about 10 exp 12 G) typically found on the surface of a neutron star. Simple analytical scaling relations for several limiting cases (e.g., large n, high B field) are derived. The binding energies of H(n) molecules are numerically calculated for various magnetic-field strengths. For a given magnetic-field strength, the binding energy per atom in the H(n) molecules is found to approach a constant value as n increases. For typical field strengths of interest, energy saturation is essentially achieved once n exceeds 3 to 4. Also considered is the structure of negative H ions in a high magnetic field. For B about 10 exp 12 G, the dissociation energy of an atom in a hydrogen chain and the ionization potential of H(-) are smaller than the ionization potential of neutral atomic hydrogen.

Lai, Dong↗

NASA NTRS1993

Imaging Genetic Molecules At Atomic Resolution

Proposed method of imaging informational polymeric biological molecules at atomic resolution enables determination of sequences of component monomers about 10 to the 3rd power to 10 to the 4th power times as fast as conventional methods do. Accelerates research on genetic structures of animals and plants. Also contributes significantly to imaging processes like scanning electron microscopy (SEM), atomic-force microscopy (AFM), and scanning tunneling microscopy (STM) in cases in which necessary to locate or identify small specimens on relatively large backgrounds and subtract background images to obtain images of specimens in isolation. V-grooves on silicon wafer laid out in square pattern, intersections of which marked to identify coordinates. Specimen molecules held in grooves for reproducible positioning and scanning by AFM or STM.

Coles, L. Stephen↗

NASA NTRS1992

Concerted hydrogen atom exchange between three HF molecules

We have investigated the termolecular reaction involving concerted hydrogen exchange between three HF molecules, with particular emphasis on the effects of correlation at the various stationary points along the reaction. Using an extended basis, we have located the geometries of the stable hydrogen-bonded trimer, which is of C(sub 3h) symmetry, and the transition state for hydrogen exchange, which is of D(sub 3h) symmetry. The energies of the exchange reation were then evaluated at the correlated level, using a large atomic natural orbital basis and correlating all valence electrons. Several correlation treatments were used, namely, configration interaction with single and double excitations, coupled-pair functional, and coupled-cluster methods. We are thus able to measure the effect of accounting for size-extensivity. Zero-point corrections to the correlated level energetics were determined using analytic second derivative techniques at the SCF level. Our best calculations, which include the effects of connected triple excitations in the coupled-cluster procedure, indicate that the trimer is bound by 9 +/- 1 kcal/mol relative to three separate monomers, in excellent agreement with previous estimates. The barrier to concerted hydrogen exchange is 15 kcal/mol above the trimer, or only 4.7 kcal/mol above three separated monomers. Thus the barrier to hydrogen exchange between HF molecules via this termolecular process is very low.

Komornicki, Andrew↗