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NASA NTRS2000

Effective Potential Energies and Transport Cross Sections for Interactions of Hydrogen and Nitrogen

The interaction energies for N2-He and N2-H2 are calculated by accurate ab initio methods. The virial coefficient and differential scattering cross section for N2-H2 are calculated; the theoretical results are compared with experimental data. The transport collision integrals for N2-H2 and N2-N2 interactions are calculated and tabulated; the results yield transport coefficients that compare well with measured data. Transport coefficients are found to be determined accurately from the interaction energies for a specific configuration of the molecule formed from the interaction partners. Comparisons with results of measurement and accurate calculations demonstrate that the transport properties of complex molecular interactions can be determined rapidly and fairly accurately from the interaction energies of simpler system using combination rules for the short-range parameters of effective interaction energies and the coefficients for the long-range forces. The coefficients for a two-parameter temperature expansion of diffusion and viscosity are tabulated for a realistic universal potential energy that is based primarily on the results of very accurate calculations of the He-He interaction energy.

Stallcop, James R.↗

NASA NTRS2000

Laboratory Studies of Interstellar PAH Analogs

Polycyclic aromatic hydrocarbons (PAHs) are now considered to be an important and ubiquitous component of the organic material in space. PAHs are found in a large variety of extraterrestrial materials such as interplanetary dust particles (IDPs) and meteoritic materials. PAHs are also good candidates to account for the infrared emission bands (UIRs) and the diffuse interstellar optical absorption bands (DIBs) detected in various regions of the interstellar medium. The recent observations made with the Infrared Space Observatory (ISO) have confirmed the ubiquitous nature of the UIR bands and their carriers. PAHs are though to form through chemical reactions in the outflow from carbon-rich stars in a process similar to soot formation. Once injected in the interstellar medium, PAHs are further processed by the interstellar radiation field, interstellar shocks and energetic particles. A major, dedicated, laboratory effort has been undertaken over the past years to measure the physical and chemical characteristics of these complex molecules and their ions under experimental conditions that mimic the interstellar conditions. These measurements require collision-free conditions where the molecules and ions are cold and chemically isolated. The spectroscopy of PAHs under controlled conditions represents an essential diagnostic tool to study the evolution of extraterrestrial PAHs. The Astrochemistry Laboratory program will be discussed through its multiple aspects: objectives, approach and techniques adopted, adaptability to the nature of the problem(s), results and implications for astronomy as well as for molecular spectroscopy. A review of the data generated through laboratory simulations of space environments and the role these data have played in our current understanding of the properties of interstellar PAHs will be presented. The discussion will also introduce the newest generation of laboratory experiments that are currently being developed in order to provide a closer simulation of space environments and a better support to space missions.

Salama, Farid↗

NASA NTRS2004

Model of Mixing Layer With Multicomponent Evaporating Drops

A mathematical model of a three-dimensional mixing layer laden with evaporating fuel drops composed of many chemical species has been derived. The study is motivated by the fact that typical real petroleum fuels contain hundreds of chemical species. Previously, for the sake of computational efficiency, spray studies were performed using either models based on a single representative species or models based on surrogate fuels of at most 15 species. The present multicomponent model makes it possible to perform more realistic simulations by accounting for hundreds of chemical species in a computationally efficient manner. The model is used to perform Direct Numerical Simulations in continuing studies directed toward understanding the behavior of liquid petroleum fuel sprays. The model includes governing equations formulated in an Eulerian and a Lagrangian reference frame for the gas and the drops, respectively. This representation is consistent with the expected volumetrically small loading of the drops in gas (of the order of 10 3), although the mass loading can be substantial because of the high ratio (of the order of 103) between the densities of liquid and gas. The drops are treated as point sources of mass, momentum, and energy; this representation is consistent with the drop size being smaller than the Kolmogorov scale. Unsteady drag, added-mass effects, Basset history forces, and collisions between the drops are neglected, and the gas is assumed calorically perfect. The model incorporates the concept of continuous thermodynamics, according to which the chemical composition of a fuel is described probabilistically, by use of a distribution function. Distribution functions generally depend on many parameters. However, for mixtures of homologous species, the distribution can be approximated with acceptable accuracy as a sole function of the molecular weight. The mixing layer is initially laden with drops in its lower stream, and the drops are colder than the gas. Drop evaporation leads to a change in the gas-phase composition, which, like the composition of the drops, is described in a probabilistic manner

Bellan, Josette↗

NASA NTRS1998

Study of Atmospheric Nitric Oxide

We investigated the contribution of energetic nitrogen atoms to the production of nitric oxide in the thermosphere and their influence on the infrared emission spectrum. The nitric oxide molecules are important contributors to the cooling of the atmosphere. We first pointed out that in determining the energy distribution of the nitrogen atoms, it is important to take into account the thermal motion of the atmospheric gases. It had been ignored in all earlier studies. The source spectra are broadened considerably by the center of mass motion of the reactants. We worked out the consequences for the production of nitric oxide at night, using as sources of energetic N atoms, NO(+) + e yield N + O, N(D-2) + O yield N + O. The high energy tail is enhanced by orders of magnitude. We had earlier suggested (Sharma et al. 1993) that the reaction of energetic nitrogen atoms with O2 was responsible for the rotationally enhanced NO identified in the infrared spectrum. Our calculations provided quantitative confirmation of the suggestion. We proceeded to explore the validity of another approximation used in earlier analyses, the hard sphere approximation for the energy loss in elastic collisions. We carried out precise quantum mechanical calculations of the elastic 2 differential scattering of nitrogen atoms in collisions with oxygen atoms and showed that although the hard sphere approximation was nowhere of high precision, reasonable results could be obtained with an effective cross section of 6 x 10(exp 15)sq cm. We also initiated a program to include inelastic energy loss processes in the determination of the energy distribution function. We began a calculation of the rotation and vibrational excitation cross sections of molecular nitrogen and nitrogen atoms and developed a method for including inelastic energy loss as a function of scattering angle in the Boltzmann equation. A procedure for obtaining the solution of the Boltzman equation was worked out.

Dalgarno, A.↗

NASA NTRS1985

Space shuttle molecular and wake vacuum measurements

The wake environment of the space shuttle is analyzed to determine whether it is feasible to perform ultrahigh vacuum experiments in or near the payload bay with the shuttle oriented such that the payload bay faces the antivelocity direction. Several mechanisms were considered by which molecules could approach the payload bay from this direction and their relative contributions to the wake environment are estimated. These mechanisms include ambient atmospheric molecules that have velocities in excess of the orbital velocity which can overtake the shuttle, ambient atmospheric molecules that are backscattered by collisions with the shuttle induced atmosphere, and self scattering from the induced atmosphere. These estimates are compared with the measurements made with the collimated mass spectrometer which was part of the Induced Environment Contamination Monitor flown on several of the early shuttle flights. Although the collimated mass spectrometer was not designed for this purpose and the instrument background for the species for which the collimator is effective is above the expected levels, upper limits can be established for these species in the wake environment which are consistent with the analysis. There was considerably more helium and argon observed in the wake direction than was predicted, however. Possible origins of these gases are discussed.

Naumann, R. J.↗

NASA NTRS1990

The Interstellar Medium in External Galaxies: Summaries of contributed papers

The Second Wyoming Conference entitled, The Interstellar Medium in External Galaxies, was held on July 3 to 7, 1989, to discuss the current understanding of the interstellar medium in external galaxies and to analyze the basic physical processes underlying interstellar phenomena. The papers covered a broad range of research on the gas and dust in external galaxies and focused on such topics as the distribution and morphology of the atomic, molecular, and dust components; the dynamics of the gas and the role of the magnetic field in the dynamics; elemental abundances and gas depletions in the atomic and ionized components; cooling flows; star formation; the correlation of the nonthermal radio continuum with the cool component of the interstellar medium; the origin and effect of hot galactic halos; the absorption line systems seen in distant quasars; and the effect of galactic collisions.

Hollenbach, David J.↗

NASA NTRS2000

Laboratory Astrochemistry: Interstellar PAHs

Polycyclic aromatic hydrocarbons (PAHs) are now considered to be an important and ubiquitous component of the organic material in space. PAHs are found in a large variety of extraterrestrial materials such as interplanetary dust particles (IDPs) and meteoritic materials. PAHs are also good candidates to account for the infrared emission bands (UIRs) and the diffuse interstellar optical absorption bands (DIBs) detected in various regions of the interstellar medium. The recent observations made with the Infrared Space Observatory (ISO) have confirmed the ubiquitous nature of the UIR bands and their carriers. PAHs are thought to form through chemical reactions in the outflow from carbon-rich stars in a process similar to soot formation. Once injected in the interstellar medium, PAHs are further processed by the interstellar radiation field, interstellar shocks and energetic particles. A major, dedicated, laboratory effort has been undertaken to measure the physical and chemical characteristics of these complex molecules and their ions under experimental conditions that mimic the interstellar conditions. These measurements require collision-free conditions where the molecules and ions are cold and chemically isolated. The spectroscopy of PAHs under controlled conditions represents an essential diagnostic tool to study the evolution of extraterrestrial PAHs. The Astrochemistry Laboratory program will be discussed through its multiple aspects: (1) objectives, (2) approach and techniques adopted, (3) adaptability to the nature of the problem(s), and (4) results and implications for astronomy as well as for molecular spectroscopy. A review of the data generated through laboratory simulations of space environments and the role these data have played in our current understanding of the properties of interstellar PAHs will be presented. The discussion will also introduce the newest generation of laboratory experiments that are currently being developed in order to provide a closer simulation of space environments and a better support to space missions.

Salama, Farid↗

NASA NTRS2008

Modifications to Axially Symmetric Simulations Using New DSMC (2007) Algorithms

Several modifications aimed at improving physical accuracy are proposed for solving axially symmetric problems building on the DSMC (2007) algorithms introduced by Bird. Originally developed to solve nonequilibrium, rarefied flows, the DSMC method is now regularly used to solve complex problems over a wide range of Knudsen numbers. These new algorithms include features such as nearest neighbor collisions excluding the previous collision partners, separate collision and sampling cells, automatically adaptive variable time steps, a modified no-time counter procedure for collisions, and discontinuous and event-driven physical processes. Axially symmetric solutions require radial weighting for the simulated molecules since the molecules near the axis represent fewer real molecules than those farther away from the axis due to the difference in volume of the cells. In the present methodology, these radial weighting factors are continuous, linear functions that vary with the radial position of each simulated molecule. It is shown that how one defines the number of tentative collisions greatly influences the mean collision time near the axis. The method by which the grid is treated for axially symmetric problems also plays an important role near the axis, especially for scalar pressure. A new method to treat how the molecules are traced through the grid is proposed to alleviate the decrease in scalar pressure at the axis near the surface. Also, a modification to the duplication buffer is proposed to vary the duplicated molecular velocities while retaining the molecular kinetic energy and axially symmetric nature of the problem.

Liechty, Derek S.↗

NASA NTRS1986

The molecular content of interacting and isolated galaxies The effect of environment on the efficiency of star formation

Molecular gas observations of merging/interacting and isolated galaxies are presented in order to study the relationship between environment and the efficiency of star formation. The two galaxy samples differ primarily in their IR properties and are quite similar in their molecular gas contents. The ratios of IR luminosity to H2 mass have a mean value of 78 and 12 solar luminosity/solar mass for interacting and isolated galaxies, respectively. The highest star formation efficiencies (SFEs) appear to occur in the merging and interacting pairs. The SFE in merging/interacting galaxies is greater than that found in the spiral arms of M51 and may be roughly proportional to the rate of cloud-cloud collisions in the interacting systems.

Young, Judith S.↗

NASA NTRS1976

Photoionization and ion cyclotron resonance studies of the ion chemistry of ethylene oxide

The formation of the ethylene oxide molecular ion and its subsequent ion-molecule reactions leading to the products C2H5O(+) and C3H5O(+) have been studied using time-resolved photoionization mass spectroscopy, ion cyclotron resonance spectroscopy, and photoelectron spectroscopy. An examination of the effects of internal energy on reactivity shows that the ratio of C3H5O(+) to C2H5O(+) increases by an order of magnitude with a single quantum of vibrational energy. The formation of (C2H4O/+/)-asterisk in a collision-induced isomerization is found which yields a ring-opened structure by C-C bond cleavage. The relaxed ring-opened C2H4O(+) ion reacts with neutral ethylene oxide by CH2(+) transfer to yield an intermediate product ion C3H6O(+) which gives C3H5O(+) by loss of H.

Corderman, R. R.↗

NASA NTRS1982

Charge exchange in the Io torus and exosphere

Charge-exchange cross sections and their velocity dependence have been estimated for the most important reactions in the Io torus and exosphere. The methods used for calculating the cross sections are given and discussed in some detail. For symmetric-resonant single and double charge exchange, the cross sections are slowly varying functions of velocity. For inelastic charge-exchange collisions, the transition probabilities into a given final state can depend critically on velocity. Models are described which can be used to estimate both the most rapid charge-exchange processes and those states which play an important role. Calculated cross sections are used to obtain reaction rates as a function of radial position, demonstrating the importance of charge exchange in the inner torus. Charge-exchange reactions of torus ions with molecular species in Io's exosphere may yield a net supply of neutrals and plasma to the torus.

Johnson, R. E.↗

NASA NTRS1986

Millimeter interferometry of the molecular gas in ARP 20

The Owens Valley Millimeter Wave Interferometer has been used to map the 2.6-mm CO emission in the ultraluminous infrared galaxy Arp 220. Approximately 70 percent of the CO emission from the galaxy originates from an unresolved region less than 4 arcsec x 6 arcsec in size (corresponding to 1500 pc diameter) centered on the near-infrared nucleus. The mass of gas within this region is 10 to the 10th solar mass, which is about 30 times greater than that in an equivalent area of the Galaxy. This concentration could result in efficient star formation via cloud-cloud collisions and provide a significant accretion flow onto a compact, central object.

Scoville, N. Z.↗

NASA NTRS1971

The formation of excited atoms during charge exchange between hydrogen ions and alkali atoms

The charge exchange cross sections for protons and various alkali atoms are calculated using the classical approximation of Gryzinski. It is assumed that the hydrogen atoms resulting from charge exchange exist in all possible excited states. Charge transfer collisions between protons and potassium as well as protons and sodium atoms are studied. The energy range investigated is between 4 and 30 keV. The theoretical calculations of the capture cross section and the cross section for the creation of metastable 2S hydrogen are compared to experimental values. Good quantitative agreement is found for the capture cross section but only qualitative agreement for the metastable cross section. Analysis of the Lyman alpha window in molecular oxygen suggests that measured values of the metastable cross section may be in error. Thick alkali target data are also presented. This allows the determination of the total electron loss cross section. Finally, some work was done with H2(+).

Nieman, R. A.↗

NASA NTRS1981

Infrared studies of the S235 molecular cloud

Infrared observations from 7.8 to 200 microns have been obtained for the S235 molecular cloud. Far-infrared maps were obtained for a region of active star formation, as marked by the presence of compact H II regions, water masers, and compact near-infrared sources. The primary heating source for the far-infrared emission appears to be the compact H II region, S235A. Detailed examination of the gas energetics in the region supports the plausibility of the picture in which the gas is heated by collisions with warm dust grains. The ratio of far-infrared optical depth to (C-13)O column density is somewhat lower in this source than is commonly found. This effect may be caused by the presence of substantial (C-13)O in regions where the dust is not warm enough to emit substantial 50-100 micron radiation.

Evans, N. J., II↗

NASA NTRS1992

Plasma-neutral interaction processes in the magnetosphere of Saturn

Models of Saturnian magnetospheric processes are reviewed emphasizing the interaction of charged and neutral particles in the gaseous phase and mentioning the role of solid matter. It is found that interpretations of different Voyager datasets regarding the Saturnian magnetosphere can vary. Specific interactions examined to resolve these discrepancies include charge exchange, ion-atom interchange, isotropizing and thermalizing collisions, and interactions between magnetospheric charged particles and surface layers of the icy satellites. The latter interactions result in sputtering of the surface or atmosphere as well as neutral injections into the magnetosphere. Constraints based on known reaction rates are shown to be useful in analyzing the abundances of the water-group molecules. The composition of the magnetospheric plasma is shown to be related to the differences between the interactions of atomic and molecular plasmas with neutral matter.

Eviatar, Aharon↗

NASA NTRS1994

The Materials Chemistry of Atomic Oxygen with Applications to Anisotropic Etching of Submicron Structures in Microelectronics and the Surface Chemistry Engineering of Porous Solids

Neutral atomic oxygen is the most abundant component of the ionospheric plasma in the low Earth orbit environment (LEO; 200 to 700 kilometers altitude) and can produce significant degradation of some spacecraft materials. In order to produce a more complete understanding of the materials chemistry of atomic oxygen, the chemistry and physics of O-atom interactions with materials were determined in three radically different environments: (1) The Space Shuttle cargo bay in low Earth orbit (the EOIM-3 space flight experiment), (2) a high-velocity neutral atom beam system (HVAB) at Los Alamos National Laboratory (LANL), and (3) a microwave-plasma flowing-discharge system at JSC. The Space Shuttle and the high velocity atom beam systems produce atom-surface collision energies ranging from 0.1 to 7 eV (hyperthermal atoms) under high-vacuum conditions, while the flowing discharge system produces a 0.065 eV surface collision energy at a total pressure of 2 Torr. Data obtained in the three different O-atom environments referred to above show that the rate of O-atom reaction with polymeric materials is strongly dependent on atom kinetic energy, obeying a reactive scattering law which suggests that atom kinetic energy is directly available for overcoming activation barriers in the reaction. General relationships between polymer reactivity with O atoms and polymer composition and molecular structure have been determined. In addition, vacuum ultraviolet photochemical effects have been shown to dominate the reaction of O atoms with fluorocarbon polymers. Finally, studies of the materials chemistry of O atoms have produced results which may be of interest to technologists outside the aerospace industry. Atomic oxygen 'spin-off' or 'dual use' technologies in the areas of anisotropic etching in microelectronic materials and device processing, as well as surface chemistry engineering of porous solid materials are described.

Koontz, Steve L.↗

NASA NTRS2003

Dissociative Ionization of Aromatic and Heterocyclic Molecules

Space radiation poses a major health hazard to humans in space flight. The high-energy charged particles in space radiation ranging from protons to high atomic number, high-energy (HZE) particles, and the secondary species they produce, attack DNA, cells, and tissues. Of the potential hazards, long-term health effects such as carcinogenesis are likely linked to the DNA lesions caused by secondary electrons in the 1 - 30 eV range. Dissociative ionization (DI) is one of the electron collision processes that can damage the DNA, either directly by causing a DNA lesion, or indirectly by producing radicals and cations that attack the DNA. To understand this process, we have developed a theoretical model for DI. Our model makes use of the fact that electron motion is much faster than nuclear motion and assumes DI proceeds through a two-step process. The first step is electron-impact ionization resulting in a particular state of the molecular ion in the geometry of the neutral molecule. In the second step the ion undergoes unimolecular dissociation. Thus the DI cross section sigma(sup DI)(sub a) for channel a is given by sigma(sup DI)(sub a) = sigma(sup I)(sub a) P(sub D) with sigma(sup I)(sub a) the ionization cross section of channel a and P(sub D) the dissociation probability. This model has been applied to study the DI of H2O, NH3, and CH4, with results in good agreement with experiment. The ionization cross section sigma(sup I)(sub a) was calculated using the improved binary encounter-dipole model and the unimolecular dissociation probability P(sub D) obtained by following the minimum energy path determined by the gradients and Hessians of the electronic energy with respect to the nuclear coordinates of the ion. This model is used to study the DI from the low-lying channels of benzene and pyridine to understand the different product formation in aromatic and heterocyclic molecules. DI study of the DNA base thymine is underway. Solvent effects will also be discussed.

Huo, Winifred M.↗

NASA NTRS1992

Experimental study and improved modeling of high-temperature air radiation

Spectral emission measurements were conducted in an atmospheric pressure air plasma. The thermodynamic state of the plasma was determined to be close to local thermodynamic equilibrium (LTE), and the temperature profile was measured. The spectrum emitted by the plasma over the range 2,000 to 8,000 A was recorded and calibrated. This spectrum comprises the major radiating molecular bands and atomic lines in air, and is therefore proposed as a benchmark to test radiative calculations. A comparison of these results with the predictions of the NEQAIR code induced several modeling improvements in the code. In particular, radiative transition probabilities and spectroscopic constants were updated, and additional band systems of NO (NO Delta, Epsilon, Beta prime and Gamma prime) were included. Since the C state from which the NO Delta transition originates is predissociated, a simplified collision-predissociation model for this state was added to the code. These changes are presented, and their effect discussed.

Laux, Christophe O.↗