Photoexcitation and ionization in molecular oxygen - Theoretical studies of electronic transitions in the discrete and continuous spectral intervals
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Gerwer, A.↗
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Need for high resolution in VUV Rydberg state spectroscopy
The paper emphasizes the necessity of high energy resolution (resolving powers greater than 100,000) in VUV spectroscopic determinations of atomic and molecular Rydberg electronic structures. Attention is given to spectral observations of Rydberg levels in Ge I near the 4p 2P(0)1/2 limit, in Sr I near the 4d 2D3/2 limit, and in diatomic hydrogen halides (especially H I) below the X2Pi3/2 limit.
Ginter, M. L.↗
UHV studies of the interaction of CO with small supported metal particles, Ni/mica
The interaction of carbon monoxide with small nickel (Ni) particles supported on UHV-cleaved mica was studied using flash thermal desorption, Auger electron spectroscopy, and transmission electron microscopy. Molecular desorption was accompanied by decomposition of CO at a rate strongly dependent on particle size. Recombination of surface-precipitated carbon with adsorbed oxygen was observed, and gas-induced morphological particle changes because of exposure to CO and O2 are analyzed in some detail.
Doering, D. L.↗
CASSCF/CI calculations for first row transition metal hydrides - The TiH(4-phi), VH(5-delta), CrH(6-sigma-plus), MnH(7-sigma-plus), FeH(4,6-delta) and NiH(2-delta) states
Calculations are performed for the predicted ground states of TiH(4-phi), VH(5-delta), CrH(6-sigma-plus), MnH(7-sigma-plus), Fett(4,6-delta) and NiH(2-delta). For FeH both the 6-delta and 4-delta states are studied, since both are likely candidates for the ground state. The ground state symmetries are predicted based on a combination of atomic coupling arguments and coupling of 4s(2)3d(n) and 4s(1)3d(n+1) terms in the molecular system. Electron correlation is included by a CASSCF/CI (SD) treatment. The CASSCF includes near-degeneracy effects, while correlation of the 3d electrons in included at the CI level.
Walch, S. P.↗
Why CO bonds side-on at low coverage and both side-on and upright at high coverage on the Cr(110) surface
An atom superposition and electron delocalization molecular orbital study of CO adsorption on the Cr(110) surface shows a high coordinate lying down orientation is favored. This is a result of the large number of empty d-band energy levels in chromium, which allows the antibonding counterparts to sigma and pi donation bonds to the surface to be empty. When lying down, backbonding to CO pi sup * orbitals is enhanced. Repulsive interactions cause additional CO to stand upright at 1/4 monolyer coverage. The results confirm the recent experimental study of Shinn and Madey.
Mehandru, S. P.↗
Adsorption of O2, SO2, and SO3 on nickel oxide. Mechanism for sulfate formation
Calculations based on the atom superposition and electron delocalization molecular orbital (ASED-MO) technique suggest that O2 will adsorb perferentially end-on at an angle 45 deg from normal on a nickel cation site on the (100) surface of NiO. SO2 adsorption is also stronger on the nickel site; SO2 bonds through the sulfur atom is a plane perpendicular to the surface. Adsorption energies for SO3 on the nickel and oxygen sites are comparable in the perferred orientation in which the SO3 plane is parallel to the surface. On activation, SO3 adsorbed to an O2(-) site forms a trigonal pyramidal SO4 species which yields, with a low barrier, a tetrahedral sulfate anion. Subsequently the anion reorients on the surface. Possibilities for alternative mechanisms which require the formation of Ni3(+) or O2(-) are discussed. NiSO4 thus formed leads to the corrosion of Ni at high temperatures in the SO2+O2/SO3 The SO2+O2/SO3 atmosphere, as discussed in the experimental literature.
Mehandru, S. P.↗
Dopant effect of yttrium and the growth and adherence of alumina on nickel-aluminum alloys
The atom superposition and electron delocalization molecular orbital theory and large cluster models have been employed to study cation vacancy diffusion in alpha-Al2O3 and the bonding of alpha-Al2O3 to nickel, aluminum, and yttrium surfaces. Al(3+) diffusion barriers in alpha-Al2O3 by the vacancy mechanism are in reasonable agreement with experiment. The barrier to Y(3+) diffusion is predicted to be much higher. Since addition of yttrium to transition metal alloys is known to reduce the growth rate and stress convolutions in protective alumina scales, this result suggests the rate-limiting step in scale growth is cation vacancy diffusion. This may partially explain the beneficial effect of yttrium dopants on scale adhesion. The theory also predicts a very strong bonding between alumina and yttrium at the surface of the alloy. This may also be important to the adhesion phenomenon. It is also found that aluminum and yttrium atoms bond very strongly to nickel because of charge transfer from their higher lying valence orbitals to the lower lying nickel s-d band.
Anderson, A. B.↗
Mechanism for chelated sulfate formation from SO2 and bis (triphenylphosphine) platinum
Structure and energy surface calculations using the atom superposition and electron delocalization molecular orbital theory show that the first step in the reaction between SO2 and the dioxygen complex (PPh3)2PtO2 is the coordination of SO2 with one oxygen atom of the complex, followed by metal-oxygen bond breaking and reorientation, leading to a five-membered cyclic structure. This then rearranges to form the bidentate coordinated sulfate. Alternative pathways are considered and are found to be less favorable.
Mehandru, S. P.↗
Adsorption of O2, SO2, and SO3, on nickel oxide - Mechanism for sulfate formation
Calculations based on the atom superposition and electron delocalization molecular orbital technique suggest that O2 will adsorb preferentially end-on at an angle 45 deg from normal on a nickel cation site on the (100) surface of NiO. SO2 adsorption is also stronger on the nickel site; SO2 bonds through the sulfur atom in a plane perpendicular to the surface. Adsorption energies for SO3 on the nickel and oxygen sites are comparable in the preferred orientation in which the SO3 plane is parallel to the surface. The calculations suggest that the strength of adsorption varies as O2 greater than SO2 greater than SO3. On activation, SO3 adsorbed to an O(2-) site forms a trigonal pyramidal SO4 species which yields, with a low barrier, a tetrahedral sulfate anion. Subsequently the anion reorients on the surface. Alternative mechanisms which require the formation of Ni(3+) or O(-) are discussed. NiSO4 thus formed may play a passivating role for the corrosion of Ni at low temperatures in the SO2 + O2 + SO3 atmospheres and an active role at high temperatures, as discussed in the experimental literature.
Mehandru, S. P.↗
Assessment of two-temperature kinetic model for ionizing air
A two-temperature chemical-kinetic model for air is assessed by comparing theoretical results with existing experimental data obtained in shock-tubes, ballistic ranges, and flight experiments. In the model, named the TTv model, one temperature (T) is assumed to characterize the heavy-particle translational and molecular rotational energies, and another temperature (Tv) to characterize the molecular vibrational, electron translational, and electronic excitation energies. The theoretical results for nonequilibrium air flow in shock tubes are obtained using the computer code STRAP (Shock-Tube Radiation Program), and for flow along the stagnation streamline in the shock layer over spherical bodies using the newly developed code STRAP (Stagnation-Point Radiation Program). Substantial agreement is shown between the theoretical and experimental results for relaxation times and radiative heat fluxes. At very high temperatures the spectral calculations need further improvement. The present agreement provides strong evidence that the two-temperature model characterizes principal features of nonequilibrium air flow. New theoretical results using the model are presented for the radiative heat fluxes at the stagnation point of a 6-m-radius sphere, representing an aeroassisted orbital transfer vehicle, over a range of free-stream conditions. Assumptions, approximations, and limitations of the model are discussed.
Park, Chul↗
Products of Dissociative Recombination in the Ionosphere
SRI International undertook a novel experimental measurement of the product states formed by dissociative recombination (DR) of O2(+), NO(+), and N2(+) as a function of both electron energy and reactant ion vibrational level. For these measurements we used a recently developed experimental technique for measuring dissociation product distributions that allows both the branching ratios to be accurately determined and the electronic and rovibrational state composition of the reactant ions to be specified. DR is the dominant electron loss mechanism in all regions of the ionosphere. In this process, electron attachment to the molecular ion produces an unstable neutral molecule that rapidly dissociates. For a molecular ion such as O2(+), the dissociation recombination reaction is (1) O2(+) + e yields O + O + W. The atomic products of this reaction, in this case two oxygen atoms, can be produced in a variety of excited states and with a variety of kinetic energies, as represented by W in Eq. (1). These atoms are not only active in the neutral chemistry of the ionosphere, but are also especially important because their optical emissions are often used to infer in situ concentrations of the parent molecular ion and ambient electron densities. Many laboratory measurements have been made of DR reaction rates under a wide range of electron temperatures, but very little is known about the actual distributions among the final states of the atomic products. This lack of knowledge seriously limits the validity and effectiveness of efforts to model both natural and man-made ionospheric disturbances. Bates recently identified major deficiencies in the currently accepted branching ratios for O2(+) as they relate to blue and green line emission measurements in the nocturnal F-region. During our two-year effort, we partially satisfied our ambitious goals. We constructed and operated a variable pressure, electron-impact ion source and a high pressure, hollow-cathode discharge ion source for O2(+), NO(+), and N2(+) beams. Translational spectroscopy of the products of dissociative charge transfer in Cs vapor was used to accurately assay the composition of the O2(+) and NO(+) beams and to develop a methodology for the vibrationally controlled preparation of the ground state ion beams. Attempts to assay the N2(+) beam revealed a novel two-electron process in the charge transfer reactions. A coaxial electron gun for the DR measurements was constructed following an extensive numerical design of the fields. Tests of the gun, however, found substantial perturbations of the magnetic fields by the soft iron (CMI-C) assembly containing the Langmuir probe that locates the electron beam. Hydrogen annealing of the iron failed to eliminate the field perturbations, necessitating the removal of the probe assembly. During this work on the coaxial electron gun, we discovered that predissociated high Rydberg states of O2 could be produced by subjecting the molecules to a sudden perturbation by an electromagnetic field. This technique allowed a measurement of the product branching to the atomic limits for the lowest seven vibrational levels of O2(+).
Cosby, Philip↗
Microwave Observations and Modeling of O2 (1-delta(sub g)) and O3 Diurnal Variation in the Mesosphere
The first microwave measurements of an electronically excited molecular species in the Earth's atmosphere are presented. Local thermodynamic equilibrium (LTE) rotational line emission from mesospheric O2(1-del(sub g)) was observed at a frequency of 255.01794 GHz (lambda is approx. 1.2 mm), employing the National Radio Astronomy Observatory (NRAO) millimeter facility at Kitt Peak, Arizona (32 N, 111 W). The pressure broadened line shapes of the O2(1-del(sub g)) spectra, which were obtained in January and April 1992 and in January and November 1993, are inverted to retrieve O2(1-del(sub g)) mixing profiles over the 50-70 km altitude region. The observed daytime abundances exceed ozone abundances in the lower mesosphere, which are separately retrieved with coincident O3 spectral line (249.7886 GHz) observations. The January and November 1993 observations are binned into 20-60 min time intervals to study O2(1-del(sub g)) diurnal behavior. Derived abundances of O2(1-del(sub g)) between 50 and 70 km for the four observation dates are 9%, 31%, 3%, and 26%, respectively, each +/- 10% higher than predicted, based on the simple photochemistry of lower mesospheric O2(1-del(sub g)). Modeled variation of [O2(1-del(sub g))] with time of day agrees with observed variation in that the observed difference between model and data abundances is constant throughout the daylight hours of each observation date. Model underprediction Of [02(lAg)] is consistent with similar model underprediction of mesospheric [O3]. A perturbation to the photochemical model that forces decreased ozone chemical loss brings brings both model [O3] and [O2(1-del(sub g))] into agreement with the observations. O2(1-del(sub g)) abundances derived from these 1.2 mm observations agree with [O2(1-del(sub g))] values derived from comparable SME observations of the 1.27 micrometers emission, with assumption of a 3880 sec O2(1-del(sub g)) radiative lifetime. The 6800 sec O2(1-del(sub g)) radiative lifetime proposed by Mlynczak and Nesbitt is ruled out by the similar comparison.
Sandor, Brad J.↗
Preferential inhibition of the plasma membrane NADH oxidase (NOX) activity by diphenyleneiodonium chloride with NADPH as donor
The cell-surface NADH oxidase (NOX) protein of plant and animal cells will utilize both NADH and NADPH as reduced electron donors for activity. The two activities are distinguished by a differential inhibition by the redox inhibitor diphenyleneiodonium chloride (DPI). Using both plasma membranes and cells, activity with NADPH as donor was markedly inhibited by DPI at submicromolar concentrations, whereas with NADH as donor, DPI was much less effective or had no effect on the activity. The possibility of the inhibition being the result of two different enzymes was eliminated by the use of a recombinant NOX protein. The findings support the concept that NOX proteins serve as terminal oxidases for plasma membrane electron transport involving cytosolic reduced pyridine nucleotides as the natural electron donors and with molecular oxygen as the electron acceptor.
Non-NASA Center↗
Multipole correction of atomic monopole models of molecular charge distribution. I. Peptides
The defects in atomic monopole models of molecular charge distribution have been analyzed for several model-blocked peptides and compared with accurate quantum chemical values. The results indicate that the angular characteristics of the molecular electrostatic potential around functional groups capable of forming hydrogen bonds can be considerably distorted within various models relying upon isotropic atomic charges only. It is shown that these defects can be corrected by augmenting the atomic point charge models by cumulative atomic multipole moments (CAMMs). Alternatively, sets of off-center atomic point charges could be automatically derived from respective multipoles, providing approximately equivalent corrections. For the first time, correlated atomic multipoles have been calculated for N-acetyl, N'-methylamide-blocked derivatives of glycine, alanine, cysteine, threonine, leucine, lysine, and serine using the MP2 method. The role of the correlation effects in the peptide molecular charge distribution are discussed.
NASA Discipline Exobiology↗
Polaribacter gen. nov., with three new species, P. irgensii sp. nov., P. franzmannii sp. nov. and P. filamentus sp. nov., gas vacuolate polar marine bacteria of the Cytophaga-Flavobacterium-Bacteroides group and reclassification of 'Flectobacillus glomeratus' as Polaribacter glomeratus comb. nov
Several psychrophilic, gas vacuolate strains of the Cytophage-Flavobacterium-Bacteroides (CFB) phylogenetic group were isolated from sea ice and water from the Arctic and the Antarctic. The closest taxonomically defined species by 16S rRNA sequence analysis is 'Flectobacillus glomeratus'. However, 'Flc. glomeratus' is phylogenetically distant from the Flectobacillus type species, Flc. major. On the basis of phenotypic, genotypic and 16S rRNA sequence analyses we propose a new genus, Polaribacter, with three new species, Polaribacter irgensii strain 23-P (ATCC 700398), Polaribacter franzmannii strain 301 (ATCC 700399) and Polaribacter filamentus strain 215 (ATCC 700397). P. filamentus is the type species of the genus. None of these species exhibits a cosmopolitan or bipolar distribution. This is the first taxonomic description of gas vacuolate bacteria in the CFB group. Additionally, we propose that 'Flc. glomeratus' be reclassified to the genus Polaribacter as P. glomeratus, comb. nov.
NASA Discipline Exobiology↗
Microwave Observations andf Modeling of O(sub 2) ((sup 1)delta-(sub g)) and O(sub 3) Diurnal Variation in the Mesosphere
The first microwave measurements of an electronically excited molecular species in the Earth's atmosphere are presented. Local thermodynamic equilibrium (LTE) rotational line emission from mesopheric O(sub 2) ((sup 1)delta-(sub g)) was observed at a frequency of 255.01794 GHz (Lambda approx. 1.2mm), employing the National Radio Astronomy Observatory (NRAO) millimeter facility at Kitt Peak, Arizona (32(deg)N, 111(deg)W).
photolysis ozone↗
Electrostatic Properties of PE and PTFE Subjected to Atmospheric Pressure Plasma Treatment; Correlation of Experimental Results with Atomistic Modeling
The use of an atmospheric pressure glow discharge (APGD) plasma was used at KSC to increase the hydrophilicity of spaceport materials to enhance their surface charge dissipation and prevent possible ESD in spaceport operations. Significant decreases in charge decay times were observed after tribocharging the materials using the standard KSC tribocharging test. The polarity and amount of charge transferred was dependent upon the effective work function differences between the respective materials. In this study, polyethylene (PE) and polytetrafluoroethylene (PTFE) were exposed to a He+O2 APGD. The pre and post treatment surface chemistry was analyzed by X-ray photoelectron spectroscopy and contact angle measurements. Semi-empirical and ab initio calculations were performed to correlate the experimental results with some plausible molecular and electronic structure features of the oxidation process. For the PE, significant surface oxidation was observed, as indicated by XPS showing C-O, C=O, and O-C=O bonding, and a decrease in the surface contact angle from 98.9 deg to 61.2 deg. For the PTFE, no C-O bonding appeared and the surface contact angle increased indicating the APGD only succeeded in cleaning the PTFE surface without affecting the surface structure. The calculations using the PM3 and DFT methods were performed on single and multiple oligomers to simulate a wide variety of oxidation scenarios. Calculated work function results suggest that regardless of oxidation mechanism, e.g. -OH, =0 or a combination thereof, the experimentally observed levels of surface oxidation are unlikely to lead to a significant change in the electronic structure of PE and that its increased hydrophilic properties are the primary reason for the observed changes in its electrostatic behavior. The calculations for PTFE argue strongly against significant oxidation of that material, as confirmed by the XPS results.
Trigwell, Steve↗
Laboratory spectra of C60 and related molecular structures
The electronic spectra of fullerene structures in high frequency discharge are studied in the plasma chemistry laboratory of the Faculty of Science of Masaryk University in Brno. The ultraviolet and visual spectra are investigated in order to be compared with the diffuse interstellar bands and interpreted within the theory of quantum mechanics. The preliminary results of the study are presented here in the form of a poster.
Janca, J.↗