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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Kinematics of the CS method for the treatment of molecular collisions

The problem of the quantum treatment of nonreactive collisions of two simple molecules or of an atom and a molecule is considered mathematically. Kinematical structure theorems analogous to the Wigner-Eckart theorem for scalar operators are derived for the transition operator (T-operator) of a two-molecule system, where, in addition to the usual conservation laws, certain additional conservation laws are presumed satisfied in a collision. The additional conservation laws were examined in order to develop a testable approximation scheme (CS method) for the specification of the kinematics of a system of two rigid diatomic molecules. The rand p-helicity basis states for a two-rigid rotor system are defined and unitary transformations established between these and conventional bases. It is found that only questionable criteria exist for testing the on-the-energy-shell matrix of a T(energy) operator for the presence of properties of two of the additional conservation laws.

Hahne, G. E.↗
NASA NTRS2016

Scattering and Diffraction of Electromagnetic Radiation: An Effective Probe to Material Structure

Scattered electromagnetic waves from material bodies of different forms contain, in an intricate way, precise information on the intrinsic, geometrical and physical properties of the objects. Scattering theories, ever deepening, aim to provide dependable interpretation and prediction to the complicated interaction of electromagnetic radiation with matter. There are well-established multiple-scattering formulations based on classical electromagnetic theories. An example is the Generalized Multi-particle Mie-solution (GMM), which has recently been extended to a special version ̶ the GMM-PA approach, applicable to finite periodic arrays consisting of a huge number (e.g., >>106) of identical scattering centers [1]. The framework of the GMM-PA is nearly complete. When the size of the constituent unit scatterers becomes considerably small in comparison with incident wavelength, an appropriate array of such small element volumes may well be a satisfactory representation of a material entity having an arbitrary structure. X-ray diffraction is a powerful characterization tool used in a variety of scientific and technical fields, including material science. A diffraction pattern is nothing more than the spatial distribution of scattered intensity, determined by the distribution of scattering matter by way of its Fourier transform [1]. Since all linear dimensions entered into Maxwell's equations are normalized by wavelength, an analogy exists between optical and X-ray diffraction patterns. A large set of optical diffraction patterns experimentally obtained can be found in the literature [e.g., 2,3]. Theoretical results from the GMM-PA have been scrutinized using a large collection of publically accessible, experimentally obtained Fraunhofer diffraction patterns. As far as characteristic structures of the patterns are concerned, theoretical and experimental results are in uniform agreement; no exception has been found so far. Closely connected with the spatial distribution of scattered intensities are cross sections, such as for extinction, scattering, absorption, and radiation pressure, as a critical type of key quantity addressed in most theoretical and experimental studies of radiative scattering. Cross sections predicted from different scattering theories are supposed to be in general agreement. For objects of irregular shape, the GMM-PA solutions can be compared with the highly flexible Discrete Dipole Approximation (DDA) [4,5] when dividing a target to no more than ~106 unit cells. Also, there are different ways to calculate the cross sections in the GMM-PA, providing an additional means to examine the accuracy of the numerical solutions and to unveil potential issues concerning the theoretical formulations and numerical aspects. To solve multiple scattering by an assembly of material volumes through classical theories such as the GMM-PA, the radiative properties of the component scatterers, the complex refractive index in particular, must be provided as input parameters. When using a PA to characterize a material body, this involves the use of an adequate theoretical tool, an effective medium theory, to connect Maxwell's phenomenogical theory with the atomistic theory of matter. In the atomic theory, one regards matter as composed of interacting particles (atoms and molecules) embedded in the vacuum [6]. However, the radiative properties of atomic-scaled particles are known to be substantially different from bulk materials. Intensive research efforts in the fields of cluster science and nanoscience attempt to bridge the gap between bulk and atom and to understand the transition from classical to quantum physics. The GMM-PA calculations, which place virtually no restriction on the component-particle size, might help to gain certain insight into the transition.

Xu, Yu-Lin↗
NASA NTRS1976

The neutral atmospheres of comets

A comprehensive review of current knowledge about the neutral gas atmospheres of comets is given, with emphasis on the task of deriving the chemical composition of the cometary nucleus. The discussion centers on the following major topics: photometric and spectrophotometric observations of the neutral atmosphere and its constituent neutral radicals, molecules, and atoms; excitation processes responsible for the observed emissions from the neutral coma; the parent molecules of the neutral radicals; the gas-phase chemistry likely to take place in the dense inner coma; dynamic models (both exospheric and hydrodynamic) of the neutral atmosphere; and the chemical compositions of comets. The primary conclusions reached on the basis of all the data considered are that: (1) the chemical composition of the dust component is most likely to be the same as that of the stream meteoroids which burn up on hitting the earth's upper atmosphere; (2) the dominant icy component in most comets is probably H2O; and (3) anomalous abundances of other volatile species are present in some comets.

Mendis, D. A.↗
NASA NTRS1986

The outer atmosphere of the carbon star TX Piscium

A high-resolution LWP IUE spectrum of the bright N-type carbon star TX Psc demonstrates that the Mg II h and k emission profiles are strongly affected by absorption from Mg II, Mn I, probably Fe I, and possibly from molecules. The indication that the absorbing matter has a column density of not less than 10 to the 20th H atoms or molecules per sq cm is consistent with absorption in a slowly expanding envelope. The integrated Mg II line flux is found to be much greater than in 1981, and the radio CO (J = 1 - 0) line from the circumstellar shell is detected. Results for a column density of not larger than 10 to the 22nd H2 molecules/sq cm, and a radial velocity close to that of the star, are in agreement with those obtained from UV data. Some dust emission from carbon grains is suggested by the far infrared flux distribution, and a mass-loss rate estimation for the star of 10 to the -6th to 10 to the -8th solar masses is obtained.

Eriksson, K.↗
NASA NTRS2015

Opportunities for Laboratory Opacity Chemistry Studies to Facilitate Characterization of Young Giant Planets and Brown Dwarfs

The thermal emission spectra of young giant planets is shaped by the opacity of atoms and molecules residing in their atmospheres. While great strides have been made in improving the opacities of important molecules, particularly NH3 and CH4, at high temperatures, much more work is needed to understand the opacity and chemistry of atomic Na and K. The highly pressure broadened fundamental band of Na and K in the optical stretches into the near-infrared, strongly influencing the shape of the Y and K spectral bands. Since young giant planets are bright in these bands it is important to understand the influences on the spectral shape. Discerning gravity and atmospheric composition is difficult, if not impossible, without both good atomic opacities as well as an excellent understanding of the relevant atmospheric chemistry. Since Na and K condense at temperatures near 500 to 600 K, the chemistry of the condensation process must be well understood as well, particularly any disequilibrium chemical pathways. Comparisons of the current generation of sophisticated atmospheric models and available data, however, reveal important shortcomings in the models. We will review the current state of observations and theory of young giant planets and will discuss these and other specific examples where improved laboratory measurements for alkali compounds have the potential of substantially improving our understanding of these atmospheres.

chemistry of the condensation process↗
NASA NTRS2002

Symmetric Resonance Charge Exchange Cross Section Based on Impact Parameter Treatment

Using a two-state impact parameter approximation, a calculation has been carried out to obtain symmetric resonance charge transfer cross sections between nine ions and their parent atoms or molecules. Calculation is based on a two-dimensional numerical integration. The method is mostly suited for hydrogenic and some closed shell atoms. Good agreement has been obtained with the results of laboratory measurements for the ion-atom pairs H+-H, He+-He, and Ar+-Ar. Several approximations in a similar published calculation have been eliminated.

Omidvar, Kazem↗
NASA NTRS1972

Models for the interaction of rarefied gas with a surface

Various theoretical models are discussed for the interaction of gaseous atoms with solids at energies of approximately 10 eV. The surface is modeled by a one- and three-dimensional aggregate of atoms with different initial conditions and lattice parameters. In calculating the accommodation coefficients, the direct integration of the equations of motion of gaseous molecules and atoms in the crystal lattice of the solid is used. An analysis of the advantages and the drawbacks of the models, as well as the possibilities of the analytical solutions in a one-dimensional case, is given. The dependence of the energy and the normal and the tangential momenta accommodation coefficients on the collision parameters is obtained.

Pyarnpuu, A. A.↗
NASA NTRS1995

Operation of the computer model for direct atomic oxygen exposure of Earth satellites

One of the primary causes of material degradation in low Earth orbit (LEO) is exposure to atomic oxygen. When atomic oxygen molecules collide with an orbiting spacecraft, the relative velocity is 7 to 8 km/sec and the collision energy is 4 to 5 eV per atom. Under these conditions, atomic oxygen may initiate a number of chemical and physical reactions with exposed materials. These reactions contribute to material degradation, surface erosion, and contamination. Interpretation of these effects on materials and the design of space hardware to withstand on-orbit conditions requires quantitative knowledge of the atomic oxygen exposure environment. Atomic oxygen flux is a function of orbit altitude, the orientation of the orbit plan to the Sun, solar and geomagnetic activity, and the angle between exposed surfaces and the spacecraft heading. We have developed a computer model to predict the atomic oxygen exposure of spacecraft in low Earth orbit. The application of this computer model is discussed.

Bourassa, R. J.↗
NASA NTRS2005

Experimental Study on Interactions Between H Atoms and Organic Haze

In Titan s atmosphere composed of N2 and CH4, irradiations of both solar ultraviolet light and charged particles induce active chemical reactions. In the processes of these reactions, a large amount of hydrogen (H) atoms are expected to be formed by dissociation of CH4 and other hydrocarbons [e.g., 1, 2]. Theoretical models suggest that these active H atoms need to be converted to stable hydrogen molecules (H2) efficiently to maintain unsaturated hydrocarbons and organic haze in Titan s atmosphere [e.g., 1]. Furthermore, molecular hydrogen is an important greenhouse effect gas in Titan s atmosphere, and small variation in its abundance strongly affects Titan s surface temperature [3]. Thus, the formation of H2 molecules from H atoms is a key reaction for both the atmospheric chemistry and the surface environment of Titan. Although several numerical calculations have been conducted to investigate the atmospheric chemistry of Titan with hypothesized recombination reactions of H atoms, such as catalytic scheme of C4H2 [e.g., 1, 2], it is still unclear what chemical reaction is responsible for the conversion of H atoms to H2 molecules in Titan s atmosphere.

Sekine, Y.↗
NASA NTRS1974

Semiclassical S-matrix theory of vibrationally inelastic collisions between two diatomic molecules

We derive a semiclassical S matrix for vibrationally inelastic collisions between two diatomic molecules, assuming a collinear geometry. Our theory incorporates a quantum mechanical superposition principle with classical dynamics and, as such, is an extension of the atom-diatomic molecule theory of Miller. The several approximations to the S matrix differ in the complexity with which the interference between various classical trajectories is treated. We report numerical calculations for H2-D2 and D2-D2 collisions based on two different interaction potentials. The cruder approximations yield transition probabilities which agree with exact quantum mechanical results to within a factor of 2. More sophisticated approximations to the S matrix yield excellent quantitative agreement with the quantum calculations.

Cohen, S. C.↗
NASA NTRS1979

Transport properties of monatomic carbon. II - Contributions from excited electronic states

The aim of the present analysis is to verify the conventional assumption that the transport properties of atoms and molecules in excited electronic states are nearly the same as in the ground state. Reasonably exact calculations of the transport properties of excited-state carbon atoms are obtained on the basis of available experimental and theoretical data on the interactions between carbon atoms in excited electronic states. A comparison with results obtained with several models that have been used to estimate the transport properties of excited species indicates that the assumption under consideration is correct, but that the models do not predict very accurately the transport properties of excited species.

Biolsi, L.↗
NASA NTRS1990

Scattering of positrons and electrons by alkali atoms

Absolute total scattering cross sections (Q sub T's) were measured for positrons and electrons colliding with sodium, potassium, and rubidium in the 1 to 102 eV range, using the same apparatus and experimental approach (a beam transmission technique) for both projectiles. The present results for positron-sodium and -rubidium collisions represent the first Q sub T measurements reported for these collision systems. Features which distinguish the present comparisons between positron- and electron-alkali atom Q sub T's from those for other atoms and molecules (room-temperature gases) which have been used as targets for positrons and electrons are the proximity of the corresponding positron- and electron-alkali atom Q sub T's over the entire energy range of overlap, with an indication of a merging or near-merging of the corresponding positron and electron Q sub T's near (and above) the relatively low energy of about 40 eV, and a general tendency for the positron-alkali atom Q sub T's to be higher than the corresponding electron values as the projectile energy is decreased below about 40 eV.

Stein, T. S.↗
NASA NTRS2013

Life and the Universe: From Astrochemistry to Astrobiology

Great strides have been made in our understanding of interstellar material thanks to advances in infrared astronomy and laboratory astrophysics. Ionized polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier astrochemical standards, are widespread and very abundant throughout much of the cosmos. In cold molecular clouds, the birthplace of planets and stars, interstellar atoms and molecules freeze onto extremely cold dust and ice particles forming mixed molecular ices dominated by simple species such as water, methanol, ammonia, and carbon monoxide. Within these clouds, and especially in the vicinity of star and planet forming regions, these ices and PAHs are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of biogenic interest. Eventually, these are delivered to primordial planets by comets and meteorites. As these materials are the building blocks of comets and related to carbonaceous micrometeorites, they are likely to be important sources of complex organic materials delivered to habitable planets (including the primordial Earth) and their composition may be related to the origin of life. This talk will focus on the chemical evolution of these cosmic materials and their relevance to astrobiology.

Atmoshperic Chemistry↗
NASA NTRS1981

Energy accommodation of 5-50 eV ions within an enclosure

Beams of 5-50 eV He(+), Ar(+), Ne(+), O(+), and N2(+) ions were directed into an aluminum sphere, and the equilibrium number density of the atom or molecules was measured inside the sphere using a quadrupole mass spectrometer and signal averaging techniques. The equilibrium number density is inversely proportional to the average speed of the atoms; thus, the results are expressed in terms of the speed ratio, R = V(i)/V(s), where V(i) is the average speed within the enclosure, and V(s) is the average speed of atoms fully accommodated to the temperature of the wall. The speed ratios vary between 1.0 and 1.8. For N2, several values of R were less than 1; this was largely due to desorbed N2. There was no detectable number density for O, which is explained by the reaction of O with the surface.

Humphris, R. R.↗
NASA NTRS2004

Vibrational Spectroscopy at Cryogenic Temperatures: From the Laboratory to the Ends of the Universe

Polycyclic aromatic compounds (PAC) have been the subject of astrophysical interest for nearly two decades. Indeed, there is now evidence that these compounds are among the most abundant interstellar molecules, comprising perhaps 10 - 30% of all the carbon in our galaxy. Ranging in size from as few as 20 to as many as hundreds or thousands of carbon atoms, these molecules are enormous by interstellar standards and impact a wide range of astrophysical phenomena including: the near- and mid-infrared emission spectrum of a wide range of astronomical objects; the ultraviolet/visible/near-infrared absorption of the diffuse medium that permeates our galaxy; and the heating and cooling mechanisms of interstellar clouds. Nevertheless, until recently a lack of laboratory spectroscopic data on PAC under astrophysically relevant conditions (i.e. isolated, ionized molecules; ionized molecular clusters, etc.) has hindered critical evaluation and extension of this model. Fortunately, the last decade has seen an explosion of experimental and theoretical techniques aimed specifically at addressing these issues. Here, we begin with a brief review of the evidence for the existence and abundance of interstellar PAC. From there, we move on to an overview of the general spectroscopic characteristics of the range of isolated, neutral and ionized PAC that we have studied over the last decade. Finally, we will conclude with a consideration of the astrophysical implications and applications of interstellar PAC. Our discussion will revolve primarily around the observed interstellar infrared emission spectra and the ability of the laboratory spectra of varying combinations of PAC to accommodate both the global appearance and the region-to-region variability of this emission. Careful models based on sound laboratory spectroscopic data such as these now have the potential to provide insight into the structure, abundance, and ionization state of the interstellar PAC population. That, in turn, will provide direct insight into the processes of carbon nucleation, growth and evolution as matter is processed through the interstellar medium.

Hudgins, Douglas M.↗
NASA NTRS1998

Computational Studies of the Interaction of H/H2 with Diamond and Silicon Surfaces

The interaction of hydrogen atoms and molecules with diamond and silicon surfaces is important in several important applications. Two areas that we are interested are: 1) tribology (molecular level friction) and 2) the role of H atoms in silicon chemical vapor deposition (CVD). In the tribology area, H atoms can be used to tie off dangling bonds, which otherwise form bonds between adjacent surfaces, and lead to resistance to sliding the surfaces by each other. Processes which are important in understanding molecular level friction include barriers to addition of H/H2 to the surface and barriers to migration of H atoms on the surface. In the silicon CVD area, we have studied the process of H2 elimination from the 100 surface of silicon. Cluster models for the dime surfaces of diamond are presented. The unrelaxed 100 surface has carbene like surface carbon atoms; however, for the relaxed surface these dimerize to give rows of surface dimers and there is a significant amount of p bonding between the radical orbitals of the dimer. The 110 surface has zig-zag rows of carbon atoms with a dangling bond on each carbon atom. These dangling bonds are hybridized away from each other and thus interact less strongly than for the 100 surface. Finally, the 111 surface has surface C atoms arranged in a triangular pattern and the surface dangling bonds are well separated from each other (second nearest neighbor distance) leading to almost no interaction between adjacent dangling bonds. These qualitative features may be quantified by computing the overlap of adjacent dangling bonds in a GVB(pp) calculation. The overlaps are 0.462, 0.292, and 0.016 for the diamond 100, 110, and 111 surfaces, respectively.

Walch, Stephen P.↗
NASA NTRS2012

Mercury's Na Exosphere from MESSENGER Data

MESSENGER entered orbit about Mercury on March 18, 2011. Since then, the Ultraviolet and Visible Spectrometer (UWS) channel of MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer (MASCS) has been observing Mercury's exosphere nearly continuously. Daily measurements of Na brightness were fitted with non-uniform exospheric models. With Monte Carlo sampling we traced the trajectories of a representative number of test particles, generally one million per run per source process, until photoionization, escape from the gravitational well, or permanent sticking at the surface removed the atom from the simulation. Atoms were assumed to partially thermally accommodate on each encounter with the surface with accommodation coefficient 0.25. Runs for different assumed source processes are run separately, scaled and co-added. Once these model results were saved onto a 3D grid, we ran lines of sight from the MESSENGER spacecraft :0 infinity using the SPICE kernels and we computed brightness integrals. Note that only particles that contribute to the measurement can be constrained with our method. Atoms and molecules produced on the nightside must escape the shadow in order to scatter light if the excitation process is resonant-light scattering, as assumed here. The aggregate distribution of Na atoms fits a 1200 K gas, with a PSD distribution, along with a hotter component. Our models constrain the hot component, assumed to be impact vaporization, to be emitted with a 2500 K Maxwellian. Most orbits show a dawnside enhancement in the hot component broadly spread over the leading hemisphere. However, on some dates there is no dawn/dusk asymmetry. The portion of the hot/cold source appears to be highly variable.

Killen, Rosemary M.↗
NASA NTRS1986

Analysis of the physical atomic forces between noble gas atoms, alkali ions and halogen ions

The physical forces between atoms and molecules are important in a number of processes of practical importance, including line broadening in radiative processes, gas and crystal properties, adhesion, and thin films. The components of the physical forces between noble gas atoms, alkali ions, and halogen ions are analyzed and a data base for the dispersion forces is developed from the literature based on evaluations with the harmonic oscillator dispersion model for higher order coefficients. The Zener model of the repulsive core is used in the context of the recent asymptotic wave functions of Handler and Smith; and an effective ionization potential within the Handler and Smith wave functions is defined to analyze the two body potential data of Waldman and Gordon, the alkali-halide molecular data, and the noble gas crystal and salt crystal data. A satisfactory global fit to this molecular and crystal data is then reproduced by the model to within several percent. Surface potentials are evaluated for noble gas atoms on noble gas and salt crystal surfaces with surface tension neglected. Within this context, the noble gas surface potentials on noble gas and salt crystals are considered to be accurate to within several percent.

Wilson, J. W.↗