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Stripping and low-energy collisions of H atoms on O and N molecules, showing target ionization to projectile stripping ratio dependence on ionization potential
The Shuttle/Payload Contamination Evaluation (SPACE) computer program was designed to three dimensionally synthesize the dynamics of the induced on-orbit molecular contaminant environment of the Shuttle Orbiter and a number of Spacelab payload carrier configurations. SPACE has the capability to accept any instrument or spacecraft configuration for contamination evaluation through manipulation of program input data. It represents a significant advancement over previous analytical or modeling techniques in that it is the first integrated systems level model of its type to collectively consider geometry (emphasizing surface shadowing), any arbitrary contaminant source(s) and the contaminant transport mechanisms of: (1) direct source-to-surface; (2) return flux due to molecular collisions with the ambient atmosphere and other contaminant molecules; and (3) reflection/reemission from structural surfaces.
Preliminary calculations suggest that collisions between instreaming atmospheric constituents and secondary backscattered molecules can generate unexpectedly large enhancements in neutral gas concentrations in the vicinity of the shuttle. This effect is a result of a rapid decrease in the mean free path length of the scattered components following the initial expected concentration enhancement. A study will be outlined for the theoretical investigation of this mechanism. The shape dependence of associated glow halos on vehicle configuration will be discussed.
A numerical model is developed for the interaction of a cometary probe, such as Giotto, with its environment, i.e., dust particles and gas. The spacecraft was set on a course to pass the comet at a velocity of 69 km/sec, so a chance existed that a potential field would form around the spacecraft and block lower energy particles from reaching the spacecraft instruments. The motion of electrons and ions is traced as a function of time to examine the evolution of the electric field, electric potential and the total space charge distributions on the surface of the spacecraft and its environment. Account is taken of the density of the particles and gas molecules at various distances from the comet, the collision energies involved, and the Giotto geometry. A solution is defined for the Poisson equation to describe the evolution of the plasma around Giotto, including the effects of ion collisions with the Al bumper protecting the spacecraft. The simulation predicts formation of an ion wake behind Giotto and the evolution of a positive potential on the order of 10 V around the spacecraft, i.e., sufficient for a positive potential barrier near the surface of the spacecraft.
We have investigated the impact of hot metastable oxygen atoms on the product yields and rate coefficients of atmospheric reactions involving O( (sup 1)D). The contribution of the metastable oxygen atoms to the thermal balance of the terrestrial atmosphere between 50 and 200 km has been determined. We found that the presence of hot O((sup l)D) atoms in the mesosphere and lower thermosphere significantly increases the production rate of the rotationally-vibrationally excited NO molecules. The computed yield of the NO molecules in N2O+ O((sup 1)D) atmospheric collisions, involving non-Maxwellian distributions of the metastable oxygen atoms, is more than two times larger than the NO-yield at a thermal equilibrium. The calculated non-equilibrium rate and yield functions are important for ozone and nitrous oxide modeling in the stratosphere, mesosphere and lower thermosphere.
The quantum mechanical problem of reactive or nonreactive scattering of atoms and molecules is formulated in terms of square-integrable basis sets with variational expressions for the reactance matrix. Several formulations involving expansions of the wave function (the Schwinger variational principle) or amplitude density (a generalization of the Newton variational principle), single-channel or multichannel distortion potentials, and primitive or contracted basis functions are presented and tested. The test results, for inelastic and reactive atom-diatom collisions, suggest that the methods may be useful for a variety of collision calculations and may allow the accurate quantal treatment of systems for which other available methods would be prohibitively expensive.
Many properties of gaseous systems such as electromagnetic absorption and emission, sound dispersion and absorption, may be elucidated if the nature of collisions between the particles in the system is understood. A procedure for the calculation of the classical trajectories of two interacting diatomic molecules is described. The dynamics of the collision will be assumed to be that of two rigid rotors moving in a specified potential. The actual outcome of a representative sample of many trajectories at 298K was computed, and the use of these values at any temperature for calculations of various molecular properties will be described. Calculations performed for the O2 microwave spectrum are given to demonstrate the use of the procedure described.
An algorithm for the random walk problem of multiple elastic collisions between newly formed non-thermal neutral cometary radicals and the outflowing cometary molecules was incorporated into the Monte Carlo particle-trajectory model. Preliminary model analysis has shown that the effects of collision on the observed spatial distribution of cometary radicals becomes important for the larger bright comets, especially at smaller values of the helicocentric distance. The model and early results are discussed herein.
Cross sections for the important rotational transitions in molecular hydrogen collisions are presented for rotational levels to j = 11 and energies to 20,000 K. These have been obtained from extensive theoretical calculations. The resulting cross sections are fitted to a simple polynomial in the energy, and the coefficients are tabulated for easy utilization.
An approximation to a previously presented rigorous description of molecular (atom-atom) collisions occurring in the presence of intense radiation is investigated. This rigorous description explicitly considers the angular momentum transferred between the molecule and the radiation field in the absorption or emission of a photon, but involves a complicated system of close-coupled equations which must be solved independently for each projection M of the initial, total molecular angular momentum. (This is a direct consequence of the lack of rotational invariance in the molecule-field problem). These equations are solved for a model system which mimics the collision of a halogen with a rare gas atom. Empirical observations made in the course of performing these calculations lead to the development of an approximation which avoids the repeated calculations for each initial M. This orientational average approximation greatly reduces the effort required to describe the system, and for the model calculation, yields accurate results for field intensities as high as 10 GW/sq cm.
Theoretical rate constants are presented for excitation of NH3 by collisions with He. The lowest 22 levels of ortho-NH3 and the lowest 16 levels of para-NH3 are considered at kinetic temperatures of 15 to 300 K.
Mass flow and compositional gradient (elemental and isotope separation) occurs when flu-id(s) or gas(es) in an enclosure is subjected to a thermal gradient, and the phenomenon is named thermal diffusion. Gas phase thermal diffusion has been theoretically and experimentally studied for more than a century, although there has not been a satisfactory theory to date. Nevertheless, for isotopic system, the Chapman-Enskog theory predicts that the mass difference is the only term in the thermal diffusion separation factors that differs one isotope pair to another,with the assumptions that the molecules are spherical and systematic (monoatomic-like structure) and the particle collision is elastic. Our previous report indicates factors may be playing a role because the Non-Mass Dependent (NMD) effect is found for both symmetric and asymmetric, linear and spherical polyatomic molecules over a wide range of temperature (-196C to +237C). The observed NMD phenomenon in the simple thermal-diffusion experiments demands quantitative validation and theoretical explanation. Besides the pressure and temperature dependency illustrated in our previous reports, efforts are made in this study to address issues such as the role of convection or molecular structure and whether it is a transient, non-equilibrium effect only.
Effect of elastic, hard sphere collisions between electrons of injected beam and natural gas molecules
Optical emissions produced in collisions of protons and hydrogen atoms incident on nitrogen molecules studied in 1200 to 6000 angstrom spectral region
A theoretical model of the radiating metallic gas produced about an iron meteor entering the earth's atmosphere is discussed. Numerical results are presented for a 0.1 cm diameter iron meteor traveling at 15 km/sec at an altitude of 100 km above the earth. It is shown that collisions between the expanding iron gas and the air molecules produce a radiating gas shell a few meters thick which is located many meters ahead of the meteor core. Temperature, pressure, and density distributions are presented as functions of radial distance and angle for several initial meteor conditions.
Investigation of different models for the collisional excitation of carbon monoxide (CO) by solving the rate equations for the ten lowest energy levels, including stimulated emission and absorption by 3 K isotropic radiation, spontaneous emission, and collisions. The results are qualitatively applicable to other linear molecules. They show that a wide range of rotational excitation temperatures, including population inversions, can be produced by the various kinetic temperatures and particle densities which are likely to occur in the interstellar regions containing molecules.
Semiclassical collisions of an atom with a rigid-rotor molecule are examined in the sudden approximation. The rotational transition probability is shown to be invariant with respect to the choice of orientation for the molecular coordinate system; this fact contradicts recently reported results of a computer analysis. The present analysis may lead to an improved interpretation of recent molecular beam measurements.
High resolution spectra of the coma of P/Halley were obtained on 25 April, 2 May, and 3 May 1986. The self-scanned Digicon detector of the 2.7 m telescope coude spectrograph was used to obtain spectra at a resolution of 0.6A. Results of the observation of the NH (A 3 Pi i yields X 3 Sigma (-)) band near 3360 A are presented. The summed NH spectrum represents the results of 190 min of integration centered on the nucleus of the comet. Eight rotational lines from the P and R branches of the band are detected. The lines from the Q branch are especially weak, and are not obviously resolved in the spectrum. The data are compared with the theoretical emission spectrum of Litvak and Kuiper (1982) and substantial discrepancies are found. These discrepancies are explained through a consideration of the relative timescales for collisions, resonance fluorescence, and radiative decays of the NH molecules in the coma.