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Atomic and Molecular Data Preparation and Evaluation for Planetary Applications, 15-9140 Printer Friendly VersionPrincipal Investigators Inclusive Dates: 04/13/99 - 08/12/99 Background - Analysis, modeling, and interpretation of ground- and space-based data of atmospheres for all solar system bodies have one common requirement: they all need basic atomic and molecular data. The circumstances differ from body to body in that they require data for different atomic and molecular species, both major and minor. An important trace species in comets is sodium (Na). It was detected in Comet Hale-Bopp by several teams of astronomers for the first time in the form of two types of tails with completely different morphologies. One sodium tail is broad and diffuse and is completely superimposed on the dust tail. The sodium atoms in this tail appear to be released in situ directly from the dust particles. The other sodium tail is sharp and narrow because the atoms are accelerated in the inner coma of the comet by solar radiation pressure. It has been suggested that the source of the sodium in the inner coma is from Na-containing molecules with short lifetimes. Determining the lifetime of potential molecules bearing sodium was one important aspect of this research. In a second effort to expand SwRI’s modeling and analysis capabilities for other planetary bodies, the research team has compiled and assessed laboratory and quantum theoretical photon and electron impact cross sections. These cross sections are used to model the plasma environment, chemistry, and emissions of Earth-like planets and moons (those atmospheres consist mainly of oxygen, nitrogen, and carbon compounds). Approach - To explore the source of sodium atoms in the narrow comet tail, the team investigated the lifetime of Na-bearing molecules exposed to unattenuated solar ultraviolet radiation. Researchers supplemented the measured molecular photo cross sections near the dissociation threshold with photo cross sections synthesized from the atomic constituents of the molecules. The resulting total photo cross sections of these molecules were then folded with the solar radiation field and integrated over wavelength to give the dissociation rate coefficients (reciprocal lifetimes). Energy-weighted integrations yield the excess energies of the photolysis products. Development of a cross-section database covering the entire energy (wavelength) ranges of interest necessitated the compilation and reduction of data from a variety of sources, which are often in poor agreement. Multiple energy-dependent measurements of the key electron impact cross sections have been fit by a nonlinear least-squares technique, using appropriate quantum basis functions. Measurements of photoabsorption and photoionization cross sections have been merged and interpolated onto a standard solar emission line grid. Accomplishments - During this project, the investigators: 1) calculated the lifetimes (inverse rate coefficients) and excess energies for the photolysis products, Ex, of several sodium-bearing molecules (in the gas phase) exposed to the unattenuated solar radiation field, and 2) developed techniques to calculate the dissociation and ionization rate coefficients, lifetimes, and excess energies from electron impact cross sections. The key cross sections for Earth-like atmospheres (CNO compounds) in the solar system have been determined. The photon cross sections have been interpolated onto a solar emission line grid. As an application of these data, the team has calculated photo-ion and photoelectron production rates, and photoelectron fluxes for Venus, Mars, and Neptune’s moon Triton. These new capabilities will be used in future investigations of data from current and upcoming planetary missions. |
