Emission line shapes produced by dissociative excitation of atmospheric gases |
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| Authors: | EC Zipf WC Wells |
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| Institution: | 1. Lockheed Palo Alto Research Laboratory, Lockheed Missiles and Space Company, Palo Alto, CA 94304 U.S.A.;2. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, U.S.A. |
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| Abstract: | Special line shapes are derived fro the λ 1356 Å () transition of atomic oxygen from metastable () time-of-flight spectra produced by electron impact dissociative excitation of O2, CO2, CO, and NO, and they are compared with the broadened λ 1304 A resonance line shapes deduced by Poland and Lawrence (1973) from atomic oxygen absorption studies. The non-thermal line shapes for both airglow emission features are shown to have an effective width comparable to a 60,000 K thermal doppler line shape for an electron impact energy of 100eV. The variation of the effective line width with electron-impact energy from threshold to 300 eV is given. Since the effective line width of the resonance radiation produced by dissociative excitation is very large compared with the doppler absorption widths of the ambient O atoms at normal exospheric temperatures, the anomalously broadened resonance lines will propagate through a planetary atmosphere as though they were optically thin. Thus, electron-impact dissociation of CO and CO2 will contribute to the observed optically thin component of the λ 1304 Å emission in the upper atmospheres of Venus and Mars. However, the process cannot account for more than 10% of the observed optically thin emission because of the small magnitude of the excitation cross-section and the comparatively high-energy threshold for the process. The possibility that the source of the kinetically energetic ) atoms is the dissociative recombination of vibrationally excited CO2+ ions is discussed. |
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