Browsing by Author "Hakes, Charles Lynn"

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    Absolute differential cross-sections for keV energy ion and atom collisions with oxygen atoms
    (1990) Hakes, Charles Lynn; Stebbings, R. F.
    Oxygen atoms are the most abundant constituent in the Earth's atmosphere between 200km and 600km, and thus are the most probable targets for precipitating ions and atoms at these altitudes. Differential cross sections for collisions involving oxygen atoms supply information necessary for numerically modeling the conditions in the Earth's upper atmosphere. Absolute cross sections, differential in angle, for electron capture by 500, 1500, and 5000eV protons in collisions with oxygen atoms are presented. The target, a mixture of atomic and molecular oxygen produced in a microwave discharge, is confined to a very short ($<$1mm) teflon-coated cell. The abundance of atomic and molecular oxygen in the target is determined using an electron-impact-ionization time-of-flight-mass spectrometer. The mass spectrometer signal, yielding the atomic oxygen density in the target cell, is calibrated by comparing the direct scattering of He(500eV) + O to the direct scattering of O(2000eV) + He. The charge transfer cross sections are integrated and compared to total cross sections reported in the literature.
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    AN ATOMIC OXYGEN TARGET FOR DIFFERENTIAL CROSS SECTION MEASUREMENTS
    (1987) Hakes, Charles Lynn
    Atomic oxygen, an abundant upper atmospheric species, is an important target in collisions involving fast ions and neutrals precipitating into the Earth's thermosphere. To facilitate the laboratory study of these collisions and to measure their differential cross sections, an appropriate atomic oxygen target has been developed. Oxygen atoms for the target are produced by electron-impact dissociation of O$\sb2$ in a discharge created in a microwave cavity resonant at 2.54 GHz. Thirty percent of the added molecular oxygen at 50 mTorr is dissociated in the discharge. The atomic and molecular oxygen mixture flows through a phosphoric acid coated pyrex tube to a fluorocarbon coated target cell. The dissociation fraction in the target cell is determined by titration with NO$\sb2$ and by analysis with a time-of-flight mass spectrometer.