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Vibrational and Electronic Energy Transfer and Dissociation of Diatomic Molecules by Electron CollisionsAt high altitudes and velocities equal to or greater than the geosynchronous return velocity (10 kilometers per second), the shock layer of a hypersonic flight will be in thermochemical nonequilibrium and partially ionized. The amount of ionization is determined by the velocity. For a trans atmospheric flight of 10 kilometers per second and at an altitude of 80 kilometers, a maximum of 1% ionization is expected. At a velocity of 12 - 17 kilometer per second, such as a Mars return mission, up to 30% of the atoms and molecules in the flow field will be ionized. Under those circumstances, electrons play an important role in determining the internal states of atoms and molecules in the flow field and hence the amount of radiative heat load and the distance it takes for the flow field to re-establish equilibrium. Electron collisions provide an effective means of transferring energy even when the electron number density is as low as 1%. Because the mass of an electron is 12,760 times smaller than the reduced mass of N2, its average speed, and hence its average collision frequency, is more than 100 times larger. Even in the slightly ionized regime with only 1% electrons, the frequency of electron-molecule collisions is equal to or larger than that of molecule-molecule collisions, an important consideration in the low density part of the atmosphere. Three electron-molecule collision processes relevant to hypersonic flows will be considered: (1) vibrational excitation/de-excitation of a diatomic molecule by electron impact, (2) electronic excitation/de-excitation, and (3) dissociative recombination in electron-diatomic ion collisions. A review of available data, both theory and experiment, will be given. Particular attention will be paid to tailoring the molecular physics to the condition of hypersonic flows. For example, the high rotational temperatures in a hypersonic flow field means that most experimental data carried out under room temperatures are not applicable. Also, the average electron temperature is expected to be between 10,000 and 20,000 K. Thus only data for low energy electrons are relevant to the model.
Document ID
20020035523
Acquisition Source
Ames Research Center
Document Type
Preprint (Draft being sent to journal)
Authors
Huo, Winifred M.
(NASA Ames Research Center Moffett Field, CA United States)
Langhoff, Stephen R.
Date Acquired
August 20, 2013
Publication Date
January 1, 1995
Subject Category
Atomic And Molecular Physics
Meeting Information
Meeting: NATO ASI Molecular Physics and Hypersonic Flows
Location: Maratea
Country: Italy
Start Date: May 21, 1995
End Date: June 3, 1995
Funding Number(s)
PROJECT: RTOP 242-80-01
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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