Engineering topics
Publications and source records attributed to James, G..
High-resolution (Delta lambda = 115 m Angstrom) emission spectra of molecular hydrogen produced by electron-impact excitation at 100 eV have been obtained in the wavelength range 900-1200 A.
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Photodissociation via discrete line absorption into predissociating Rydberg and valence states is the dominant destruction mechanism of CO and other molecules in the interstellar medium and molecular clouds.
The intensity of UV resonance transitions excited by electron impact is determined by direct and cascade processes. The lifetime of decay by spontaneous emission is short (<10 ns); the lifetimes for cascade from higher-lying states is longer (>30 ns).
In 1996 and 1997 the Galileo Ultraviolet Spectrometer (UVS) obtained the first measurements of Jupiter's nightside midultraviolet (MUV) polar auroral spectrum from 1620 to 3231 A at 13 A resolution.
A new generation of high resolution UV imaging spacecraft (Polar, Galileo, HST) are studying the airglow and aurora of the Earth and the jovian planets. To keep pace with these technological improvements we have developed a laboratory program to provide electron collision cross sections of the major molecular planetary gasses. Molecular spectra under optically thin conditions have been measured with a high resolution UV spectrometer in tandem with an electron impact collision chamber. The results of various studies are reported in this paper.
The next generation of high resolution UV imaging spacecraft are being prepared for studying the airglow and aurora of the Earth, the other terrestrial planets and the Jovian planets. To keep pace with these technological improvements we have developed a laboratory program to provide electron impact collision cross sections of the major molecular planetary gases (H2, N2, CO2, O2, and CO). Spectra under optically thin conditions have been measured with a high resolution (lambda/delta(lambda) = 50000) UV spectrometer in tandem with electron impact collision chamber. High resolution spectra of the Lyman and Wemer band systems of H2 have been obtained and modeled. Synthetic spectral intensities based on the J-dependent transition probabilities that include ro-vibronic perturbations are in very good agreement with experimental intensities. The kinetic energy distribution of H(2p,3p) atoms resulting from electron impact dissociation of H2 has been measured. The distribution is based on the first measurement of the H Lyman-alpha (H L(alpha)) and H Lyman-beta (H L(beta)) emission line Doppler profiles. Electron impact dissociation of H2 is believed to be one of the major mechanisms leading to the observed wide profile of H L-alpha from Jupiter aurora by the Hubble Space Telescope (HST). Analysis of the deconvolved line profile of H L-alpha reveals the existence of a narrow line peak (40 mA FWHM) and a broad pedestal base (240 mA FWHM). The band strengths of the electron excited N2 (C(sup 3) Pi(sub(upsilon) - B(sup 3)Pi(sub g)) second positive system have been measured in the middle ultraviolet. We report a quantitative measurement of the predissociation fraction 0.15 +/- 01(sup .045, sub .01) at 300 K in the N2 c'(sub )4 (1)sigma(sup +, sub g) - x(1)sigma(sup +, sub g)(00) band, with an experimental determination of rotational line strengths to be used to understand N2 EUV emission from Titan, Triton and the Earth.
In the electron-impact-induced fluorescence spectrum on neon in the wavelength range of 120 270 nm at a spectral resolution of 0.43 nm, the strongest lines in the far ultraviolet (FUV) spectrum of neon are assigned to items of the Doublet system of Ne II and the Triplet system of Ne III. FUV spectral data at 300 eV electron impact energy provide absolute emission cross sections of these Ne II and Ne III lines and are compared to previous measurements.
The next generation of high resolution UV imaging Spacecraft are being prepared for studying the airglow and aurora of the Earth, the other terrestrial planets and the Jovian planets. To keep pace with these technological improvements we have developed a laboratory program to provide electron impact collision cross sections of the major molecular planetary gases. This program is described in detail.
We report medium resolution electron-impact induced emission spectra of CO for 20, 100 1nd 200 eV impact energies.
In the upper atmospheres of the Jovian and Terrestrial planets a dominant mechanism for energy transfer occurs through electron collisional processes with neutral species leading to UV radiation. In response to the need for accurate collision cross sections to model spectroscopic observations of planetary systems, JPL has measured in the laboratory emission cross sections and medium resolution spectra of H, H(sub 2), N(sub 2), SO(sub 2), and other important planetary gases.Voyager and International Ultraviolet Explorer (IUE) spacecraft have established that band systems of H(sub 2) and N(sub 2) are the dominant UV molecular emissions in the solar system produced by electron impact. Applications of our data to models of Voyager, IUE, Galileo, and Hubble Space Telescope observations of the planets will be described.