Engineering topics
Publications and source records attributed to Ciocca, M..
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.
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. Accurate values for the rovibronic oscillator strengths of these transitions and predissociation yields of the excited states are required for input into the photochemical models that attempt to reproduce observed abundances. We report here on our latest experimental results of the electron collisional properties of CO and N2 obtained using the 3-meter high resolution single-scattering spectroscopic facility at JPL.
The kinetic-energy distribution of D(2p) atoms resulting from electron-impact dissociation of D2 has been measured. A high-resolution vacuum ultraviolet spectrometer was employed for the first measurement of the D Lyman-alpha (D L(alpha)) emission line profiles at 20- and 100-eV excitation energies. Analysis of the deconvoluted line profile of D L(alpha) at 100 eV reveals the existence of a narrow line central peak of 29+/-2 mA full width at half maximum and a broad pedestal wing structure about 190 mA wide. The wings of the line can be used to determine the fast atom distribution. The wings of D L(alpha) arise from dissociative excitation of a series of doubly excited states that cross the Franck-Condon region between 23 and 40 eV. The fast atom distribution at 100-eV electron impact energy spans the energy range from 1 to 10 eV with a peak value near 6 eV. Slow D(2p) atoms characterized by a distribution function with peak energy near 100 meV produce the central peak profile, which is nearly independent of the impact energy. The deconvoluted line profiles of the central peak at 20 eV for dissociative excitation of D2 and H2 are fitted with an analytical function for use in calibration of space flight instrumentation equipped with a D/H absorption cell. The kinetic-energy and line profile results are compared to similar measurements for H2. The absolute cross sections for the line center (slow atoms) and wings (fast atoms) and total emission line profile were measured from threshold to 400 eV. Analytical model coefficients are given for the energy dependence of the measured slow atom cross section.
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.
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.