Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt.........2m&link_type=abstract
Thesis (PhD). THE UNIVERSITY OF CHICAGO, Source DAI-B 62/02, p. 916, Jun 2001, 266 pages.
Astronomy and Astrophysics
Astronomy
6
Scientific paper
H+3 is the simplest and most fundamental polyatomic molecule, consisting of only three protons and two electrons. As such, it plays important roles in the laboratory spectroscopy of hydrogen-rich plasmas, the theoretical calculation of rotation-vibration energy levels, and also the chemistry of interstellar clouds. This dissertation touches on all three of these areas. High resolution spectroscopy of H+3 has been performed in a positive column discharge. The combination bands ν1 + 2ν2 <-- 0 and 2ν1 + ν2 <-- 0 have been observed with a diode laser, and reach the highest energy vibrational states yet studied. The initial detection of the fourth overtone band (5ν2 <-- 0, which reaches above the barrier to linearity) with a Titanium-Sapphire laser is also discussed. A comprehensive re-evaluation of all previous laboratory spectroscopy of H+3 has been conducted in order to obtain a reliable linelist and derive experimentally determined energy levels. These energy levels have been compared with the most recent variational calculations on ab initio potential energy surfaces. It is hoped that this comparison will permit further refinements in the theoretical calculations. H+3 has been detected (using absorption lines of the ν2 fundamental band) in several dense interstellar clouds, where it serves as the universal protonator, initiating a chain of ion-neutral reactions that is responsible for the production of the variety of molecules observed by radioastronomers. In dense clouds, measurements of H+3 provide direct estimates of the clouds' path lengths, average number densities, and kinetic temperatures. H+3 has also been observed in several diffuse interstellar clouds, where it is supposed to be two to three orders of magnitude less abundant due to the efficiency of electron recombination. This observational result suggests a serious general problem with the models of diffuse cloud chemistry. The most likely solution is that the ratio of the cosmic ray ionization rate (ζ) to the dissociative recombination rate (ke) is at least one to two orders of magnitude higher than has been generally assumed. This implies that the laboratory measurement of ke is not applicable to interstellar conditions, and/or that H2 ionization is enhanced in diffuse clouds relative to dense clouds.
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