Gas-Phase Ion Chemistry in Interstellar, Circumstellar, and Planetary Environments

Demarais, Nicholas J.

Graduate Thesis Or Dissertation

Gas-Phase Ion Chemistry in Interstellar, Circumstellar, and Planetary Environments Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/qj72p7495

Abstract

In the last century, astronomers, physicists, and chemists have shown that the environments of space are complex. Although we have learned a great amount about the interstellar medium, circumstellar medium, and atmospheres of other planets and moons, many mysteries still remain unsolved. The cooperation of astronomers, modelers, and chemists has lead to the detection of over 180 molecules in the interstellar and circumstellar medium, and the evolution of the new scientific field of astrochemistry. Gas-phase ion chemistry can determine the stability of ions in these complex environments, provide chemical networks, and guide searches for new interstellar molecules.

Using the flowing afterglow-selected ion flow tube (FA-SIFT), we have characterized the reactions of positive and negative ions that are important in a variety of astrochemical environments. The detection of CF⁺ in photodissociation regions highlights the importance of fluorinated species in the interstellar medium. The viability of CF⁺ as a possible diffuse interstellar band (DIB) carrier is discussed as related to reactions with neutral molecules in various interstellar conditions; the reactions of CF⁺ with twenty-two molecules of interstellar relevance were investigated.

The chemical reactions of HCNH⁺ with H₂, CH₄, C₂H₂, and C₂H₄ were reexamined to provide insight into the overprediction of HCNH⁺ in Titan’s ionosphere by current astrochemical models. In addition, this work suggests other chemical reactions that should be included in the current models to fully describe the destruction rates of HCNH⁺ in Titan’s ionosphere.

The reactions of polycyclic aromatic hydrocarbon (PAH) ions with H atoms and other small molecules were carried out to determine the stability of these species. In diffuse regions, where the photon flux is high, PAH cations are the dominant ionization state. This work continues our previous research to include PAHs of differing geometries as well as nitrogencontaining PAHs. Extension to larger PAH cations was made possible by the integration of the laser induced acoustic desorption (LIAD) source with the FA-SIFT.

In addition, in dense environments, where the photon flux is low, anionic PAHs may exist. The detection of negative ions in the past 10 years has highlighted the importance of their inclusion in astrochemical models. We have investigated the chemistry of deprotonated PAHs with molecules of interstellar relevance to determine their chemical stability in dense regions of the interstellar and circumstellar medium.

In addition to PAH anions, H⁻ is an important species in dense interstellar environments. While the reaction of hydride anion has been recognized as a critical mechanism in the initial cooling immediately after the Big Bang, H⁻ + H → H₂ + e⁻, chemistry with neutral molecules was largely unknown. The chemistry of H⁻ with various classes of organic molecules was investigated and conclusions are drawn based on reaction mechanisms.

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