Recently, several negative molecular ions, CnN- (n = 1, 3, 5) and CnH- (n = 4, 6, 8), were observed in the interstellar medium (ISM). It was suggested that the anions are formed in the ISM by the process of radiative electron attachment (REA). A simple statistical model was developed in 1980's to estimate rate coefficients of the REA reactions. Some of the rate coefficients obtained in the model are consistent with the observations, the others are not. More importantly, some of the approximations employed in the model are not physically justified. The aim of this thesis is a development of a quantum-mechanical approach to study the process of radiative electron attachment to linear molecules of astrophysical interest. The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. Cross sections and rate coefficients for formation of the following molecular negative ions by REA were determined: CN-, C2H-, C3N-, C4H-, C5N-, C6H-, and C8H-. All the calculations presented in the thesis were carried out using the MOLPRO and UK R-matrix (Quantemol)suites of programs. Uncertainty quantification of the results, obtained for each studied system, was performed. A second process, closely related to the radiative electron attachment, photodetachment (PD), was also studied in the thesis. Photodetachment cross sections for the CnN- (n = 1, 3, 5), CnH- (n = 4, 6, 8) and C2- molecules were determined using an approach similar to the one employed for REA from the same transition dipole moment matrix elements. The obtained REA cross sections and rate coefficients were validated by comparing the present theoretical results with the experimental data from recent photodetachment experiments. The present results suggest that the observed abundance of these ions in the ISM can hardly be explained by the REA process. In other words, these anions are formed in the interstellar medium by a process different than radiative electron attachment. Dissociative electron attachment (DEA) is another process of anion formation, which could possibly explain formation of certain molecular anions in the ISM. The ClF attachment was studied using a first principle approach.A good agreement with experimental data was demonstrated. A theoretical approach to evaluate cross sections for rotational excitation of linear neutral molecules by an electron impact was developed and applied to acetylene, HCCH. The differential cross sections for e-HCCH scattering were calculated between energies 0.1 eV and 10 eV. The momentum transfer cross section and eigenphases for e-HCCH scattering were also calculated.
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Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Khamesian, Marjan, "Theoretical Study of Negative Molecular Ions Relevant to the Interstellar and Laboratory Plasma" (2017). Electronic Theses and Dissertations. 5408.