A two-dimensional model of the resonant electron-molecule collision processes with one nuclear and one electronic degree of freedom introduced by Houfek, Rescigno and McCurdy [Phys. Rev. A 73, 032721 (2006)] and a similar two- dimensional model of the dissociative recombination with potential proposed by Hamilton [Ph.D. thesis, University of Colorado, (2003)] are formulated within the time-dependent framework and solved numerically using the finite-element method with the discrete variable representation basis, the exterior complex scaling method and the generalized Crank-Nicolson method. On the model of electron-molecule collisions we illustrate how the time-dependent calculations can provide a deep insight into the origin of oscillatory structures in the vibrational excitation cross sections if one evaluates the cross sections not only at sufficiently large time to obtain the final cross sections, but rather at several characteristic times which are given by the evolution of the system. With use of the time- dependent calculations we demonstrate the complex nature of the dissociative recombination model dynamics and we propose the interpretation of the recom- bination process mechanism. We also propose few techniques for the explanation of the sharp structures in the dissociative recombination cross sections...

The purpose of this work and the project under which it was created is to develop, compare and validate several theoretical approaches and computation methods used to calculate the cross sections of dissociative recombination. For the most part it is con- cerned with the indirect dissociative recombination of molecular ions of H+ 2 in the singlet ungerade channels computed with three distinct approaches. First, the fully numerically solvable two-dimensional approach developed at ÚTF MFF UK as a part of my master's thesis. Second, a vibrational frame transformation method based on the work of Chang and Fano [E. S. Chang and U. Fano, Phys. Rev. A 6, 173 (1972)] and then enhanced into a full energy-dependent form by Gao and Greene [H. Gao and C. H. Greene, J. Chem. Phys. 91, 3988 (1989)], [H. Gao and C. H. Greene, Phys. Rev. A 42, 6946 (1990)], fur- ther improved by our own revisions. Third, a two-dimensional R-matrix method based on matching exact 2D solutions from a small interaction region to asymptotic solutions in the non-interacting region. We thoroughly discuss the various advantages and caveats of these methods and, in the later chapters, present our work on employing them for the realistic recombination of HeH+ + e− . Furthermore, we attempt to extend the presented models to the description of the...

In this dissertation, different approaches have been employed to address the quest of understanding the formation and growth mechanisms of carbon-containing molecular ions with relevance to astrochemistry. Ion mobility mass spectrometry and DFT computations were used to investigate how a second nitrogen in the pyrimidine ring will affect the formation of a covalent bond between the benzene radical cation and the neutral pyrimidine molecule, after it was shown that a stable covalent adduct can be formed between benzene radical cation and the neutral pyridine. Evidence for the formation of a more stable covalent adduct between the benzene radical cation and the pyrimidine is reported here. The effect of substituents on substituted-benzene cations on their solvation by an HCN solvent was also investigated using ion mobility mass spectrometry and DFT computations were also investigated. We looked at the effect of the presence of electron-withdrawing substituents in fluorobenzene, 1,4 di- fluorobenzene, and benzonitrile on their solvation by up to four HCN ligands, and compared it to previous work done to determine the solvation chemistry of benzene and phenylacetylene by HCN. We report here the observed increase in the binding of the HCN molecule to the aromatic ring as the electronegativity of the substituent increased. We also show in this dissertation, DFT calculations that reveal the formation of both hydrogen-bonded and electrostatic isomers, of similar energies for each addition to the ions respectively. The catalytic activity of the 1st and 2nd row TM ions towards the polymerization of acetylene done using the reflectron time of flight mass spectrometry and DFT calculations is also reported in this dissertation. We explain the variation in the observed trend in C-H/C-C activity of these ions. We also report the formation of carbide complexes by Zr+, Nb+, and Mo+, with the acetylene ligands, and show the thermodynamic considerations that influence the formation of these dehydrogenated ion-ligand complexes. Finally, we show in this dissertation, a novel ionization technique that we employed to generate ions that could be relevant to the interstellar and circumstellar media using the reflectron time of flight mass spectrometry.

The purpose of this thesis is to construct a numerically solvable quantum mechanical model describing the dynamics of the indirect mechanism of the dissociative recombination process of a molecular cation by electron impact. The model also describes vibrational excitation of a molecular cation by electron impact. The solution of this model is carried out by implementing a combination of finite elements, discrete variable representation and exterior complex scaling methods. This is then specifically applied to the dissociative recombination and vibrational excitation of H$_2^+$ by an incoming electron. The results can be used to test the accuracy of approximative methods and the programs expanded to cover the cases of other diatomics. Powered by TCPDF (www.tcpdf.org)

Les colonnes de plasma entretenues par un champ électrique (continu ou alternatif) à haute pression (p > 10 Torr) sont affectées par les phénomènes de contraction (réduction de la section radiale de la décharge) et de filamentation (fragmentation de la section de plasma en plusieurs filaments). La compréhension de ces phénomènes ainsi que le développement d’une méthode pouvant les supprimer demeurent une étape essentielle pour l’optimisation de certains procédés plasma. Dans cette optique, un premier objectif de notre travail était de déterminer les mécanismes à l’origine de la contraction et de la filamentation dans les décharges créées dans des gaz rares. Ainsi, nous avons montré que dans les plasmas micro-ondes contractés la cinétique de la décharge est contrôlée par les ions moléculaires et que la contraction est liée à l’influence du gradient de la température du gaz sur la concentration de ces ions. De plus, nous avons mis en évidence que la filamentation apparaît lorsque l’inhomogénéité radiale du champ électrique devient importante. Dans un second temps, nous avons développé une méthode de décontraction et de défilamentation de la décharge, qui consiste à ajouter à une décharge initiale de gaz rare des traces d’un autre gaz rare de plus faible potentiel d’ionisation. Dans le cas des plasmas décontractés, nous avons démontré que la cinétique de la décharge n’est plus contrôlée par les ions moléculaires, ce qui confirme bien l’importance de ces ions dans la description de la contraction. Pour terminer, nous avons étendu à la pression atmosphérique la technique d’absorption optique de mesure de densité des états métastables et résonnants à l’aide d’une lampe spectrale, ce qui n’avait été réalisé jusqu’ici que pour des pressions inférieures à 10 Torr. Ces états jouent un rôle essentiel dans l’ionisation des décharges contractées alors que dans les décharges décontractées leur désexcitation par les atomes du gaz adjuvant est l’étape fondamentale du processus de changement de cinétique menant à la décontraction. / Plasma columns sustained at high pressures (p > 10 Torr) by an electric field (of constant or varying intensity) are affected by the contraction phenomena (reduction of the radial section of the discharge) and filamentation (breaking of the plasma column into several filaments). The understanding of these phenomena and the development of methods to suppress them are essential steps in the optimization of some plasma processes. In this context, the initial objective of our work was to determine the mechanisms at the origin of plasma contraction and filamentation in rare gas discharges. Along that line, we have shown that the discharge kinetics of micro-wave contracted plasmas is controlled by the presence of molecular ions and that contraction relates to the influence of the radial gradient of gas temperature on the concentration of these ions. In addition, we have evidenced that filamentation shows up whenever the radial inhomogeneity of the electric field intensity becomes important enough. In a second step, we have developed a method for eliminating plasma contraction and filamentation. It consists in adding to a contracted rare-gas discharge, a small amount of another rare gas having a lower ionization potential. In the case of the expanded plasmas obtained in this way, the discharge kinetics has been shown to be no longer controlled by molecular ions, thereby confirming their essential role in the contraction mechanism. Finally, we have extended to atmospheric pressure the technique of optical absorption that uses a spectral lamp to measure metastable-atom and resonant-atom densities. Until now, this technique has been used only at gas pressures lower than 10 Torr. Our interest in measuring metastable-state atom density is related to their participation in the step-wise ionization of contracted plasmas while, in expanded discharges, the fact that are desexcited by collisions with the added gas atoms is the essential step in modifying the kinetics of the discharge and preventing it to contract.

Les colonnes de plasma entretenues par un champ électrique (continu ou alternatif) à haute pression (p > 10 Torr) sont affectées par les phénomènes de contraction (réduction de la section radiale de la décharge) et de filamentation (fragmentation de la section de plasma en plusieurs filaments). La compréhension de ces phénomènes ainsi que le développement d’une méthode pouvant les supprimer demeurent une étape essentielle pour l’optimisation de certains procédés plasma. Dans cette optique, un premier objectif de notre travail était de déterminer les mécanismes à l’origine de la contraction et de la filamentation dans les décharges créées dans des gaz rares. Ainsi, nous avons montré que dans les plasmas micro-ondes contractés la cinétique de la décharge est contrôlée par les ions moléculaires et que la contraction est liée à l’influence du gradient de la température du gaz sur la concentration de ces ions. De plus, nous avons mis en évidence que la filamentation apparaît lorsque l’inhomogénéité radiale du champ électrique devient importante. Dans un second temps, nous avons développé une méthode de décontraction et de défilamentation de la décharge, qui consiste à ajouter à une décharge initiale de gaz rare des traces d’un autre gaz rare de plus faible potentiel d’ionisation. Dans le cas des plasmas décontractés, nous avons démontré que la cinétique de la décharge n’est plus contrôlée par les ions moléculaires, ce qui confirme bien l’importance de ces ions dans la description de la contraction. Pour terminer, nous avons étendu à la pression atmosphérique la technique d’absorption optique de mesure de densité des états métastables et résonnants à l’aide d’une lampe spectrale, ce qui n’avait été réalisé jusqu’ici que pour des pressions inférieures à 10 Torr. Ces états jouent un rôle essentiel dans l’ionisation des décharges contractées alors que dans les décharges décontractées leur désexcitation par les atomes du gaz adjuvant est l’étape fondamentale du processus de changement de cinétique menant à la décontraction. / Plasma columns sustained at high pressures (p > 10 Torr) by an electric field (of constant or varying intensity) are affected by the contraction phenomena (reduction of the radial section of the discharge) and filamentation (breaking of the plasma column into several filaments). The understanding of these phenomena and the development of methods to suppress them are essential steps in the optimization of some plasma processes. In this context, the initial objective of our work was to determine the mechanisms at the origin of plasma contraction and filamentation in rare gas discharges. Along that line, we have shown that the discharge kinetics of micro-wave contracted plasmas is controlled by the presence of molecular ions and that contraction relates to the influence of the radial gradient of gas temperature on the concentration of these ions. In addition, we have evidenced that filamentation shows up whenever the radial inhomogeneity of the electric field intensity becomes important enough. In a second step, we have developed a method for eliminating plasma contraction and filamentation. It consists in adding to a contracted rare-gas discharge, a small amount of another rare gas having a lower ionization potential. In the case of the expanded plasmas obtained in this way, the discharge kinetics has been shown to be no longer controlled by molecular ions, thereby confirming their essential role in the contraction mechanism. Finally, we have extended to atmospheric pressure the technique of optical absorption that uses a spectral lamp to measure metastable-atom and resonant-atom densities. Until now, this technique has been used only at gas pressures lower than 10 Torr. Our interest in measuring metastable-state atom density is related to their participation in the step-wise ionization of contracted plasmas while, in expanded discharges, the fact that are desexcited by collisions with the added gas atoms is the essential step in modifying the kinetics of the discharge and preventing it to contract.