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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Electron Coincidence Studies of Molecules

Atkins, Danielle S, N/A January 2007 (has links)
The electron-electron coincidence (e,2e) technique yields complete kinematical information about the electron impact ionization process. The (e,2e) technique has been widely used to study dynamical effects in ionizing collisions with atomic targets, however studies of molecular ionization using this technique have been very limited. Recently further experimental studies of small molecules have been proposed, as the cross sections of small molecules are now computable using sophisticated theoretical approaches [77, 24]. This thesis presents dynamical investigations for the electron impact ionization of the molecular targets H2O and H2, employing the (e,2e) technique to experimentally measure the triple differential cross section (TDCS). The TDCS is defined as the probability that a bound electron will be ejected from the target atom or molecule (into a particular direction with a defined energy) and the initial electron will be scattered into a particular direction with a particular energy. All TDCSs presented within this thesis were performed using an electron coincidence spectrometer in the coplanar asymmetric geometry at intermediate incident electron energies. This thesis presents the electron impact ionization TDCSs of H2O. A series of measurements were performed using H2O in the vapour form. Measurements of the TDCS are presented for the 2a1 atomic-like orbital and the 1b2, 3a1 and 1b1 molecular orbitals at a common incident electron energy of 250eV, ejected electron energy of 10eV and scattering angle of -15o. The experimental TDCSs are compared with theoretical cross sections that were calculated by Champion et al [25, 26] using a distorted wave Born approach (DWBA). TDCS measurements for the single ionization of the hydrogen molecule, H2 were performed as in recent years there has been evidence that indicates the ejected electron angular distribution is perturbed due to Young-type interference effects. The oscillatory structure which is predicted in the cross section is due to the two-centred nature of the molecule [27, 29]. This thesis presents experimental TDCSs for the ionization of H2 which are compared to TDCSs of helium. A series of measurements for the TDCSs of H2 and He are presented at a common incident electron energy of 250eV and scattering angle of -15o, for a range of ejected electron energies between 10eV and 100eV. The experimental TDCSs are compared with two types of theoretical calculations.
2

Monte Carlo simulations and a theoretical study of the damage induced by ionizing particles at the macroscopic scale as well as the molecular scale / Simulations Monte Carlo et étude théorique des dommages induits par les particules ionisantes à l’échelle macroscopique ainsi qu’à l’échelle moléculaire

Mouawad, Lena 16 December 2017 (has links)
Le travail présenté dans cette thèse se place dans le contexte de la simulation de dommages biologiques. D'abord une étude macroscopique met en question la pertinence des plans de traitement basés sur la dose absorbée et le passage à une étude de micro-dosimétrie permet l'utilisation de paramètres biologiques plus pertinents, tels que les cassures de brins d'ADN. La validité des sections efficaces d'interaction sur lesquelles se basent ces simulations est discutée en plus de détails. Suite à la complexité du milieu biologique, les sections efficaces d'interaction avec l'eau sont souvent utilisées. Nous développons un algorithme qui permet de fournir les sections efficaces d'ionisation pour n'importe quelle cible moléculaire, en utilisant des outils qui permettent de surmonter les difficultés de calcul, ce qui rend notre programme particulièrement intéressant pour les molécules complexes. Nous fournissons des résultats pour l'eau, l'ammoniac, l'acide formique et le tétrahydrofurane. / The work presented in this thesis can be placed in the context of biological damage simulation. Webegin with a macroscopic study where we question the relevance of absorbed-dose-based treatmentplanning. Then we move on to a micro-dosimetry study where we suggest the use of morebiologically relevant probes for damage, such as DNA strand breaks. More focus is given to thefundamental considerations on which the simulations are based, particularly the interaction crosssections. Due to the complexity of the biological medium, the interaction cross sections with waterare often used to simulate the behavior of particles. We develop a parallel user-friendly algorithmthat can provide the ionization cross sections for any molecular target, making use of particular toolsthat allow to overcome the computational difficulties, which makes our program particularlyinteresting for complex molecules. We provide preliminary results for water, ammonia, formic acidand Tetrahydrofuran.
3

Étude détaillée du deuxième terme de l'approximation de Born : applications à l'ionisation de l'atome d’Hydrogène et à la double ionisation de l'atome d’Hélium par impact d’électrons et de positrons / Full study of the second term of the Born approximation : applications on the ionization of the hydrogen atom and the helium atom by electron and positron impacts

Hmouda, Bassem 10 September 2014 (has links)
Les méthodes perturbatives, telle que l’approximation de Born, sont nécessaires pour résoudre les problèmes inhérents à l’ionisation d’atomes et de molécules par impact d’électrons ou de positrons. Afin d’optimiser les calculs nécessaires pour le second terme de l’approximation de Born, nous avons commencé par étudier l’ionisation de l’atome le plus simple : celui d’hydrogène. Nous avons utilisé une base contenant un grand nombre d’états (294) nous permettant d’éviter la relation de fermeture qui nécessite l’introduction d’un paramètre qui représente la valeur moyenne d’excitation. Nos résultats ont montré un bon accord avec l’expérience surtout pour les faibles énergies des électrons éjectés. Nous avons ainsi pu montrer l’importance de la contribution des états du continuum (représentés par des pseudo-états), en particulier pour les transitions de type dipolaire. Pour la double ionisation de l’atome d’hélium, nous avons appliqué la même méthodologie de calcul numérique complet tout en incluant 20 états et pseudo-états intermédiaires et en utilisant une fonction d’onde corrélée d’interaction de configuration, on a trouvé pour les grandes énergies d’incidence (5 keV) que l’effet du terme Born 2 est presque nul. Par contre l’application de « SBA » avec la relation de fermeture pour l’état fondamental et les premiers états excités montre une petite différence avec « FBA » en particulier en dehors de la région du transfert. Dans le cas des faibles énergies d’incidence (601 eV) il était attendu d’obtenir un effet important de « SBA » surtout que des études sur les (e,3-1e) de l’hélium montrent un déplacement du pic principal par rapport à « FBA ». Donc on peut dire que les 20 états intermédiaires contribuent de façon insuffisante et qu’il faudra considérer beaucoup plus d’états / The perturbative methods, such as Born approximation, are necessary to solve the problems concerning the ionization of atoms and molecules by electrons or positrons impacts. In order to use Born approximation in an optimized way, we tested it on the simplest atom « Hydrogen » by using a basis of large amount of intermediate states (294) and complete numerical calculation without using the closure approximation whose application needs the introduction of a parameter which is the excitation mean value. Our results proved a significant agreement with the experiment particularly for small energies of the ejected electrons. We also proved an important contribution of the continuum (represented by the pseudo-states), and particularly the dipolar transition. For the double ionization of Helium atom, we applied the same methodology of complete calculation by including 20 intermediate states and pseudo-states and by using a configuration interaction wave function, we found that for high incident energy (5 keV) the effect of the second term of Born is almost zero. However, the application of the « SBA » with the closure approximation by using the fundamental state and the first excited states show a slight difference relative to the « FBA » particularly outside the transfer region. In case of low incident energy (601 eV), it was expected a crucial effect of the « SBA » especially that previous studies of (e, 3-1e) of Helium show a significant shift of the main peak relative to the « FBA ». So we can say that 20 intermediate states are not enough and the application of the « SBA » needs more states

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