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The properties of molecular ions

O'Connor, Caroline Sophie Scott; (1999) The properties of molecular ions. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

This thesis presents the results of a series of experimental investigations of the single and double ionization of atmospheric molecules. In particular, the formation and dissociation of singly and doubly charged molecular ions are studied using electron-impact time-of-flight mass spectrometry and ion-ion coincidence techniques. The target atmospheric molecules under investigation are dinitrogen pentoxide (N2O5), nitric acid (HNO3), chlorine dioxide (OCIO) and chlorine monoxide (CI2O). These molecules are thought to play significant roles in the atmospheric ozone cycle as the sources of ozone-destroying radicals. However, despite the potential atmospheric importance of these reactive molecules and the interest in their photodissociation and spectroscopic properties, there have been relatively few investigations of their ionization. In the single ionization studies, electron-impact time-of-flight mass spectrometry is used to determine the relative partial single ionization cross sections of the ions produced upon single ionization of the target molecules from near threshold to 500 eV. In the case of N2O5 and HNO3, the appearance energies of previously unobserved fragment ions are also reported. The first investigations of the double ionization of the target species are also reported in this thesis. In order to study the formation and fragmentation of the doubly charged molecular ions (dications), ion-ion coincidence techniques coupled with time-of-flight mass spectrometry have been developed. One and two-dimensional coincidence experiments have been performed, where the distinction between these experiments arises from the dimensionality of the data set recorded. In the one-dimensional experiments, comparisons of the experimentally determined appearance energies for the dication dissociation reactions with the energetics derived from the kinetic energy release involved in these dissociation processes give (i) an indication of the mechanisms by which the doubly-charged molecules dissociate and (ii) the energy of the dication electronic states which are the source of the fragment ions. From the 2D coincidence spectra, analysis of the ion pair intensities and peak slopes can also be inteipreted to yield information concerning dication dissociation mechanisms. These investigations have led to the first evaluation of the ratio of double-to-single ionization cross sections for CI2O, OCIO and HNO3. In addition, first estimates of the double ionization energy of N2O5, HNO3, OCIO and CI2O and the development of the first models of their decay dynamics have also been made.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The properties of molecular ions
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Atmospheric molecules
URI: https://discovery.ucl.ac.uk/id/eprint/10097624
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