Douglas, K.M.; (2012) The formation, dissociation and reactivity of gaseous ions. Doctoral thesis , UCL (University College London).

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Abstract

This thesis presents the results of a series of experiential investigations into the formation, dissociation and reactivity of gaseous ions. Firstly, using a time-of-flight mass spectrometer coupled with a 2D ion coincidence technique, studies of the electron ionization of a number of small gas-phase molecules are presented. Relative partial ionization cross-sections (PICS) are derived for the formation of positively charged fragment ions, following electron ionization of H2S, CH3OH and CF3I. The 2D ion coincidence technique enables fragment ions formed by dissociative single, double, triple and quadruple ionization to be distinguished and quantified. This information also allows precursor specific relative PICS to be determined. While the relative PICS quantify the overall yield of each fragment ion, the precursor specific relative PICS quantify the contribution from single, double, triple and quadruple ionization to the relative yields of each fragment ion. Such information is essential for the accurate modelling, and the understanding, of the chemical processes occurring in energetic environments, such as industrial plasmas and planetary atmospheres. Comparison of the relative PICS data to existing measurements of the PICS for these molecules generally shows good agreement for experiments in which the efficient collection of translationally energetic ions is demonstrated. In addition, information on the energetics and dissociation dynamics involved in the fragmentation of H2S2+, CH3OH2+, CF3I2+ and CF3I3+ are provided by interpretation of ion pair peaks recorded in the 2D ion coincidence spectra. Secondly, this thesis also presents the results of an investigation into the photoionization of CF3I, using the threshold photoelectron-photoion-photoion coincidence (iPEPICO) endstation on the vacuum-ultraviolet beamline at the Swiss Light Source. These experiments were part of a scoping study to see if this existing apparatus could be used to study multiple ionization. The photoionization spectra are interpreted and discussed, and issues with the current experimental arrangement, which may be improved for future visits, are addressed. Finally, studies of I2+ collisions with OCS, carried out using a crossed ion beam experiment with a time-of-flight mass spectrometer, are presented. Two bond-forming reactions producing IO+ and IS+ are observed, together with the more ubiquitous electron transfer reactions. These electron transfer reactions are rationalised using the Reaction Window model.

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