Studies of gas phase electron, ion and atom collision processes

• Main Author: Harland, Peter W.
• Published: University of Edinburgh 1995
• Subjects: 539.6
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652117

The research papers submitted in this thesis describe experimental and theoretical investigations of particle collisions in which the projectiles have been electrons, ions and atoms, and the targets have been atoms and molecules. Non-reactive and reactive collisions have been studied in order to explore the fundamental nature of the collision event, to understand the dynamics, and to facilitate the determination of thermochemical parameters and reaction properties. The formation of positive and negative ions under single collision conditions as a function of electron impact energy has been investigated for small molecules and for molecular clusters. The measurement of accurate ionization efficiency curves and ionization thresholds has been achieved using custom designed near-monochromatic electron sources or analytical deconvolution. In many cases, detailed energy balancing has been attempted through the measurement of the recoil energies of fragment ions using retarding electric fields. Ionization mechanisms for associative and dissociative resonance electron capture and the formation of isomeric positive ions have been deduced. Thermochemical parameters, including electron affinities, ionization potentials, enthalpies of formation and bond dissociation energies, have been determined. Experiments in which the molecular targets were spatially oriented have shown, for the first time, that the mass spectrum and the ionization efficiency are orientation dependent. A theoretical model has been developed which accounts for the experimental measurements. Investigations of ion-molecule chemistry and non-reactive ion-molecule interactions have been carried out using a custom designed drift-tube mass spectrometer. It has been shown that isomeric ions can be distinguished by their ion transport properties and that the isomeric form of an ion-molecule reaction product ion can be directly measured. A theoretical model based on a generalised ion-helium interaction potential was developed which quantitatively accounted for the relative ion mobilities of a wide range of ions according to their physical properties.