Electron impact ionisation of molecular clusters and spatially oriented molecules
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A pulsed supersonic molecular beam apparatus has been used to determine the appearance energies of the cluster ions (CO₂)n+ (2≤n≤4), (N₂O)n+ (2≤n≤4), (NH₃)nNH⁺₄ (0≤n≤7) and the cluster ion fragments (N₂O.O)⁺ and (N₂O.NO)⁺ by electron impact ionisation. The measured appearance energies were used to estimate cluster ion binding energies and to deduce possible mechanisms for the formation of the cluster fragment ions (N₂O.O)⁺ and (N₂O.NO)⁺. A discussion is given on the transformation of the molecular beam machine to accommodate the study of electron impact ionisation of spatially oriented molecules. Characterisation of the inhomogeneous electrostatic hexapole field is described. Electron impact ionisation asymmetry results were measured for CH₃Cl, CH₃Br, CF₃Br and CHCl₃ using a quadrupole mass spectrometer and mass insensitive rotatable particle multiplier. Measurement of the asymmetry effect for formation of the molecular ions CH₃Cr, CH₃Br⁺ and the fragment ions CH₃+ from CH₃Cl and CH₃Br, CF₃⁺ from CF₃Br and CHCl₂+ from CHCl₃ are determined and discussed. Each molecule showed a preference for electron impact to occur at the positive end of the molecule. The effect of hexapole voltage and electron energy on the asymmetry effect is also discussed. A simple model is presented for electron impact ionisation at either end of a molecule which accounts qualitatively with the asymmetry results determined.