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Positronium is the lightest known atom, consisting of an electron and its antiparticle the positron. Because of its light mass (comparable with that of the electron and positron, rather than conventional atoms), recoil effects are expected to play an important role in its scattering from atomic and molecular targets. Up until recently positronium beam experiments have been restricted to total cross-section measurements from simple target atoms and molecules i.e. molecular hydrogen, molecular nitrogen, helium and argon where trends have been noted. Therefore, the aim of the project has been to look at more complex targets to supplement existing data and to investigate further positronium total cross-section measurements for molecular oxygen and xenon started by Gamer et al (1998) and Leslie (PhD thesis, 2005), as well as to compare with corresponding data for other projectiles and theoretical determinations. This has been achieved by looking at neon, xenon, molecular oxygen and water. The total cross-section for positron-water has also been measured in order to investigate the general performance of the system in the presence of water vapour. Also, as an extension of the fragmentation study by Armitage et al (2002) and Leslie (2005), the yields of residual electrons and positrons from positronium-xenon impact at an energy of 30eV have been examined to ascertain contributions from target ionization. Both the integrated cross-section and longitudinal energy distributions have been measured. Finally, due to the lack of data for water interacting with positrons and positronium, and to complete the work of Arcidiacono (PhD thesis, 2006), both doubly differential ionization cross-sections of water molecules by 100 and 153eV positron impact and total cross-sections of water molecules for positronium impact have been measured.

This thesis presents an evaluation of the model potential and pseudopotential methods for the calculation of atom-atom interaction curves. At University College London, Peach has computed interatomic potential curves for several alkali-rare gas pairs using both the model potential and pseudopotential methods in a completely self-consistent manner. The value of these calculations is that any differences between the interatomic potential curves obtained from the model potential and pseudopotential methods can be attributed unambiguously to the theoretical representations of the Pauli exclusion principle which differentiate the two methods. The work of Peach thus allows the utility of these two representations to be compared directly and their effect on the quality of the resulting interatomic potentials to be assessed. The present study has developed a suite of computer programs designed to calculate observable quantities which are sensitive to the interatomic potentials. Calculations of the profiles, widths and shifts of the alkali resonance line when perturbed by rare gases, together with cross sections for fine structure state changing transitions within the alkali excited state multiplet, are reported and comparisons are made with experimental measurements of these observables. It is demonstrated that the theoretical representation of the Pauli exclusion principle has a significant effect on the quality of the resulting interatomic potentials. Observables calculated on the basis of interatomic potentials obtained via the model potential method are in better agreement with the available experimental data than those calculated on the basis of pseudopotential interaction curves. From this it may be inferred that the interatomic potentials obtained by the model potential method represent the actual interaction between alkali and rare gas atoms more accurately than do interatomic potentials obtained by the pseudopotential method. The results of this study provide strong evidence for the superiority of the model potential method for the calculation of interatomic potentials.

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