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The work contained within this thesis is concerned with the interactions of intense ultrashort laser fields with atoms and diatomic molecules and associated phenomena. The study of 2-colour 2-photon photoelectron spectroscopy on a range of noble gases was performed at the FLASH facility, Hamburg. This experiment sought to cross a gaS jet with temporally and spatially overlapped laser pulses from the FEL and an optical laser to observe sidebands to photoelectron flight time peaks measured using a magnetic bottle mass spectrometry device. Sidebands were observed for a Helium target, the intensity of which varied with scanning the te~poral overlap and thus yielded a temporal profile of the two overlapped beams. As well as observing sidebands, the experiments at FLASH were able to yield details of the spectral composition of the FEL beam revealing evidence for 2nd and 3rd harmonics of the fundamental frequency contained within the beam. At Queens University Belfast a Linear Electrostatic Ion Beam Trap (LEIDT) has been built, tested and used to store a range ofboth positive and negative ions with low keY energies for up to 10 seconds. Using both an on-axis channeltron and an electrostatic pick-up ring, the time evolution of injected ion bunches has been studied. By applying. a train of AC pulses to the trapped ions, it has been shown that bunching can be enhanced to longer times. The LEIDT has also been used to store molecular ions of H/, HD+ and D/. At ASTRA, RAL, the stored ion bunches were interacted with a laser to obtain time of flights as a function of storage time. From these time of flights, cooling of HD+ to the ground vibrational state has been observed and from interaction of this ground state with the laser, evidence of Above Threshold Dissociation has been obtained.

Collisions of positronium (Ps) with the noble gases helium, neon, argon, krypton and xenon are considered in a two stage approach. Firstly, 'target elastic' collisions, where the noble gas target does not change state, are dealt with within the Impulse approximation. Secondly, collisions where the atom does change state, 'target inelastic' collisions, are considered within the first Born approximation. In both cases it is the spectra of ejected positrons and electrons that is the focus. Finally, isomorphism of the dipositronium (Ps2 ) symmetry group with the point symmetry group D2d is shown. Projection operators are obtained for collisional calculations of Ps with Ps.

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