| Summary: | This thesis concerns the study of electronic excitation in ion/atom-molecule collisions. An extensive review of the subject is given first. At high energies, the quasidiatomic correlation diagram, that is so useful in the interpretation of atomic inelastic collisions, can also be applied in the case of molecular collisions. The model breaks down at small impact parameters where orientation of the molecule begins to play a role, and diabatic potential energy surfaces must be calculated instead. Recent developments in this area are reviewed. At lower energies, a host of new theoretical techniques can now be used, notably the infinite order sudden approximation, and time-dependent semiclassical methods. With these recent theoretical developments, it is concluded that the experimentalist must increasingly turn to the coincidence technique to probe the orientation of molecules during collisions, and to supply state-specific data. Such an experiment on the K + CH<SUB>3</SUB>I collision system is described next. In these collisions, CH<SUB>3</SUB>I is excited to Rydberg levels and subsequently ionises and fragments. The process is known to onset at low scattering angles (500 eV<SUP>o</SUP>), and the aim here was to observe these fragment ions in coincidence with the scattered atoms. No coincidence signal was observed, from which an upper estimate of the cross section ratio igma<SUB>i</SUB>(θ)/igma_itot could be set. Previous workers have interpreted the excitation mechanism as involving excited ionic intermediate surfaces, and a discussion here using a correlation diagram confirms this view. A comparison with the analogous atomic systems suggest an alternative interpretation, in which the collision is viewed as a scattering of the potassium valence electron off the molecule. In the final part of the thesis, work on an apparatus designed specifically for photon-ion coincidence measurements is described. The apparatus features a multi-angle particle detector, that will allow 45 angular measurements to be made simultaneously. Here the performance of the apparatus is critically assessed, and suggestions are made for improvement. Two pulsing devices are described. The first generate bunching fields for pulsing the ion beam, and will allow us to perform time-of-flight measurements. The second is positioned after the collision zone, and is designed to deflect away the elastically scattered ions that would otherwise contribute to noise in the coincidence experiment.
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