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Two materials, magnesium oxide and magnesium aluminate spinel, have been studied using the ab initio methodology, density functional theory. In the case of MgO, energetics of a variety of point defects were considered. These defects were isolated Schottky and Frenkel defects and interstitial pairs, along with a number of Schottky defects and di-interstitials. Comparisons were made between the density functional theory results and results obtained from empirical potential simulations and these generally showed good agreement. Both methodologies predicted the first nearest neighbour Schottky defects to be the most energetically favourable of the considered Schottky defects and that the first, second, and fifth nearest neighbour di-interstitials were of similar energy and were favoured over the other di-interstitial configurations. Relaxed structures of the defects were analysed, which showed that empirical potential simulations were accurately predicting the displacements of atoms surrounding di-interstitials, but overestimated O atom displacement for Schottky defects.

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An innovative optical-optical double resonance scheme (bound-free-bound) which employs resonant continuum intermediate states has been developed. A full quantum explanation has been suggested for the development of the vibronic envelope of the final states excited through such bound-free-bound pathways with two-colour pulsed layers. This scheme has been applied to observing extended vibrational progressions of some 0<SUP>+</SUP><SUB>g</SUB> and 1<SUB>g</SUB> ion-pair states of I<SUB>2</SUB> and C1<SUB>2</SUB>, and the [<SUP>2</SUP>II<SUB>1/2</SUB>]6s;0 Rydberg state of methyl iodide (along the C-I stretching mode, <I>v</I><SUB>3</SUB>). The vibrational structures of the E0<SUP>+</SUP><SUB>g</SUB> and f0<SUP>+</SUP><SUB>g</SUB> ion-pair states of I<SUB>2</SUB> are simulated by using the resultant Franck-Condon calculation. The vibrational structure of the 0<SUP>+</SUP><SUB>g</SUB> and 1<SUB>g</SUB> ion-pair states of C1<SUB>2</SUB> also agrees with the predictions from the Franck-Condon calculations. The curtailment at <I>v</I><SUP>1</SUP> = 5 of the <I>v</I><SUB>3</SUB> vibrational progression of the methyl iodide [<SUP>2</SUP>II<SUB>1/2</SUB>]6s;0 Rydberg state has been attributed to the sudden onset of predissociation. The two-photon Hönl-London factors for (a) a well defined intermediate (<I>J<SUB>i</SUB>, M<SUB>i</SUB>),</I> (b) a coherently superposed (<I>J<SUB>i</SUB>, M<SUB>i</SUB></I>) and (c) a full set of coherent virtual states, are discussed. The simulation of the observed rotational contours of the 0<SUP>+</SUP><SUB>g</SUB> ion-pair states of I<SUB>2</SUB> and C1<SUB>2</SUB> in the bound-free-bound scheme reveals the dominant character of the intermediate electronic states. Similar deductions cannot be made for the 1<SUB>g</SUB> ion-pair states of C1<SUB>2</SUB> and the reason is discussed. The two-colour two-photon nonresonant excitation of the 0<SUP>0</SUP><SUB>0</SUB> vibronic band of the (<SUP>2</SUP><I>A"<SUB>2</SUB></I>)3p<I><SUB>z</SUB></I> Rydberg state of CD<SUB>3</SUB> has been observed and its rotational contour has also been simulated on the basis of established Hönl-London factors. The dynamics of the virtual intermediate states can be revealed by considering the observed parallel and perpendicular polarization spectra. A relative resonant enhancement of the 2<SUP>1</SUP><SUB>3</SUB> vibronic band of the (<SUP>2</SUP><I>A<SUP>11</SUP><SUB>2</SUB>)</I>4p<I><SUB>z</SUB></I> Rydberg state, in the out-of-plane mode (<I>v</I><SUB>2</SUB>), occurred when the protons were turned on and off resonances with the intermediate (<SUP>2</SUP><I>A<SUP>1</SUP><SUB>1</SUB></I>)3s state which is strongly predissocated by tunnelling.

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The work described in this thesis comprises two studies related to gas phase processes: a theoretical study of negative ion mass spectral processes at low electron energies, and an experimental investigation into the hydropyrolysis of ethylbenzene. In Part One, the MINDO/3 quantum chemical method has been used to study the fragmentation processes of open and closed shell organic anions. The principal aims of the study were threefold: to assess the accuracy of MINDO/3 for carbanions; to determine the relevance of the calculations to mass spectral processes, and to deduce the mechanisms giving rise to the spectra for some alcohols and carbonyl compounds. The following are the main conclusions drawn from the study. (1) No obvious correlations exist between the equilibrium properties of the ions and their fragmentation modes. (2) The accuracy of calculated activation energies is as good as would be expected of the species for which MINDO/3 was parameterized. (3) The charge is localized in the products such that the lowest enthalpy of reaction obtains. (4) The mass spectra arc derived from molecular ions formed initially in excited electronic states; but these states are not necessarily isolated from the ground state. (5) There is evidence from the observed spectra and the MINDO/3 results that unimolecular rate theory is applicable to dissociative resonance capture. (6) Fragmentation rate calculations corroborate the anomalous result that some simple fission processes have exceptionally high reverse activation energies. (7) The deduced mechanisms are able to account for the observed spectra, except at high energies; which implies that activation energies cease to be pertinent factors at these energies. The hydropyrolysis of ethylbenzene was investigated over the temperature range 530-581°C. Six products were identified and quantitatively determined: benzene, toluene, styrene, ethane, echene and mccyane_1 The following Arrhenius parameters were estimated: E ° 71.0 kcal mol- and A - 10 .Is-1. On the basis of the experimental results and thonmochemical data, the mechanism for this complex reaction is discussed.