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This work investigates interacting states in polyatomic molecules, concentrating on two examples, the analysis of interacting states in the linear C3 radical and the study of perturbations resulting from interacting states in Formaldehyde. Work on the C₃ radical started with ab initio potential energy surface calculations to outline the lowest 8 singlet electronic states, followed by vibrational energy level calculations corresponding to these electronic states. On the experimental front, the vibronic states of the radical were probed by laser spectroscopy, particularly by double resonance techniques.

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Investigations of the kinetics and dynamics of elementary gas-phase collisional processes and reactions have proved to be of major significance in many areas of both pure and applied physics and chemistry. A thorough understanding, both experimental and theoretical, of the collisional processes of defined quantum states of atoms and molecules is required. This includes physical processes such as energy transfer, collisional quenching, diffusion, charge transfer and ionisation, and chemical processes such as reaction rates, mechanism branching ratios and reaction potential energy surfaces. These topics have long been important in a variety of systems, including atmospheric chemistry, gaseous plasmas, combustion science and developing novel chemical laser systems. The primary motivation for the present work is to extend our knowledge about the collisional behaviour and reaction kinetics of alkaline-earth atoms in specific electronic states. The first part of this thesis (chapters 1-5) is directed towards a kinetic investigation of ground state atoms, in particular, Ca[4s<SUP>2</SUP>(<SUP>1</SUP>S<SUB>0</SUB>)], generated by flash photolysis of gaseous iodide molecules and whose decay is monitored by time-resolved atomic resonance absorption spectroscopy in the 'single-shot mode'. A new body of data for the absolute rate constants for the F, Cl, Br and I-atom abstraction reactions of Ca[4s<SUP>2</SUP>(<SUP>1</SUP>S<SUB>0</SUB>)] with a range of halide molecules are reported. The diffusional behaviour of this atom in noble gases, He, Kr and Xe and also N<SUB>2</SUB>, is also investigated in this work and the diffusion coefficients are consequently presented here. It is envisaged in broader terms that these studies have not only provided some new kinetic data for use in related areas but also contribute to form a framework for theoretical considerations and comparisons between the ground state and excited state reactions involving this atom. It is also considered that this type of research can be helpful in the area of exploring the relationship between reaction rate constants and the atomic or molecular electronic structures. In this work, the influence of molecular structure on the reaction rate when increasing the number of carbon atoms in the alkyl-halide chains is also investigated. Some measurements of the absorption cross-sections of gaseous MI<SUB>2</SUB> (M = Ca, Sr, Ba) molecules by UV radiation as a function of wavelength are also made in this work but are presented in the appendices of this thesis.

A range of tunnelling phenomena were observed in clean Nb-Si interfaces. In the most non-transmissive Nb-Si interfaces, the Schottky barrier acted as an insulating tunnel barrier. Thus, electrical characteristics similar to those obtained from normal-insulator-superconductor junctions were observed with a conductance dip about the Nb superconducting energy gap. As the transmission through the interface was increased, analysis of differential resistance bias dependence measurements indicated that only a small percentage of the contact contributed to conduction. This was attributed to the formation of a highly non-uniform Schottky barrier when the depletion width is comparable with the inter-dopant spacing. Evidence of constructive quantum interference between electrons and Andreev reflected holes was observed and its insensitivity to an applied magnetic indicated that it was caused by the presence of a disordered elastic scattering region adjacent to the Nb-Si interface. Subsequent enhancement in the transmissivity of the junctions indicated that Andreev reflection induced weak localization was occurring as predicted by theory. The fundamental limit in Nb-Si barrier transmissivity was found to be caused by the Fermi velocity mismatch between the Nb and the Si. When Nb<SUB>3</SUB>Si particles were present at the Nb-Si interface, a dramatic enhancement in conductance was observed when the temperature was lowered past the Nb<SUB>3</SUB>Si superconducting critical temperature. The presence of Nb<SUB>3</SUB>Si particles reduced the tunnelling path length through the Schottky barrier and opened new conduction channels. As the temperature was lowered further, the normal coherence length exceeded the Nb-Nb<SUB>3</SUB>Si separation distance so that interfacial Josephson junctions were formed. The critical current of these Josephson junctions could be measured directly by applying a large d.c. voltage bias across the interface. The critical current of the Josephson junctions was quantized as a function of temperature because the junction cross-sectional area increased with decreasing temperature. This phenomenon was successfully modelled by assuming that superconductivity was suppressed at the edge of a Nb<SUB>3</SUB>Si particle over a temperature dependent coherence length and that the Josephson junction cross-sectional width is equivalent to the width of the Nb<SUB>3</SUB>Si particle. This provides a temperature dependent expression for the number of one-dimensional quantized modes passing through the Josephson junction which is in good agreement with the experimentally observed critical current dependence.

This thesis is concerned with the computational simulation of weakly bound molecules. Two general methods of producing potential energy surfaces, Neural Networks and Gaussian processes, are described and evaluated. The Neural Network method is used to generate potential functions for HF-HCI, H<SUB>2</SUB>-HF and H<SUB>2</SUB>-HCI from good quality <I>ab initio </I>data. These surfaces are used by the diffusion Monte Carlo algorithm to solve the vibrational Schrödinger equation for the ground state of the above clusters. In addition, the DMC method is used in a size selective study of the N<SUP>+</SUP><SUB>2</SUB>-He<I><SUB>n</SUB></I> system. Good agreement is obtained with previous theoretical calculations and with the small amount of experimental data available and it is hoped that the predictions made will aid future studies of these clusters. The combined <I>ab initio-</I>Neural-DMC approach is shown to be an efficient method of studying weakly bound molecules and as such will prove to be a useful step towards understanding the structure and bonding of these systems.

In this thesis we address the phenomenology of SUSY breaking from the superstring scale to the electroweak scale in order to produce a guide map for discovery at the LHC. From the top-down approach we study SUSY breaking at low-energies from a well defined superstring theory scenario and extract its phenomenological implications. From the bottom-up approach (independent of string theory), using Bayesian inference techniques, we make the first complete model-independent scan of the low energy phenomenological MSSM (pMSSM) parameter space. We perform the first statistically convergent pMSSM global fit to all current collider and DM data. We demonstrate the application of Bayesian model selection methodology to high energy SUSY breaking models with modest number of free parameters. We found that SUSY breaking from superstring theory should eventually help test its reality and that direct data is required in order to assign unambiguous preferences between different high energy SUSY breaking models. We predict that the Higgs boson mass lies between 117 GeV and 128 GeV at 95% confidence level. We believe this is a robust prediction that should be confirmed once SUSY is discovered at the LHC. Our pMSSM parameters fit provides an appropriate arena for the LHC studies of the MSSM which we wish to pursue further in future work.

Cavity polaritons are particles which are formed from photons confined to a cavity coupled to electronic excitations such as semiconductor excitons. Since the observation of cavity polaritons in 1992, there has been considerable interest in the quantum statistical behaviour of cavity polaritons. This thesis is a theoretical study of one of the most spectacular quantum statistical behaviours, Bose condensation, for cavity polaritons. In this thesis, we investigate Bose condensation of cavity polaritons in a generic model of photons interacting with electronic excitations. The model we consider is a generalisation of the Dicke model, familiar from quantum optics. It consists of a single bosonic oscillator, describing the electromagnetic field in a cavity, interacting with a large number of two-state oscillators with a distribution of energies. These oscillators could represent, for example, excitons bound to traps in a disordered semiconductor. Bose condensation is a phenomenon associated with conserved particles in thermal equilibrium. Thus to investigate Bose condensed polaritons we study the thermodynamics of the model at a fixed number of polaritons. We do this using two techniques: a variational approach, and a more powerful path-integral technique. The latter allows us to give an essentially exact description of both the thermodynamics and the excitations of the model.

Most conventional techniques for measuring adsorption at the liquid/air interface on fast timescales rely on measuring the change in the interfacial tension as adsorption takes place. This approach works well for solvents such as water, which has a large interfacial tension, but becomes more problematic for less polar solvents such as oil. It is therefore useful to investigate other, more direct, techniques which have the potential to extend the range of systems currently accessible. In this dissertation, Second Harmonic Generation (SHG) and Ellipsometry were applied to the problem of surfactant adsorption at the liquid/air interface, on the millisecond timescale. To measure dynamics on this timescale, SHG and ellipsometry experiments were performed on surfactants adsorbing at the surface of a liquid jet. The principle behind these experiments is that on emergence from the nozzle of the jet, the surfactant surface concentration is not at its equilibrium value and will therefore equilibrate over time. By measuring the surface surfactant concentrations at different distances from the orifice it was therefore possible to obtain a measure of adsorption as a function of time for a given jet speed. In the ellipsometry experiments, the adsorption kinetics of the cationic surfactant, CTAB, were measured. Interpretation of the data was made easier by converting the coefficient of ellipticity directly to a surface concentration using a calibration curve obtained from neutron reflection data. The experimental data obtained were compared with theoretical models for diffusion limited adsoprtion and found to give good agreement. In the SHG experiments, a number of systems were studied, including Nitrophenol, Triton X-100 and the Alkyl-Anilines. For each system, the SHG signal was found to be invariant with time and possible reasons for this finding are discussed in each case.