In this thesis, we present Kohn variational calculations of scattering and annihilation parameters for very low energy interactions of positrons with molecular hydrogen. Our analysis includes the first application of the Kohn method for this system in which the interelectronic potential in the molecular target is treated explicitly. All previous Kohn calculations on positron hydrogen molecule scattering have avoided this complication by the use of the method of models. The advantage of the explicit treatment over the method of models is that it allows approximate target wavefunctions of a very high accuracy to be admitted more easily to the Kohn calculations. We find that the accuracy of the approximate target wavefunction is an extremely important factor in obtaining reliable results from the calculations. We carry out an extensive investigation of anomalous, nonphysical behaviour in the results of our Kohn calculations. Our explanations of how these anomalies arise and how they may be avoided significantly improves upon the discussions of these phenomena given in earlier accounts of positron hydrogen molecule scattering calculations by other authors. As with all previous models of positron hydrogen molecule scattering, we find discrepancies between the experimental value of the annihilation parameter, Z effective, and the theoretical value of this quantity as determined from our Kohn calculations. Limitations of the model that could explain these discrepancies are discussed and suggestions for future improvements are proposed.

The Acoustic Paramagnetic Resonance (APR) of the chromous ion in two cubic host lattices, MgO and CaO, has been investigated for phonons of frequency around 9.5 GHz and temperatures in the range 1.51 K to 4.2 K. An iron-cored electromagnet produced magnetic fields continuously variable from 0 to 2 Tesla, the direction of magnetic field could be varied through 90 degree in the same plane as the phonon direction. Experiments were performed in which a variable uniaxial stress could be applied to a crystal of Cr2+ in MgO, giving information about the effects of local lattice strain on the APR spectrum. A change in the g-value of one resonance line with applied stress was observed, and, from this, the value of the strain coupling constant could be obtained. The experimental results enabled values for the parameters 3r (Jahn-Teller tunnelling splitting) and K (spin-orbit splitting) to be obtained, which were compared with those obtained by other methods. Further experiments were performed on Cr2+ in CaO in which the direction of the applied magnetic field could be changed by rotating the sample crystal about the phonon direction, enabling the field to be aligned with the crystal <111> direction. The theoretical prediction (Fletcher (1971)) that strain broadening of the resonance lines should be reduced under these conditions was demonstrated. Experiments were performed at two separate frequencies and two separate temperatures, and the results used as data for a computer programme which gave the values of the various parameters of the system, and thus allowed a low-lying energy level diagram to be predicted. There were similarities between this diagram and the accepted energy-level diagram of Cr2+ in MgO, as would be expected. The differences were attributed to die difference in the lattice constants of the host materials.

This thesis is concerned with the study of the excitation of Hydrogen atoms to the n = 3 states from the ground state by electrons with incident energies ranging from just above the ionization threshold to energies where the first Born approximation is expected to be valid. The major physical effects in this region are exchange, the distortion of the wave describing the external electron, and the distortion of the atomic system. A model which includes these effects - the Distorted Wave Polarized Orbital (DWPO) approximation - is generalized for any excitation and used to investigate the excitation process for n = 3 in particular. Total (integrated) and differential cross sections, not previously calculated using this model, are presented and compared, where possible, with other theoretical and experimental work. Other sensitive indicators of the effects of the model are considered. These include the polarization of Balmer-a (Ha) radiation and the parameters which describe the orientation and alignment of the atomic system after collision and the coincidence rate for the observation of emitted photons with the ejected electrons (Fano-Macek and Macek-Jaecks parameters). Also studied is the asymmetry in the observed intensity of Ha radiation arising on sign reversal of an applied electric field along the interaction direction. There is a serious disparity between the results in this model or the Born approximation and the experimental observations. A number of reasons for this are discussed. The work here indicates a need for further theoretical and experimental study but that high levels of sensitivity are required in any experimental work particularly with regard to the polarization and asymmetry measurements. Addition any, this work illustrates a very serious failure in the DWPO model caused by the use of the adiabatic polarization potential rather than an energy dependent potential especially at higher energies in the 3d excitation where we found that for the total cross section, the results obtained by including full allowance for polarizationlie a factor of about 2.5 below the Bom result at 200 eV and do not approach the Born cross section even for impact energies measuredin keV. The most useful line of future research is expected to be the allowance for coupling to adjacent states by the unitarizaticn method and some preliminary work for this is included.

This thesis is concerned with a study of inelastic electron-atom collisions with the incident energy ranging from just above the first ionization threshold to some energy where the First Born approximation becomes valid. The main physical effects which need to be included in the theoretical treatment of such collisions are electron exchange and distortion of both the atomic system and the wave describing the external electron. A method which takes account of these effects, to be referred to as the Distorted Wave Polarized Orbital (DWPO) approximation, is described. Three models based on this approximation are developed and applied to electron collisions with the light atoms hydrogen and helium. In particular the models are applied to the following collision processes: e + H(ls) &rarr; e + H(2s, 2p, 2s + 2p); e + He(11S) &rarr; e+ He(n1,3L), n = 2,3,4,5, L= S,P. Results are presented for the total (integral)and differential cross sections and also, where appropriate, for the parameters describing the orientation and alignment properties of the excited atom. The results are compared with those of other theoretical methods and with experimental measurements.

This thesis presents studies on the adsorption of chlorobenzene using STS, non-local desorption of chlorobenzene and the atomic manipulation of the PCB molecule using STM. The atomic manipulation of the mucin molecule on HOPG surfaces under different conditions has also been investigated. Chlorobenzene adsorbate on the Si(111)-(7×7) surface has been investigated using STS. The missing rest atom state at -0.8 V confirmed the rest atom involvement in the bonding geometry. Two adsorbate states located at -1.3 V and +1 to +2 V have been identified. The effect of the surface step and the temperature on the non-local chlorobenzene desorption process has been investigated. Different reactions generated by STM of the PCB molecule on the Si(111)-(7×7) surface have been studied. While molecular desorption is maximized by electron injection into the chemisorbed molecular ring at low voltage, injection into the physisorbed molecular ring at high voltage favours the reconfiguration of the bonding. The mucin molecule has been studied by AFM and STM. An unraveling manipulation has been achieved over a folded mucin polymer on the bare HOPG surface. Enhanced mucin-substrate binding has also been achieved in the liquid state on the size-selected Au55 cluster-decorated HOPG surface.

This thesis presents two studies into molecular manipulation using scanning tunnelling microscopy (STM), dissociation of chrolobenzene (PhCl) and of 4,4´ dichlorobiphenyl (PCB) manipulation both on Si(111)-7×7 surface. An ab initio investigation is presented on a possible candidate for future atomic manipulation. The dissociation of PhCl process on the Si(111)-7x7 surface was studied using an STM. STM induced dissociation was found to be temperature dependent and requires 1.4 \(\pm\) 0.1 electrons per C-Cl bond breaking event. Based on work and previous work, we suggest this is a mixture of a two-electron temperature independent and one-electron temperature dependent processes. The activation barrier for the thermally assisted one-electron dissociation was measured to be 0.8 \(\pm\) 0.2 eV. This appears to reflect the measured energy barrier of diffusion of 0.84 \(\pm\) 0.08 eV, suggesting that the thermally promoted dissociation process proceeds via a precursor physisorbed state. Electron injection from STM was used to induce atomic manipulation of PCB on Si(111)- 7×7 surface. Four types of manipulation outcomes were observed; desorption, double dark feature, single dark feature and the generation of a bistable adsorbed switching. The PCB bistable switching was voltage independent, but injection tip-site dependent. This suggests that the switching process is driven by thermal excitation, but that the tip-adsorbate interactions play a role. The computational study of naphthalene 1,8-disulfide (NDS) in gas phase was performed by the density functional theory calculations using molecular orbitals. The S-S bond lengths of NDS\(^2\)\(^-\), NDS\(^-\), NDS, NDS\(^+\) and NDS\(^2\)\(^+\) were predicted to be 3.46 Å, 2.74 Å, 2.13 Å, 2.09 Å and 2.06 Å. Therefore the disulfide bond is a strong candidate to undergo vibrational excitation due to respectively short lived ionic molecules created by charge injection from an STM tip.

Two separate experimental techniques have been used to investigate the fundamental properties of small polyatomic molecules in the gas phase. Selected ion flow tube mass spectrometry has been used to study the reactions of cations and anions with ethene, monofluoroethene, 1,1-difluoroethene, trifluoroethene and tetrafluoroethene. Calculated collisional reaction rate coefficients are compared to those measured by the experiment. The product ions from these reactions have been detected and their branching ratios measured. Many of these results have been explained using arrow-pushing mechanisms. Using tunable vacuum-ultraviolet radiation from a synchrotron, negative ions have been detected following photoexcitation of 24 gaseous molecules. The majority of the molecules studied are halogen-substituted methanes. Product anions resulting from unimolecular ion-pair dissociation reactions were detected, and their ion yields recorded in the range 8-35 eV. Absolute cross sections for ion-pair formation and resulting quantum yields are calculated. This vast collection of data is summarised and ion-pair formation from polyatomic molecules is reviewed.

This thesis deals with grey soliton dynamics in confining potentials and their association with the trapped Lieb II mode. There has been a large body of work into the study of grey solitons and their dissipation in ultra cold trapped Bose gases. It is well known at finite temperature that the standard mechanism for soliton decay in uniform ultra cold gases is scattering of thermal phonons, where the soliton lifetime was found to be proportional to inverse temperature for temperatures greater than the chemical potential [54]. The scattering of thermal phonons becomes no longer efficient for the exactly integrable Gross-Pitaevski equation. The case of non-integrable interactions has been considered and for this case the lifetime of the soliton was found to diverge as the inverse fourth power of temperature T[25]. Thus solitons propagating in a uniform background are found to be stable at zero temperature. In the presence of a trapping potential, momentum conservation is lost due to the loss of translational invariance and this leads to soliton decay even at zero temperature. Classically it has been shown that as long as the confinement is slowly varying on the length scale of the soliton then conservative dynamics are found [10], [41]. To extend previous findings we use the Born-Sommerfeld rule to quantise the grey soliton in any smooth potential with one minimum. We find a ladder of energy levels for the case of harmonic confinement and associate these states with the trapped Lieb 11 mode. These trapped Lieb 11 states are not eigenstates but are to be considered as quasiparticle resonances. The finite lifetime is due to the radiation of phonons by an accelerating grey soliton, we are able to calculate this lifetime in the semiclassical limit. We show that for a large number of trapped atoms that the quasiparticle states are long lived and can be resolved experimentally.

High resolution crossed-beam laser spectroscopy has been used to measure the isotope shifts and hyperfine parameters of nineteen transitions in atomic ruthenium. These results have been used in conjunction with four other existing transition measurements to determine accurate values for the change in mean-square charge radius between the isotopes of ruthenium. The new charge radii measurements exhibit up to an order of magnitude improvement in accuracy compared to the previously published results. These accurate charge radii systematics in ruthenium provide additional data for the interesting N=60 region of the nuclear chart. The transitions measured have been assessed in terms of their suitability for use in future collinear laser spectroscopy measurements of radioactive ruthenium isotopes. One transition in particular, the 349.8942nm Ocm-1 to 28571.890cm-1 transition, has the potential to be highly efficient.

STM measurements of large size selected clusters of Pd on HOPG show that the aspect ratio increases with increasing cluster size. The non-spherical nature of these clusters is traced back to the aggregation regime in the gas condensation cluster source. The size and temperature of the clusters create a slow coalescence process that leads to non-spherical clusters. The molecule methylphenyldisulphide is studied on Si(111)-7x7 by STM. The molecule is found to bond either as an intact molecule or as fragments after fission of the disulphide bond upon landing. The molecule exhibits a 2:1 preference for the middle adatom of the 7x7 unit cell suggesting it enters a precursor state before adsorbing on the surface. The new SPEAR technique uses an STM tip to collect photoelectrons generated at a surface. The detection of a photocurrent with a W tip is demonstrated. In the case of a bare HOPG surface electronic states of the tip are resolved. However, the large signal generated by photoelectrons from the tip restricts the resolution of states of the surface. To resolve spectra on the nanometre scale a coaxial tip is required, this is demonstrated through the use of simulations.