No description available.

The main objective of this work was to develop a predictive model for nanofiltration (NF) membrane processes. This was accomplished by development of a model which describes the transport of electrolytes in NF membranes in terms of three mechanisms: diffusion, convection and electromigration. The model includes the description of concentration polarisation for binary and more complex mixtures of charged electrolytes. The application and utility of the model were studied by identifying the key characteristics of NF membranes, modelling of a selected process, validation using experimental data and finally using the model for prediction including process optimisation. For one membrane (PES5), atomic force microscopy (AFM) showed that the membrane consisted of discrete pores of nanometre dimensions. Characterisation of the membrane using salts and uncharged solutes showed that it is more appropriate to model the membrane as porous rather than homogenous. The membrane was characterised in terms of the structural parameters: the effective pore radius, <I>r<SUB>p</SUB></I><SUB>, </SUB>and the effective ratio of thickness over porosity, <I>Δx/A<SUB>k</SUB></I>, and an electrical parameter: the effective charge density, <I>X<SUB>d</SUB>. </I>Such characterisation for a further membrane (CA30) was found to be very useful in predicting the process of diafiltration of dye/salt solutions. The prediction required that X<SUB>d</SUB> varied as the salt concentration decreased during processing. The model was then used to predict the processing conditions for the whole diafiltration process which includes the pre/post-concentration phases and the diafiltration phase. Finally, a simplified characterisation method was proposed whereby the membranes that are available in the market are characterised using the information given by the membrane manufacturers. Using the range of parameters obtained, analysis of the model showed that the contributions of all three transport mechanisms are very significant and should not be neglected in any predictive models.

No description available.

In this work an attempt has been made to develop a computer program package that can be used to simulate/model spectra and other properties of the majority of diatomic molecules. Specifically, the program package aims at: <I>(i)</I> providing a general tool for the interpretation of experimental spectra and for pointing out any perturbation that may exist in them; and <I>(ii)</I> to help researchers in setting up their experiment and know in advance optimum conditions to perform it. The program package under development at present includes the following features: 1. Generation of molecular potentials; 2. Calculation of Frank-Condon factors and transition probabilities; 3. Simulation of spectra in adsorption, emission, laser-induced fluorescence and resonance ionisation; 4. The inclusion of thermal and non-thermal level population distribution; 5. In case of perturbations, the ability to read the energies of the levels as input parameters, rather than calculating them for inadequate formulas based on standard spectroscopic constants. The program package was tested for a few diatomic molecules, selected specifically to be representative for typical cases in laser absorption and emission spectroscopy.

The major objective of these investigations was to develop a high-resolution double-charge-transfer spectrometer in order to measure vertical double-ionization energies of molecules, and to develop a thoeretical method to predict these energies economically. An overview of experimental techniques that have been used for the measurement of double-ionization energies is given in chapter 1; their advantages and restrictions are discussed and a brief review of double-charge-transfer spectroscopy studies to date is made. The various theoretical methods that are available for the studies of these processes are discussed in chapter 2, and a semi-empirical method based on the MSXα method is developed and tested for its effectiveness in predicting the complex singlet- and triplet-state energy distributions that exist in molecular dications; NH_3 and NO_2 were chosen for this purpose. Chapter 3 then describes in more detail the double-charge-transfer experiment, the spectrometers used in the studies presented in this thesis, and the modifications to that equipment. The next three chapters are devoted to the studies of double ionization of a wide variety of molecular dications. In chapter 4, theoretical predictions and interpretations of previous experimental results are presented; double-ionization energies of water, chloromethanes, iodomethanes and three metal hexacarbonyls (Cr, Mo, W) are predicted. This is followed in chapter 5 by joint theoretical and experimental studies initially of the triplet-state energies of ethane, allene and 1,1-dimethyl allene with the MS9 spectrometer, and then of ethyne, propyne and various alkyl-substituted amines with the high-resolution Finnigan 8230 spectrometer. The results are interpreted using the above semi-empirical theoretical technique. Finally, the possibility of studying singlet-state energies at high resolution is investigated in chapter 6, with joint experimental measurements and theoretical predictions of singlet-state energies of ethene, ethyne, propyne and allene being presented.

No description available.

A theory of double-charge-transfer processes in ion-atom collisions has been developed. The collision energies of interest, in the range 3-10 keV permit the use of semi-classical impact parameter formalism within which Landau-Zener theory has been adapted to describe the coupling that arises indirectly from 2 virtual 1-electron transitions, necessary because the direct 2-electron coupling amplitudes are insignificant in comparison. A consistent treatment of adiabatic and diabatic potential energy curves and techniques needed to locate crossings of the latter type are given. Application to scattering of OH<SUP>+</SUP> and F<SUP>+</SUP> by inert gas atoms require a systematic treatment of non spherical <I>p</I> atomic orbitals in the theory. Explicit rotations from space-fixed to body-fixed co-ordinates at each point in the scattering trajectory revealed that the theory can predict effects, which in theories involving only <I>s</I> orbitals require a rotational coupling operator, in the absence of such coupling. Cross sections for various double-charge-transfer scattering processes are calculated, employing established model 1-electron transfer scattering amplitudes and reproduced well the experimentally observed "reaction windows" of projectile energy loss within which the cross sections are significant. General kinematic factors, rather than the inclusion of a systematic treatment of the non-spherical <I>p</I> orbitals involved in the reactions studied, are primarily responsible for the form of the reaction window.

The two major objectives of these investigations were: i) to measure vertical double-ionization energies of molecules using a high-resolution double-charge-transfer spectrometer and, in order to interpret the experimental data, to calculate these energies using theoretical methods; ii) to develop a new experimental technique in order to measure vertical triple-ionization energies of molecules. The experimental techniques for the measurement of double- and triple-ionization energies are defined and discussed in Chapter 1 together with a review of double-charge-transfer studies to date. Two theoretical methods were applied to calculate double-ionization energies, these are discussed in Chapter 2. Double-ionization energies to ground states of dications were calculated using <I>ab initio</I> methods and double-ionization energies to ground and excited states were calculated using a semi-empirical multiple-scattering X-alpha method. Chapter 3 describes the principles of double-charge-transfer spectrometry, the double-charge-transfer experiment and the modified Finnigan 8230 mass spectrometer. The next three chapters are devoted to the studies of double ionization and a variety of molecular dications. Chapter 4 presents theoretical predictions of double-ionization energies of eleven fluorobenzene molecules and five alkyne molecules in order to interpret the experimental data of others. Chapter 5 presents high-resolution experimental studies of ethyne and ethene, and Chapter 6 presents a joint theoretical and experimental study of the double-ionization energies of six amine molecules. The final chapter in this thesis (Chapter 7) is devoted to the study of triple-ionization energies of molecules. A new experimental technique, given the name triple-charge-transfer spectrometry, was developed. The Finnigan 8230 spectrometer was adapted for triple-charge-transfer and the experiment was tested for effectiveness using krypton. Following favourable results for krypton, triple-charge-transfer studies of carbon disulphide, allene and ethene were presented. In order to interpret the spectra in terms of electronic transitions, the semi-empirical multiple-scattering X-alpha theoretical method was developed to predict triple-ionization energies.

No description available.

The research described in this thesis was concerned with the experimental determination of total scattering cross-sections, Qt, for the collision of low-energy positrons with noble gas atoms, viz: helium, neon and argon. To ensure the highest accuracy in measurement, considerable effort was directed into the development and generation of intense low-energy positron beams, the effective discrimination against forward scattering and the elimination of all systematic experimental errors. Only since 1969 have successful methods of producing monoenergetic positron beams of low-energies been reported. Since then considerable theoretical and experimental interest in low-energy positron scattering has grown, particularly in the noble gases, and is reviewed in Chapter II. In the present experiment approximately 1 in 106 positrons from a 3 mCi source of 22Na were moderated to near thermal energies when they impinged on an assembly of gold vanes coated with magnesium oxide. The moderated positrons were then accelerated and momentum selected in a 180° magnetic spectrometer, which also acted as the scattering chamber. After momentum selection, the positrons annihilated at a target. The coincident gamma-ray count rate was proportional to the transmitted beam intensity. The reduction in count rate as a function of gas pressure was measured, from which Qt could be obtained. Values of Qt, for positron-helium scattering, were obtained using positrons having energies in the range 13 to 1000 eV and it was demonstrated in initial measurements that cross-sections determined at higher energies (> 400 eV) were significantly lower than values calculated using the Born approximation. Earlier theoretical work had predicted that the Born values were likely to be correct in helium at approximately 400 eV and above. To overcome this discrepancy, which was due to positrons scattered in the forward direction reaching the target, two changes were necessary: a) to position a retarding-field spectrometer in the front of the target to prevent the transmission of inelastically scattered positrons; b) to use narrower resolving slits. These resulted in a progressive increase in values of Q for increasing energies above 200 eV. The final set of results are in good agreement with recent theory and experimentally demonstrated for the first time that the Born approximation is valid in helium above 500 eV. Examination of these results shows them to be consistent with the "sum rule" based on a forward dispersion relation. After the study with helium, neon and argon were studied using positrons having energies in the ranges 10 to 1000 eV and 200 to 1000 eV respectively. The overall impression gained in the study of these gases is that the discrepancy between theory and experiment above 200 eV is now not greater than 10%. Evaluation of the "sum rule" using the neon data shows that the rule again holds.