Browsing by Author "Couling, Vincent William."

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Experimental investigation of electric-field-induced birefringence in fluids.
Mthembu, Mzungezi Cyprian.; Couling, Vincent William.
Measurements of the quadratic electro-optical (Kerr) effect of uids can provide knowledge of fundamental molecular properties, such as (hyper)polarizabilities. They also provide valuable information about intermolecular forces through the measured Kerr-effect virial coefficients. These properties play a considerable role in aspects of physics, chemistry and biology, and so are of inherent value. They also provide benchmarks against which to evaluate ab inito quantum mechanical calculations of the properties, especially for larger atoms and molecules, where the need for large basis sets and adequate account of electron correlation places onerous demands on computational resources. This thesis reports the development of an apparatus to measure the electric-field-induced birefringence (Kerr effect) in a fluid. The apparatus has been designed in an attempt to increase the precision and absolute accuracy of the measured Kerr effect, with a long-term view to obtaining precise new data for a range of molecular species. The apparatus has been fully automated, using a personal computer containing an IEEE interfacing card to communicate with a data-acquisition and control unit, which in turn controls the experiment and collects the data from which the Kerr constant can be determined. The apparatus has been used to measure the Kerr constant of gaseous helium at two different temperatures, namely 399.4 K and 445.7 K. Helium was chosen because extremely precise and accurate ab initio calculations of this two-electron system have yielded very precise knowledge of the Kerr constant (to within 0.1%), so that it provides an ideal benchmark against which to assess the performance of the apparatus. Once the Kerr apparatus is yielding precise data for helium, it will be possible to measure the Kerr effect of other gaseous species with confidence. A full Jones calculus analysis of the optical cascade is presented, providing useful insights into the best experimental procedure to be employed in the gathering of data. In addition, the molecular-tensor theory of the second Kerr-effect virial coefficient BK is reviewed. Measurements of the Kerr-effect of helium gas have yielded the second Kerr hyperpolarizability at the experimental wavelength of 632.8 nm.
Hyperpolarizability contributions to the second Kerr-effect virial coefficients of non-dipolar molecules.
(2020) Mhlongo, Mbukeni Mzwandile.; Couling, Vincent William.
The molecular theory of the second Kerr-effect virial coeffcient, BK, describing the effects of interacting pairs of molecules on the molecular Kerr constant for molecules with non-linear symmetry is reviewed, and then extended to include second hyperpolarizability contributions in the molecular interactions. The classical long-range dipole{induced-dipole model is used to describe the interactions between pairs of molecules. This investigation has been limited to non-dipolar species, where the permanent electric quadrupole moment is the leading multipole moment, since for dipolar species, the hyperpolarizability contributions will likely be masked by the generally much-larger contributions arising from the permanent electric dipole moment. The resulting expressions for contributions to BK are evaluated numerically (using Gaussian quadrature) for nitrogen (N2), carbon dioxide (CO2) and ethene (C2H4), these molecules having measured data against which to assess the theoretical predictions. N2 and CO2 are axially-symmetric molecules, while C2H4 is of lower symmetry, belonging to the D2h point group. Previous attempts to approximate the molecular properties of C2H4 to axial symmetry in calculations of BK have produced theoretical results which signifi cantly underestimate the measured data. Inclusion of the full molecular symmetry has been shown to be essential if the molecular-tensor theory is to yield reasonable agreement with experimental data. For CO2 the quadrupole{induced-dipole contribution dominates, and the interaction induced hyperpolarizability contribution to BK is only 0.3% at 200 K rising to 1.5% at 500 K. For the N2 and C2H4 molecules, the collision-induced hyperpolarizability contributes just under 2% at 200 K, rising to 4% at 500 K for N2, and 5.5% for C2H4. These contributions are non-negligible, and are hence worth refi ning in future work through full ab initio quantum mechanical computation of the interaction-induced hyperpolarizability contribution where dispersion force and electron cloud overlap effects can be included.
Interferometric measurement of induced birefringence in polycrystalline ZnSe.
(2011) Govender, Patricia.; Couling, Vincent William.
The aim of this research project was to assemble an apparatus capable of measuring field-induced birefringence in polycrystalline ZnSe. A Michelson interferometric apparatus was assembled, and care was taken to actively stabilize the interferometer against the effects of environmental noise by using electronic feedback techniques. This greatly enhanced the sensitivity of the interferometric measurements. In addition, the applied electric field was modulated, allowing sophisticated phase-sensitive detection techniques to be used to extract the induced birefringences. Once assembled, the interferometer was tested using electric-field induced birefringence in a perspex sample, since there is Kerr-effect data in the literature against which to compare our measured quadratic electro-optic coefficients. The interferometer was then used to measure the quadratic electrooptic coefficients of polycrystalline ZnSe, these being, to the best of our knowledge, the first such measurements for this species. The theory of electro-optic and photoelastic phenomena is comprehensively reviewed. This has permitted a critical discussion of the measured Kerr coefficients obtained in this project. It is demonstrated how quadratic electro-optic coefficients measured using the traditional technique of static polarimetry might include contributions arising from the linear electro-optic effect, these data being rendered suspect. In addition, suggestions are made as regards to the future possibilities for extending the apparatus to allow for direct measurement of stress-induced birefringences.
An investigation of electric-field-gradient-induced Birefringence in fluids.
(2014) Ntombela, Siyabonga Sibusiso.; Couling, Vincent William.
The theory of electric-field-gradient-induced birefringence (EFGIB, the Buckingham effect) is briefly reviewed, and modifications to the Buckingham-effect apparatus are described. These modifications have increased the throughput of the light reaching the photodiode detector, thereby considerably enhancing the sensitivity of the measurements. In addition, a new PID voltage controller has been built for the high-voltage power amplifier, ensuring stability to better than 0.1% over long periods of time. Room-temperature measurements of the Buckingham effect for gaseous oxygen have been undertaken, allowing for an estimation of the traceless electric quadrupole moment £ of O2. The quadrupole moment has been extracted from the measured data by making the assumption that the temperature-independent hyperpolarizability contribution to the EFGIB is negligible. The limitations of this assumption are discussed. The value obtained for the electric quadrupole moment of O2 is £ = (¡1.033 § 0.027) £ 10¡40 Cm2. The available ab initio quantum-computed values of£found forO2 in the literature are tabulated, and are comparedwith our measured value. Potential future work, where a full temperature-dependent study of the EFGIB of O2 is envisioned, is briefly described.
Measurement and computational modelling of intermolecular interactions in fluids.
(2000) Nhlebela, Richard V.; Couling, Vincent William.
The molecular theory of the second light-scattering virial coefficient Bp describing the effects of interacting pairs of molecules on the depolarization ratio p of Rayleigh-scattered light is reviewed, both for interacting linear and nonlinear molecules. The molecular tensor theory of Bp for nonlinear molecules is extended for the first time to include in the scattered intensity p those contributions arising from field gradient effects and induced quadrupole moments in the molecular interactions. The expressions for contributions to Bp are evaluated numerically for the nonlinear polar molecule dimethyl ether. We have used an existing light-scattering apparatus to investigate the pressure-dependence of the depolarization ratio p for dimethyl ether, allowing Bp to be extracted. The measured value is compared with the calculated value, theory and experiment being found to agree to within 9%. This success in modelling Bp for dimethyl ether spurred us on to extend our investigation to the second Kerr-effect virial coefficient BK • The molecular-tensor theory of BK for nonlinear molecules is reviewed, and is applied in this work to dimethyl ether. The calculated BK values generally lie within the uncertainty limits of the available measured data over their full range of temperatures. We have used a recently-commissioned Kerr cell to undertake our own measurement of BK for dimethyl ether at room temperature. This value is in good agreement with the findings of our molecular model, and is in reasonable agreement with the other measured data. This thesis serves to reaffirm recent claims that comprehensive dipole-induced-dipole theories of molecular interaction effects explain the observed phenomena adequately provided one works to higher orders in the molecular tensors so that the series of contributing terms has converged to a meaningful numerical result, and provided the full symmetry of the molecules is allowed for.
Measurement of the Kerr electro-optic effect by induced birefringence.
(2005) Singh, Ann.; Couling, Vincent William.
During the period January 2001 to January 2003, M Sc student Mr Tleyane Jonas Sono developed an apparatus to measure the pressure- and temperature-dependence of the electro-optic Kerr effect (electric-field-induced birefringence) in gases. Mr Sono presented experimental results for dimethyl ether at a wavelength of 632.8 nm, extracting polarizability tensor components, first and second Kerr hyperpolarizabilities, and second Kerr-effect virial coefficients for this molecular species. This thesis has been primarily concerned with a thorough re-investigation of the Keneffect for the dimethyl ether molecule. Of primary concern is the reproducibility of the measured data, which depend upon precise and accurate knowledge of various experimental parameters. These include calibrations of the high-voltage power supply which is used to establish the electric field across the medium, the pressure transducer, the platinum thermistors, as well as the Faraday cell which forms the heart of the compensator. There is also a possibility of the 316-stainless-steel electrodes buckling and warping as they are cycled over ±200°C, leading to variations in the applied field and a consequent hysteresis in the results. In essence, we have been loath to publish our Kerr-effect investigation of dimethyl ether before making a thorough investigation of the reproducibility of our measured data. Here we present our investigations, and compare our new Kerr virial coefficients and the molecular (hyper)polarizability data extracted from them against the previous work of Sono. It will become apparent agreement is excellent, and that the findings for dimethyl ether are now ready for publication. The molecular-tensor theory of the Kerr-effect; including the second Kerr-effect virial coefficient BK, which describes the effects of intermolecular collisions on the molecular Kerr constant; is reviewed. The computed data is compared with the experimental data, yielding good agreement over the full experimental temperature range of 280 to 450 K. Attempts to obtain measured data at 260 K proved fruitless in the present study, though efforts are underway to complete this task.
Second Buckingham-effect virial coeffcients of non-dipolar molecules
(2019) Moodley, Verlan.; Couling, Vincent William.
A molecular-tensor theory of the second electric-field-gradient-induced birefringence (EFGIB) virial coeffcient BQ, which describes the effects of molecular pair interactions on the molar Buckingham constant mQ, is developed for non-dipolar molecules with axial and higher symmetry. The resulting expressions for contributions to BQ are evaluated numerically for the molecules CO2, C2H4 and C2H6. These molecules were chosen since previously developed molecular-tensor theories of the second light-scattering virial coeffcient Bp and the second Kerr-effect virial coeffcient BK have yielded calculated values for these species which are in close agreement with the available measured data. The BQ values calculated for CO2, C2H4 and C2H6 reveal that, for the uids behaving as gases, the pair-interaction contributions to mQ are generally at or below the threshold of resolution of the EFGIB apparatus, so that the measured mQ values reported in the literature have not been contaminated by pair-interaction effects. In addition, it is seen that if the precision of measured mQ data can be increased by around an order of magnitude, it should in principle become possible to resolve BQ contributions, particularly for higher gas densities.
Second Kerr-effect virial coefficients of non-dipolar molecules with axial and lower symmetry.
(2016) Naidoo, Preshen.; Couling, Vincent William.
Abstract available in PDF file.
Second light-scattering and Kerr-effect virial coefficients of molecules with linear and lower symmetry.
(1995) Couling, Vincent William.; Graham, Clive.
Theoretical and experimental investigations of the Kerr Effect and Cotton-Mouton Effect.
(2008) Janse Van Rensburg, Angela Louise.; Couling, Vincent William.
Mr T. J. Sono, an MSc student during the period January 2001 to January 2003, developed an apparatus to measure the pressure and temperature dependence of the electric-field induced birefringence (or electro-optic Kerr effect) in gases. Mr Sono obtained experimental results for dimethyl ether at a wavelength of 632.8 nm resulting in polarizability tensor components, first and second Kerr hyperpolarizabilities, and second Kerr-effect virial coefficients for this particular molecular species. One of the primary concerns of this thesis has been to obtain new measured Kerr-effect data for dimethyl ether and for trifluoromethane over a range of temperature. The cell has been calibrated using hydrogen as a primary standard, and has been carefully aligned to avoid multiple reflections of the incident laser beam off the closely-spaced electrode surfaces. The data has been analyzed to extract values of the polarizability anisotropy and the second Kerr hyperpolarizability for these molecules. In addition, precise values for the second Kerr-effect virial coefficients have been obtained from measurements of the Kerr effect a function of pressure. The molecular-tensor theory of the second Kerr-effect virial coefficient BK is reviewed. This theory describes the effects of intermolecular interactions on the molar Kerr constant, and it has been used to compute BK for dimethyl ether and trifluoromethane over the experimental temperature range. Agreement between experiment and theory is generally good. BK for ammonia has also been calculated, and compared to recent measured data found in the literature. The theory of the Cotton-Mouton effect (the magnetic analogue of the Kerr-effect) in a dilute gas is reviewed, and a new molecular-tensor theory describing the effects of molecular pair-interactions is developed. Calculations for a test molecule, namely chloromethane, indicate that density-dependent effects for this molecule are extremely tiny (of the order of 1% for typical experimental pressures). This new theory could be profitably used in selecting molecules which might demonstrate a larger effect which might be more readily measured in the laboratory.