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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
81

Numerical simulations of plasma dynamics and chemistry in dual radio-frequency and pulse driven capacitively coupled atmosphere pressure plasmas

O'Neill, Colm Philip January 2015 (has links)
This thesis presents the results of numerical models used to investigate the influence of the operating conditions of a micro atmospheric pressure plasma jet, on the electron dynamics, ionisation/sustainment mechanisms and resultant plasma chemistry. The aim is to determine the, optimum operating conditions of this discharge for enhancing and controlling the underlying plasma processes and production/composition of useful reactive species. Both nonlinear frequency coupling and pulsed excitation have been shown to influence the underlying processes governing the electron dynamics in radio-frequency driven atmospheric pressure plasmas, here the effects of operating a micro atmospheric pressure 'plasma jet (μ-APPJ) using dual frequency (2F) and pulsed excitation are explored. Several multi-scale numerical models based on hydrodynamic equations with a semi-kinetic treatment of the electrons are used to investigate the influence of the operating mode on the plasma dynamics and chemistry. The models consider a helium background gas with a small molecular admixture of either Nitrogen or Oxygen, and range in complexity, with the most complex model accounting for 184 reactions amongst 20 species. Each model is found to agree well with experimental benchmarks. Using 2F excitation, it is found that variations of power density, voltage ratio and phase relationship provide separate control over the electron density, mean electron energy and electro-negativity. Using Pulsed excitation, variations of the pulse width and repetition rate are also found to directly influence the electron density, mean electron energy and electro-negativity. In both cases this is exploited to directly influence the phase dependent and time averaged effective EEPF, which enables tailoring of the EEPF for enhanced control over the plasma chemical kinetics. This is shown to allow control over the production and composition of useful reactive species, namely reactive oxygen species.
82

Collision dynamics as a probe of gas-liquid and related interfaces

Waring, Carla January 2011 (has links)
This thesis presents a body of work aimed at increasing the current understanding of the dynamics of collisions at the gas-liquid and related interfaces. In particular, collisions of open-shell radicals (oxygen atoms in the electronic ground state and hydroxyl radicals) with hydrocarbon surfaces have been investigated. Translationally hot radicals were scattered from liquid hydrocarbons and related surfaces. The products of inelastic scattering and hydrogen abstraction reactions (OH radicals) were detected, with translational and internal-state resolution, by laser-induced fluorescence (LIF). The radicals were used as a ‘chemical probe’ of the interface, providing unique information on the structure and reactivity of the surface, and how this relates to the bulk composition. Key results include the establishment of the relative reactivity towards oxygen atoms of liquid hydrocarbons and alkylthiol SAMs and the penetration depth of O atoms into the SAM surfaces. This was achieved through systematic studies involving custom-synthesised site-selectively deuterated SAMs. It was found that hydrogen abstraction can occur deeper within the monolayer than previously believed but still emerge as OH (or OD) without reacting further to form water. The reactivity of O(3P) atoms towards a technologically important family of ionic liquids ([Cnmim][Im]) was measured. It was found that the reactivity increased non-linearly as a function of alkyl chain length, to an extent which far exceeded any anticipated increase in reactivity based on stoichiometry alone. The interface in this case was found to differ greatly from the bulk composition, with preferential occupation by the alkyl units at the surface. The energy transfer and reactive uptake of OH radicals at a variety of hydrocarbon surfaces was investigated. There was significant transfer of the initial translational energy to all the surfaces as well as substantial translational-to-rotational energy conversion. This energy conversion was dependent on the functional groups present in the liquids as was the reactive uptake of OH. Reactive uptake coefficients (γ) were obtained for alkane, alkene and carboxylic acid-containing organic liquids. The results are discussed in terms of the relevance to the aging of organic particles in the troposphere.
83

Towards understanding the formation of water on interstellar dust grains

Frankland, Victoria Louise January 2011 (has links)
Although several research groups have studied the formation of H2, both experimentally and computationally, few have explored the surface formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface and, over time, lead to the formation of icy mantles coating interstellar dust grains. The most abundant of these species within the ice is H2O. The first half of this thesis introduces the construction and characterisation of the new dual atomic beam apparatus built to explore the surface formation mechanism of H2O. The apparatus has been designed to enable singular or dual atomic or molecular oxygen and hydrogen beams to be adsorbed onto a range of astronomically relevant substrates. Analysis of the surface chemistry can be performed using a combination of temperature programmed desorption, molecular beam modulation spectrometry, quartz crystal microgravimetry and reflection-adsorption infrared spectroscopy techniques. The remainder of this thesis discusses the results obtained by performing temperature programmed desorption experiments. Kinetic analysis was deduced for: H2O on bare silica; O2 on bare silica; O2 on compact amorphous solid water on silica; and O2 on porous amorphous solid water on silica. The results obtained were used towards constructing a simulation model mimicking the desorption of O2 from the icy mantles of interstellar dust grains under dense molecular cloud environments. The analysis revealed that sub-monolayer coverages of O2 followed first order desorption kinetics with a range of desorption activation energies from all of the surfaces studied. Multilayer coverages of O2 from silica were unexpectedly found to follow fraction order kinetics. Further experiments were performed to explore the origins of this multilayer fractional desorption order. The results obtained revealed that the kinetic order decreased roughly by half as the species change from O2 to CO to N2 suggesting the underlying amorphous silica surface appeared to be the cause for this unusual observation. Preliminary atomic O beam TPD experiments had also been performed from a range of astronomical relevant surfaces. The initial results indicated that O2, O3, H2O2 and 13CO2 were formed on the surface. However, the exact surface formation mechanism could not be concluded from these single experiments.
84

Theoretical investigations of the electronic spectroscopy and ultrafast photochemistry of binary transition metal carbonyl complexes

McKinlay, Russell Graham January 2011 (has links)
This thesis is concerned with the electronic absorption spectroscopy and photochemical relaxation mechanisms of binary metal carbonyl complexes. These paradigm complexes exhibit a wide range of photoinduced vibronic coupling related phenomena that are only recently beginning to be understood with the development of modern experimental and computational techniques. These experiments have shown that after irradiation using ultrafast (femtosecond) laser pulses an unsaturated photoproduct is produced, and possibly relaxes through a conical intersection at a Jahn-Teller active geometry, on the same ultrafast timescale. However while experiment can imply the presence of conical intersection, only theoretical methods can confirm this and accurately probe the appropriate part of the potential energy surfaces relevant to this mechanism. The accurate assignment of the electronic excited states of these carbonyls is also a matter of debate with different theoretical and experimental techniques applied to these systems over the years. The large density of excited states of different character within a small energy range and the high computational expense of studying transition metal complexes with highly correlated methods presents a considerable challenge to the theoretical chemist. The research presented in this thesis falls into two main parts, firstly the electronic excited states of the binary transition metal carbonyl complexes Fe(CO)5, Cr(CO)6 and Ni(CO)4 were studied with highly correlated coupled cluster methods as well as their one-photon and two-photon absorption spectra. These results were compared with previous experimental and theoretical results. The electronic excited states and one-photon absorption spectra were also studied for the group 7 mixed-metal bimetallic carbonyls (MnTc(CO)10, MnRe(CO)10 and TcRe(CO)10) for the first time with time-dependent density functional theory (TD-DFT), the ability of TD-DFT methods to describe charge-transfer states was also investigated here. The second part of this thesis focussed on the relaxation pathways of the 2Mn(CO)5 and 1Fe(CO)4 initial photoproducts of the photodissociation of Mn2(CO)10 and Fe(CO)5 respectively using CASSCF. Both were found to relax to their lowest energy state through a Jahn-Teller induced conical intersection at a Jahn-Teller active geometry in agreement with experimental observation.
85

Proton transfer, electron binding and electronegativity in ammonium-containing systems

Whiteside, Alexander January 2012 (has links)
Using modern electronic structure methods, the ammonia-hydrogen halide complexes and their anions are characterised to determine the number, type, and properties of their minima, and their electron binding energies. Methodological issues of determining the potential energy surfaces of reactive monomers are addressed in the course of this investigation. The energetic origins of the hydrogen-bonded minima are determined by evaluation of the one-body and two-body terms composing the total energy of the complexes, and a rationale for the drive to proton transfer is presented. It is concluded that the systems have qualitatively similar potential surfaces, and that the balance of the one-body and two-body forces determines the number and depth of minima, while the electron acts as a perturbing agent on the one- or two-body energy, depending upon the nature of the minimum encountered. The halogen-bonded structures of ammonia-hydrogen bromide, iodide, and astatide complexes are shown to be stable, and one may perhaps bind an electron. The concept of the ammonium radical as a pseudo-atom is presented and tested. It is found to show competing pseudo-atomic and molecular properties.
86

Computational investigations of the spectroscopy, vibronic coupling, and photo(stereo)chemistry in inorganic systems

Zurek, Justyna M. January 2012 (has links)
This thesis focuses on the spectroscopy and photo-stereochemistry of relatively large closed-shell and open-shell transition metal complexes, investigated with an array of modern computational methodologies. The presence of the metal electrons/orbitals results in a greater number of low-lying excited states, and these states are vibronically coupled resulting in Jahn-Teller or pseudo-Jahn-Teller (pJT) effects, or general surface crossings. These features are very challenging to calculate but are vitally important to explain the observed behavior in such systems. Computational investigations using the multiconfigurational CASSCF method on the pJT effect occurring in ammonia, and Mo2(DXylF)2(O2CCH3)2(μ2-O)2 complex are presented. These definitively show that in the latter case the experimentally observed structure is due to a vibronic coupling of the ground electronic state with that of a nondegenerate 1πδ* state, resulting in a rhomboidal rather than square motif at the bimetallic centre. The (BQA)PtMe2I (BQA= bis(8-quinolinyl)amide) complex has been found to undergo unexpected meridial to facial isomerisation induced by light. The TD-DFT method was used to examine the spectroscopy of this system, and the CASSCF method was used to examine excited state relaxation pathways. The system relaxes on an excited state potential energy surface, of an essentially localised ππ* excited state of the BQA ligand, and reaches a facial excited minimum that is located adjacent to a sloped conical intersection connecting the excited and ground electronic states. Chromium (III) complexes have been investigated for many years and many aspects of their photochemistry are still not very well understood. The photochemistry of paradigm Cr (III) complexes, such as chromium oxalate [Cr(C2O4)3]3-, chromium tris- (1,3diaminopropane) [Cr(tn)3]3+ and Cr(tn)2(CN)2, have been investigated using TDDFT and CASSCF methods. Non-radiative relaxation pathways have been documented showing mechanism of both internal conversion in the quartet manifold, as well as inter-system crossing into the doublet manifold. The results explain photostereochemical features of the photo-induced racemization of [Cr(C2O4)3]3- and the photoaquation of [Cr(tn)3]3+ and Cr(tn)2(CN)2.
87

The construction of molecular dynamics model for the response of NaxCrxTi8-xO16(x=1.7) to electric fields of up to 100ps duration

Khoo, Kien Ling January 2003 (has links)
The hollandite Nax(Ti8--xCrx)O 16, (x = 1.7) is a non-stoichiometric material that is based on a family of compounds of general formula AxM4--xNy O8. The chromium and sodium ions are randomly placed in unit cells according to the relative proportions of titanium and chromium ions. Charge compensating sodium ions are located in interstitial sites along tunnels. Because of the non-stoichiometric nature, there are vacant sites into which the sodium ions may move.;A molecular dynamics (MD) simulation was used to determine the motion of the sodium ions in the tunnel, when the surrounding ions were held rigid. The dielectric response was calculated from the rate of change of polarisation, dP/dt under the action of a de step-field of 7.43MV/m to 74.3GV/m, at temperature between 200K and 373K.;A resonance behaviour has been observed. The absorption peaks in chi" at the resonance frequency lie between 4.5x1010 and 8.8x10 10 Hz at 297K which matches very well with the Poley absorption which is typically observed in polar liquids in the 1.2 - 70 cm--1 (3.6x1010 -- 2.1x1012 Hz) region at room temperature. The resonance frequency and the resonance peak height are independent of temperature. The peaks show a non-linear dependence upon electric field. At the very high field of 74.3GV/m, the sodium ions move in collective modes of vibration.
88

Application of spectrochemical techniques to electrochemistry

Clarke, F. S. January 1973 (has links)
No description available.
89

Magnetic anisotropy of triple bonds

Collinge, D. A. January 1974 (has links)
No description available.
90

Ultrasonic relaxation studies in some polymeric systems

Cochran, M. A. January 1974 (has links)
No description available.

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