No / Exploring nucleation processes by molecular simulation can provide a mechanistic understanding at the atomic level and also enables kinetic and thermodynamic quantities to be estimated. However, whilst the potential for modeling crystal nucleation and growth processes is immense, there are specific technical challenges to modeling [that need to be tackled]. In general, rare events, such as nucleation cannot be simulated using a direct ¿brute force¿ molecular dynamics approach. In recent years, the limited time and length scales that are accessible by conventional molecular dynamics simulations have inspired a number of advances to tackle problems that were hitherto considered outside the scope of molecular simulation. While general insights and features could be explored from efficient generic models, The newer methods have paved the way to realistic crystal nucleation scenarios. The association of single ions in solvent environments, the mechanisms of motif formation in solvents, the nucleation process itself, ripening reactions, role of additives, as well as the self-organization of nanocrystals can now all be investigated at the molecular level. The insights gained should complement experiments and enhance our fundamental understanding of the processes involved and facilitate the rational design of new materials.

This thesis presents algorithm development in computational chemistry, and applies new computer science concepts to voltammetric simulation. To begin, this thesis discusses why algorithm development is necessary, and inherent problems found in commercial simulation solvers. As a result of this discussion, this thesis describes the need for simulators to keep abreast of recent computational developments. Algorithm development in this thesis is taken through stages. Chapter 3 applies known theory relating to the stripping voltammetry at a macroelectrode to the diffusional model of a microdisk, using finite difference and alternating direction implicit simulation techniques. Chapter 4 introduces the concept of parallel computing, and how computational hardware has developed recently to take advantage of out-of-order calculations, by processing them in parallel to reduce simulation time. The novel area of graphics card simulation for highly parallel algorithms is also explained in detail. Chapter 5 discusses the adaptation of voltammetric finite difference algorithms to a purely parallel format for simulation by explicit solution. Through explicit solution, finite difference algorithms are applied to electrode geometries which necessitate a three-dimensional solution – elliptical electrodes; square, rectangular, and microband electrodes; and dual microdisk electrodes in collector-generator mode. Chapter 6 introduces 'Random Walk' simulations, whereby individual particles in the simulation are modelled and their trajectories over time are calculated. The random walk technique in this thesis is improved for pure three-dimensional diffusion, and adapted to graphics cards, allowing up to a factor 4000 increase in speed over previous computational methods. This method is adapted to various systems of low concentration confined voltammetry (chapter 6.4) and single molecule detection, ultra low concentration cyclic voltammetry (chapter 6.5), and underpotential deposition of thallium on mobile silver nanoparticles (chapter 6.6). Overall, this thesis presents, and applies, a series of algorithm development concepts in computational electrochemistry.

We study the low-temperature physics arising in models of a strongly correlated, tunnel-coupled double quantum dot (DQD), particularly the two-impurity Anderson model (2AIM) and the two-impurity Kondo model (2IKM), employing a combination of physical arguments and the Numerical Renormalisation Group. These models exhibit a rich range of Kondo physics. In the regime with essentially one electron on each dot, there is a competition between the Kondo effect and the interdot exchange interaction. This competition gives rise to a quantum phase transition (QPT) between local singlet and Kondo singlet phases in the 2IKM, which becomes a continuous crossover in the 2AIM as a result of the interlead charge transfer present. The 2IKM is known to exhibit two-channel Kondo (2CK) physics at the QPT, and we investigate whether this is also the case for the 2AIM at the crossover. We find that while in principle 2CK physics can be observed in the 2AIM, extremely low temperatures are required, such that it is unlikely that 2CK physics will be observed in an experimental DQD system in the near future. We have studied the effect of a magnetic field on the 2AIM and the 2IKM, finding that both the zero-field QPT in the 2IKM and the zero-field crossover in the 2AIM, persist to finite field. This presents the possibility of observing 2CK physics in an experimental DQD at finite field, but we find that the temperatures required to do so are extremely low. We show that longer even-numbered chains of spins also exhibit QPTs at finite field, and argue that a 2N-spin chain should undergo N QPTs as field is increased (starting deep in the local singlet phase at zero field). We have also carried out a joint theoretical-experimental study of a carbon nanotube based DQD, in collaboration with Dr. Mark Buitelaar et al. The agreement between experimental and theoretical results is good, and the experiments are able to access the crossover present in the 2AIM at finite field. Furthermore, the experiments show the wide range of physics exhibited by DQD systems, and illustrate the utility of such systems in probing correlated electron physics.

The question posed in this work is how one would model and predict the rotational spectrum of open-shell open-shell van der Waals complexes. There are two secondary questions that arise: the nature of radical-radical interactions in such systems and the modelling of the large amplitude motion of the constituent molecules. Four different systems were studied in this work, each providing part of the answer to the main question. Starting with the large amplitude motion, there are two theoretical approaches that may be adopted: to either model the whole complex as a semi-rigid molecule, or to perform quantum dynamical calculations. We recorded and analysed the rotational spectrum (using Fourier transform microwave spectroscopy) of the molecule of tertiary butyl acetate (TBAc) which exhibits a high degree of internal rotation; and of the weakly-bound complex between a neon atom and a nitrogen dioxide molecule (Ne-NO2). We used the semi-rigid approach for TBAc and the quantum dynamical approach for Ne-NO2. We also explored the compatibility of these two approaches. Moreover, we were able to predict and analyse the fine and hyperfine structure of the Ne-NO2 spectrum using spherical tensor operator algebra and the results of our dynamics calculations. To explore the nature of the interactions in an radical-radical van der Waals complex we calculated the PESs of the possible states that the complex may be formed in, when an oxygen and a nitrogen monoxide molecule meet on a plane using a number of high level ab initio methods. Finally, our conclusions were tested and applied when we performed the angular quantum dynamics to predict the rotational spectrum of the complex between an oxygen and a nitrogen dioxide molecule, and account for the effect of nuclear spin statistics in that system.

This thesis describes the study of a range of ion-molecule reactions at very low collision energies using a newly developed experimental technique which involves the reaction of velocity-selected beams of translationally cold neutral molecules with very low kinetic energy ion ensembles. These studies have been enabled by the construction of a new apparatus for trapping and laser-cooling gas phase atomic ions (⁴⁰Ca⁺). The laser-cooling process results in the formation of ordered, low kinetic energy, lattice-like ion structures, also known as "Coulomb crystals". The properties of single and multicomponent Coulomb crystals (which may also involve molecular ions), and their manipulation via modulation of the applied fields, are explored experimentally and with the use of molecular dynamics simulations. Variations in the laser-cooling parameters are shown to result in different steady-state populations of the electronic states of ⁴⁰Ca⁺ involved with the laser cooling cycle, and these are modelled within an appropriate theoretical framework. The imaging of ⁴⁰Ca⁺ fluorescence as a function of time allows the study of various ion-molecule reactions at collision energies around 300 K, with single ion sensitivity. These reaction studies are extended to low-temperature (collision energies close to 1 K), by combination of the ion trap apparatus with a bent quadrupole guide velocity-selector. Ion-molecule collision energies are shown to be variable over a short range through a change in the quadrupole guide voltage, or the ion trapping parameters; the effect of these modulations on the rate constant is explored for Ca⁺ + CH₃F. Bimolecular rate constants for the reactions of ⁴⁰Ca⁺ with CH₃F, CH₂F₂ and CH₃Cl have been determined for a range of ⁴⁰Ca⁺ state populations, allowing resolution of the global rate contributions from the ground and combined excited states. These results are analysed in the context of capture theories and ab initio electronic structure calculations. In each case, suppression of the ground state rate constant is explained by the presence of either a submerged or real barrier on the ground state potential surface. Rates of reaction from the combined excited states are generally found to be in line with capture theories, and in some cases variation is found between the high and low collision energy regimes. Molecular product ions generated in these experiments have been shown to be sympathetically-cooled into the crystal structure, and subsequently identified through resonance-excitation mass spectrometry. Molecular ions were also produced by multiphoton laser ionisation of a thermal background gas of OCS molecules. An ion-molecule reaction involving a molecular ion, that of charge transfer between OCS⁺ and ND₃, has been studied at a collision energy near 1 K for the first time using sympathetically-cooled OCS⁺ and velocity-selected ND₃. These experiments illustrate the generality of the techniques described herein, and should lead to many possibilities for future studies.

In this work, we wish to determine the free energy landscape and the nucleation rate associated with the process of homogeneous ice nucleation. To do this, we simulate the homogeneous nucleation of ice with the mW monatomic model of water and with all-atom models of water using primarily the umbrella sampling rare event method. We find that the use of the mW model of water, which has simpler dynamics compared to all-atom models of water, but is nevertheless surprisingly good at reproducing experimental data, results in very reasonable agreement with classical nucleation theory, in contrast to some previous simulations of homogeneous ice nucleation. We suggest that previous simulations did not observe the lowest free energy pathway in order parameter space because of their use of global order parameters, leading to a deviation from classical nucleation theory predictions. Whilst monatomic water can nucleate reasonably quickly, all-atom models of water are considerably more difficult to simulate, primarily because of their slow dynamics of ice growth and the fact that standard order parameters do not work well in driving nucleation when such models are being used. In this thesis, we describe a local, rotationally invariant order parameter that is capable of growing ice homogeneously in a biassed simulation without the unnatural effects introduced by global order parameters, and without leading to non-physical chain-like growth of 'ice' clusters that results from a naïve implementation of the standard Steinhardt-Ten Wolde order parameter. We have successfully used this order parameter to force the growth of ice clusters in simulations of all-atom models of water. However, although ice growth can be achieved, equilibrating simulations with all-atom models of water is extremely difficult. We describe several approaches to speeding up the equilibration in all-atom models of water to enable the computation of free energy profiles for homogeneous ice nucleation.

A detecção nos meios interestelar e circunstelar de espécies diatômicas contendo silício como SiC, SiN, SiO e SiS, assim como espécies contendo fósforo, sugere que a molécula SiP seja também uma forte candidata a ser observada nesses meios devido à abundância relativa de seus átomos componentes. A importância tecnológica do silício é também motivadora para a caracterização estrutural e espectroscópica de seus derivados. Neste particular, complementando estudos anteriores de espécies contendo átomos de silício em nosso grupo, este trabalho está voltado para uma descrição teórica detalhada de cerca de duas dúzias de estados eletrônicos dupletos e quartetos da espécie SiP que se correlacionam com os quatro primeiros canais de dissociação. Essa descrição compreende a construção de curvas de energia potencial, da função momento de dipolo e da função momento de transição. Os estados ligados são também caracterizados por suas constantes espectroscópicas e as possíveis transições vibrônicas expressas em termos de probabilidades de transições radiativas calculadas pelo coeficiente Av\'v\" de Einstein; tempos de vida radiativa complementam esses cálculos. Na descrição dos estados eletrônicos e de suas propriedades usamos a metodologia interação de configurações com referências múltiplas, onde a função de onda foi gerada como excitações simples e duplas a partir de um conjunto de funções de configurações de referência determinado por um cálculo prévio do tipo campo auto-consistente multi-configuracional com espaço ativo completo. Funções atômicas do tipo aug-cc-pVQZ foram usadas na construção dos orbitais moleculares. Dentre todos os estados caracterizados, destacamos a descoberta de um estado 2π abaixo do estado anteriormente rotulado de B 2Σ+, o que nos levou a uma nova renomeação da ordem energética. Esse novo estado oferece uma possibilidade realista de se acessar estados vibracionais mais altos dos estado X 2π e A 2Σ+ e com isso obter uma descrição experimental melhorada desses dois estados. Além do conjunto de estados dupletos possíveis descritos, salientamos também as possibilidades aqui apontadas e quantificadas de se detectar experimentalmente transições eletrônicas entre os estados quartetos. No seu todo a descrição detalhada e rigorosa de cerca de duas dúzias de estados eletrônicos fornece um conjunto de dados energéticos e espectroscópicos que certamente contribuirá para um melhor entendimento de sistemas diatômicos contendo silício e fósforo. / The detection in interstellar and circumstellar spaces of diatomic species containing silicon like SiC, SiN, SiO, and SiS, as well as species containing phosphorous suggests that the molecule SiP be also a strong candidate is observed in these regions due to the relative abundance of its constituting atoms. The technological importance of silicon is also a motivation for the structural and spectroscopic characterization of its derivatives. In this particular, complementing previous studies in our group of species containing silicon atoms, this work is focused on a detailed description of about two dozen doublet and quartet electronic states of the SiP species correlating with the first four dissociation channels. This description comprises the construction of potential energy curves, and dipole and transition moment functions. The bound states are also characterized by their spectroscopic constants and the possible vibronic transitions expressed in terms of the radiative transition probabilities calculated as Einstein Av\'v\'\' coefficients; radiative lifetimes also complement these calculations. In the description of the electronic states and of their properties we used the multi-reference configuration interaction method, with the wavefunction generated as all single and doubles excitations from a set of reference configuration state functions determined from a previous complete active space multiconfigurational selfconsistent field calculation. Atomic functions of the type aug-cc-pVQZ were used in the construction of the molecular orbitais. Of all the electronic states c haracterized, we call the attention to the discovery of a 2π state below a state previously know as B 2Σ+, a fact that led us to a new labelling of the energetic order. This new state offers a realistic possibility of accessing higher vibrational states of the states X 2π and A 2Σ+, thus providing the possibility of obtaining an improved experimental description of these two states. Besides the set of possible states described, we also note the possibilities here pointed out and quantitatively characterized of experimentally detecting electronic transitions between quartet states. On the whole, the detailed and rigorous description of about two dozen electronic states provides a set of energetic and spectroscopic data that certainly will contribute to a better understanding of diatomic systems containing sillicon and phosphorus.

Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance. The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (35<x<45 wt %) showed the best performance. The mass-transport limitation was essentially due to O2 diffusion in the agglomerates. The performance of cathodes with low Nafion content (<30 wt %) is limited by poor kinetics owing to incomplete wetting of platinum (Pt) by Nafion, by proton migration throughout the cathode as well as by O2 diffusion in the agglomerates. At large Nafion content (>45 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results. / QC 20101014

<p>Polymer electrolyte fuel cells (PEFC) convert the chemically bound energy in a fuel, e.g. hydrogen, directly into electricity by an electrochemical process. Examples of future applications are energy conversion such as combined heat and power generation (CHP), zero emission vehicles (ZEV) and consumer electronics. One of the key components in the PEFC is the membrane / electrode assembly (MEA). Both the membrane and the electrodes consist of proton conducting polymers (ionomers). In the membrane, properties such as gas permeability, high proton conductivity and sufficient mechanical and chemical stability are of crucial importance. In the electrodes, the morphology and electrochemical characteristics are strongly affected by the ionomer content. The primary purpose of the present thesis was to develop experimental techniques and to use them to characterise proton conducting polymers and membranes for PEFC applications electrochemically at, or close to, fuel cell operating conditions. The work presented ranges from polymer synthesis to electrochemical characterisation of the MEA performance.</p><p>The use of a sulfonated dendritic polymer as the acidic component in proton conducting membranes was demonstrated. Proton conducting membranes were prepared by chemical cross-linking or in conjunction with a basic functionalised polymer, PSU-pyridine, to produce acid-base blend membranes. In order to study gas permeability a new in-situ method based on cylindrical microelectrodes was developed. An advantage of this method is that the measurements can be carried out at close to real fuel cell operating conditions, at elevated temperature and a wide range of relative humidities. The durability testing of membranes for use in a polymer electrolyte fuel cell (PEFC) has been studied in situ by a combination of galvanostatic steady-state and electrochemical impedance measurements (EIS). Long-term experiments have been compared to fast ex situ testing in 3 % H2O2 solution. For the direct assessment of membrane degradation, micro-Raman spectroscopy and determination of ion exchange capacity (IEC) have been used. PVDF-based membranes, radiation grafted with styrene and sulfonated, were used as model membranes. The influence of ionomer content on the structure and electrochemical characteristics of Nafion-based PEFC cathodes was also demonstrated. The electrodes were thoroughly investigated using various materials and electrochemical characterisation techniques. Electrodes having medium Nafion contents (35<x<45 wt %) showed the best performance. The mass-transport limitation was essentially due to O2 diffusion in the agglomerates. The performance of cathodes with low Nafion content (<30 wt %) is limited by poor kinetics owing to incomplete wetting of platinum (Pt) by Nafion, by proton migration throughout the cathode as well as by O2 diffusion in the agglomerates. At large Nafion content (>45 wt %), the cathode becomes limited by diffusion of O2 both in the agglomerates and throughout the cathode. Furthermore, models for the membrane coupled with kinetics for the hydrogen electrode, including water concentration dependence, were developed. The models were experimentally validated using a new reference electrode approach. The membrane, as well as the hydrogen anode and cathode characteristics, was studied experimentally using steady-state measurements, current interrupt and EIS. Data obtained with the experiments were in good agreement with the modelled results. Keywords: polymer electrolyte fuel cell, proton conducting membrane, porous electrode, gas permeability, degradation, water transport</p>

A detecção nos meios interestelar e circunstelar de espécies diatômicas contendo silício como SiC, SiN, SiO e SiS, assim como espécies contendo fósforo, sugere que a molécula SiP seja também uma forte candidata a ser observada nesses meios devido à abundância relativa de seus átomos componentes. A importância tecnológica do silício é também motivadora para a caracterização estrutural e espectroscópica de seus derivados. Neste particular, complementando estudos anteriores de espécies contendo átomos de silício em nosso grupo, este trabalho está voltado para uma descrição teórica detalhada de cerca de duas dúzias de estados eletrônicos dupletos e quartetos da espécie SiP que se correlacionam com os quatro primeiros canais de dissociação. Essa descrição compreende a construção de curvas de energia potencial, da função momento de dipolo e da função momento de transição. Os estados ligados são também caracterizados por suas constantes espectroscópicas e as possíveis transições vibrônicas expressas em termos de probabilidades de transições radiativas calculadas pelo coeficiente Av\'v\" de Einstein; tempos de vida radiativa complementam esses cálculos. Na descrição dos estados eletrônicos e de suas propriedades usamos a metodologia interação de configurações com referências múltiplas, onde a função de onda foi gerada como excitações simples e duplas a partir de um conjunto de funções de configurações de referência determinado por um cálculo prévio do tipo campo auto-consistente multi-configuracional com espaço ativo completo. Funções atômicas do tipo aug-cc-pVQZ foram usadas na construção dos orbitais moleculares. Dentre todos os estados caracterizados, destacamos a descoberta de um estado 2&#960; abaixo do estado anteriormente rotulado de B 2&#931;+, o que nos levou a uma nova renomeação da ordem energética. Esse novo estado oferece uma possibilidade realista de se acessar estados vibracionais mais altos dos estado X 2&#960; e A 2&#931;+ e com isso obter uma descrição experimental melhorada desses dois estados. Além do conjunto de estados dupletos possíveis descritos, salientamos também as possibilidades aqui apontadas e quantificadas de se detectar experimentalmente transições eletrônicas entre os estados quartetos. No seu todo a descrição detalhada e rigorosa de cerca de duas dúzias de estados eletrônicos fornece um conjunto de dados energéticos e espectroscópicos que certamente contribuirá para um melhor entendimento de sistemas diatômicos contendo silício e fósforo. / The detection in interstellar and circumstellar spaces of diatomic species containing silicon like SiC, SiN, SiO, and SiS, as well as species containing phosphorous suggests that the molecule SiP be also a strong candidate is observed in these regions due to the relative abundance of its constituting atoms. The technological importance of silicon is also a motivation for the structural and spectroscopic characterization of its derivatives. In this particular, complementing previous studies in our group of species containing silicon atoms, this work is focused on a detailed description of about two dozen doublet and quartet electronic states of the SiP species correlating with the first four dissociation channels. This description comprises the construction of potential energy curves, and dipole and transition moment functions. The bound states are also characterized by their spectroscopic constants and the possible vibronic transitions expressed in terms of the radiative transition probabilities calculated as Einstein Av\'v\'\' coefficients; radiative lifetimes also complement these calculations. In the description of the electronic states and of their properties we used the multi-reference configuration interaction method, with the wavefunction generated as all single and doubles excitations from a set of reference configuration state functions determined from a previous complete active space multiconfigurational selfconsistent field calculation. Atomic functions of the type aug-cc-pVQZ were used in the construction of the molecular orbitais. Of all the electronic states c haracterized, we call the attention to the discovery of a 2&#960; state below a state previously know as B 2&#931;+, a fact that led us to a new labelling of the energetic order. This new state offers a realistic possibility of accessing higher vibrational states of the states X 2&#960; and A 2&#931;+, thus providing the possibility of obtaining an improved experimental description of these two states. Besides the set of possible states described, we also note the possibilities here pointed out and quantitatively characterized of experimentally detecting electronic transitions between quartet states. On the whole, the detailed and rigorous description of about two dozen electronic states provides a set of energetic and spectroscopic data that certainly will contribute to a better understanding of diatomic systems containing sillicon and phosphorus.