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Thesis (Ph. D.)--Missouri University of Science and Technology, 2010. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 21, 2010) Includes bibliographical references (p. 82-87).

This thesis presents a theoretical and experimental investigation of the interaction of electronically excited Rydberg hydrogen atoms with metal surfaces and the associated charge-transfer process. As a Rydberg atom approaches a metal surface, the energies of the Rydberg states are perturbed by the surface potential generated by the image charges of the Rydberg electron and core. At small atom-surface separations, the Rydberg atom may be ionised by resonant charge transfer of the Rydberg electron to the continuum of delocalised unoccupied metal states, with which the Rydberg electron is degenerate in energy. Typically, this ‘surface ionisation’ can be measured by extracting the remaining positively charged ion-cores with externally applied electric fields. By applying various levels of theory, from classical to fully time-dependent quantum calculations, this thesis explores various experimentally relevant effects on the charge-transfer process, such as the magnitude and direction of the externally applied electric field, the atom collisional velocity, the presence of local surface stray fields and electronically structured surfaces. The theoretical results give insight into the previous experimental work carried out for the xenon atom and hydrogen molecule, and point out some of the fundamental differences from the hydrogen atom system. Experiments involving Rydberg hydrogen atoms incident on an atomically flat gold surface, a rough machined aluminium surface and a single crystal copper (100) surface are presented, providing for the first time the opportunity to make a quantitative comparison of theory and experiments. The ability to control the critical distance at which charge-transfer occurs is demonstrated by using Rydberg states of varying dimensions and collisional velocities. By changing the collisional angle of the incident Rydberg beam, the effect of Rydberg trajectory is also investigated. By manipulating the polarisation of the Rydberg electron with electric fields, genuine control over the orientation of the electron density distribution in the charge-transfer process is demonstrated. This property was predicted by the theory and should be unique to the hydrogen atom due to its intrinsic symmetry. By reversing the direction of the electric field with respect to the metal surface, electrons rather than positive ions are detected, with ionisation dynamics that appear to be very different, as predicted by quantum calculations. Experiments involving the single crystal Cu(100) surface also suggests possible resonance effects from image states embedded in the projected bandgap which are shown from quantum calculations to play an important role in the surface charge transfer of electronically structured metal substrates. The experimental technique developed in this work provides some exciting future applications to study quantum confinement effects with thin films, nanoparticles and other bandgap surfaces. The ability to control the Rydberg orbital size, electronic energy, collisional velocity and orientation in the charge-transfer process will provide novel ways of probing the surface’s electronic and physical structure, as well as being a valuable feature in offering new opportunities for controlling reactive processes at metallic surfaces.

Zeeman deceleration is an experimental technique which allows for the manipulation of open-shell atoms and molecules in a supersonic beam thus producing mK-cold, velocity-tunable beams of particles in selected quantum states. The method relies on the Zeeman interaction between paramagnetic particles and time-varying, inhomogeneous magnetic fields generated by pulsing high currents through an array of solenoid coils. This thesis describes the construction and implementation of a supersonic beam setup including a 12-stage Zeeman decelerator. The Zeeman decelerator follows an original design that makes it possible to replace individual deceleration coils. Using ground-state hydrogen atoms as a test system, it is shown that the transverse acceptance in a Zeeman decelerator can be significantly increased by generating a rather low, temporally varying quadrupole field in one of the solenoid coils. An electron-impact source was constructed and optimised enabling, for the first time, the Zeeman deceleration of metastable helium atoms in the 23S1 state, with an up to 40 % decrease in the kinetic energy of the beam. It is shown that the pulse duration for electron-impact excitation needs to be matched to the acceptance of the decelerator in order to attain a good contrast between the decelerated and undecelerated parts of the beam. Experimental results are rigorously analysed and interpreted using three-dimensional numerical particle trajectory simulations. A phase-space model provides, for the first time, a means to estimate the six-dimensional phase-space acceptance in a Zeeman decelerator and to find optimum parameter sets for improved Zeeman deceleration schemes. The approach also reveals a hitherto unconsidered velocity dependence of the phase stability which is ascribed mainly to the rise and fall times of the current pulses that generate the magnetic fields inside the deceleration coils. In the future, it is planned to combine the Zeeman decelerator with a source of cold atomic and molecular ions to study chemical collisions at low temperatures. A hybrid magnetic guide consisting of permanent magnet assemblies (Halbach arrays) in hexapole configuration and a set of current-carrying wires is proposed and simulated as an interface between these setups. The design promises very efficient velocity selection, a high degree of quantum-state selection and a nearly complete removal of residual carrier gas. Prospects for using magnetic hexapole focusing in front of the Zeeman decelerator are discussed. The work represents a major step towards the study and control of chemical reactivity of paramagnetic species in the low-temperature regime and it will help in the testing of fundamental chemical reaction theories.

Nesta tese, utilizamos técnicas de química quântica para o estudo de sistemas contendo átomos de silício e nitrogênio. Nesse contexto, o trabalho aqui apresentado procurou concentrar-se em três tópicos principais: aglomerados de silício e nitrogênio, a superfície Si(100) e a interação de um átomo de nitrogênio com essa superfície. Estudamos inicialmente os aglomerados de fórmula SiNN e Si3N2, onde procuramos caracterizar de forma rigorosa a estabilidade e a natureza das ligações químicas das várias espécies. Com relação ao sistema SiNN, os resultados utilizando cálculos de alto nível mostraram a necessidade de novos dados experimentais para a caracterização inequívoca dessa espécie. Além disso, obtivemos os primeiros resultados para as propriedades de quatro novas estruturas. Estudamos a estrutura de várias espécies com fórmula Si3N2, e obtivemos como mínimo global uma estrutura planar contendo apenas ligações Si-N. Para a simulação da superfície Si(100) utilizamos os aglomerados de fórmula Si9H12 e Si15H16. Apenas a metodologia do Funcional da Densidade indicou a existência de uma estrutura distorcida para o Si9H12. Os cálculos CASSCF mostraram a necessidade do uso de funções multideterminantais. Com relação ao aglomerado Si15H16, obteve-se uma estrutura distorcida com os dímeros alternados como a forma mais estável. Entretanto, o uso do CASSCF mostrou ser a forma simétrica a mais estável. Com base no estudo dos aglomerados Si9H12 e Si15H16, partimos para a investigação dos mecanismos envolvidos no processo de interação e incorporação de átomos de nitrogênio na superfície de silício. Estruturas com simetria de spin quarteto e dupleto foram estudadas, sendo os mecanismos bastante semelhantes, com o nitrogênio interagindo inicialmente com um dos silícios dímeros para em seguida poder formar duas outras estruturas, uma com o nitrogênio ligado aos dois silícios dímeros e outra com o nitrogênio inserido no interior do aglomerado. Esta última estrutura corresponde à forma mais estável. Os resultados mostram claramente que a reação é bastante favorável do ponto de vista energético. Questionamos também o uso de vínculos durante a otimização das estruturas por impedir um devido relaxamento apropriado dos átomos de silício da primeira e segunda camadas. / In this thesis, quantum chemical techniques were used to study molecular systems containing silicon and nitrogen atoms. ln this context, the work was concentrated on three main topics: silicon-nitrogen clusters, the Si(100) surface, and the interaction of a nitrogen atom with that surface. We studied initially the clusters SiNN and Si3N2, where we tried to characterize with rigour the stability and the nature of the chemical bonds of the various species. Concerning the system SiNN, using high level calculation, the results showed the necessity of new experimental data for the unequivocal characterization of this species. Moreover, the properties of four new structures were described for the first time in this study. The structures of various species with formula Si3N2 were studied, and the global minimum corresponds to a planar structure containing only Si-N bonds. For the simulation of Si(100) surface, the clusters Si9H12 and Si15H16 were used. Density Functional Theory predicted the existence of an asymmetric geometry for the Si9H12 cluster. The CASSCF calculations showed the necessity of using multideterminantal wave functions. Concerning the Si15H16 cluster, the most stable structure corresponds to a distorted one with alternated dimers. However, the CASSCF methodology showed that the symmetric structure is the most stable. Based on the results for the Si9H12 and Si15H16 clusters, we started to investigate the mechanism involved in the interaction and incorporation of a nitrogen atom into the silicon surface. Structures with quartet and doublet spin symmetry were studied, being their mechanisms very similar, with the nitrogen atom interacting initially with one silicon dimer; after that two other structures can be formed, one with the nitrogen bonded to two silicon dimers, and the other with the nitrogen bonded to one silicon dimer and two internal silicons. This last structure corresponds to the most stable species. The results show clearly that the reaction is very favorable energetically. The use of geometrical constraints during the optimization was questioned since it prevents the proper relaxation of the silicon atoms in the first and second layers.

Made available in DSpace on 2014-12-17T15:15:05Z (GMT). No. of bitstreams: 1 HidalynTCMS.pdf: 3137452 bytes, checksum: c9101961e25b2af76cdd7d74bfe14bb5 (MD5) Previous issue date: 2007-04-26 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Understanding the way in which large-scale structures, like galaxies, form remains one of the most challenging problems in cosmology today. The standard theory for the origin of these structures is that they grew by gravitational instability from small, perhaps quantum generated, ?uctuations in the density of dark matter, baryons and photons over an uniform primordial Universe. After the recombination, the baryons began to fall into the pre-existing gravitational potential wells of the dark matter. In this dissertation a study is initially made of the primordial recombination era, the epoch of the formation of the neutral hydrogen atoms. Besides, we analyzed the evolution of the density contrast (of baryonic and dark matter), in clouds of dark matter with masses among 104M? ? 1010M?. In particular, we take into account the several physical mechanisms that act in the baryonic component, during and after the recombination era. The analysis of the formation of these primordial objects was made in the context of three models of dark energy as background: Quintessence, ?CDM(Cosmological Constant plus Cold Dark Matter) and Phantom. We show that the dark matter is the fundamental agent for the formation of the structures observed today. The dark energy has great importance at that epoch of its formation / Entender o modo pelo qual estruturas em grande-escala, como gal?xias, se formam permanece um dos problemas mais desafiadores em cosmologia hoje. A teoria padr?o para a origem destas estruturas ? que elas cresceram por instabilidade gravitacional de pequenas, talvez geradas quanticamente, flutua??es na densidade de mat?ria escura, b?rions e f?tons sobre um Universo primordial uniforme. Depois da recombina??o, os b?rions come?aram a cair nos po?os de potencial gravitacional pr?-existentes de mat?ria escura. Nesta disserta??o ? feito inicialmente um estudo da era da recombina??o primordial, a ?poca da forma??o dos ?tomos de hidrog?nio neutro. Al?m disso, analisamos a evolu??o do contraste de densidade (de mat?ria bari?nica e escura), em nuvens de mat?ria escura com massas entre milhares e bilh?es de massas solares. Em particular, na an?lise da componente bari?nica, levamos em conta os v?rios mecanismos f?sicos que ocorrem nela durante e depois da era da recombina??o. A an?lise da forma??o desses objetos primordiais foi feita no contexto de tr?s modelos de energia escura como background ': Quintess?ncia, CDM (Constante Cosmol?gica mais Mat?ria Escura Fria) e Fantasma. Mostramos que a mat?ria escura ? o agente fundamental para forma??o das estruturas observadas hoje. A energia escura tem grande import?ncia na ?poca de sua forma??o

Nesta tese, utilizamos técnicas de química quântica para o estudo de sistemas contendo átomos de silício e nitrogênio. Nesse contexto, o trabalho aqui apresentado procurou concentrar-se em três tópicos principais: aglomerados de silício e nitrogênio, a superfície Si(100) e a interação de um átomo de nitrogênio com essa superfície. Estudamos inicialmente os aglomerados de fórmula SiNN e Si3N2, onde procuramos caracterizar de forma rigorosa a estabilidade e a natureza das ligações químicas das várias espécies. Com relação ao sistema SiNN, os resultados utilizando cálculos de alto nível mostraram a necessidade de novos dados experimentais para a caracterização inequívoca dessa espécie. Além disso, obtivemos os primeiros resultados para as propriedades de quatro novas estruturas. Estudamos a estrutura de várias espécies com fórmula Si3N2, e obtivemos como mínimo global uma estrutura planar contendo apenas ligações Si-N. Para a simulação da superfície Si(100) utilizamos os aglomerados de fórmula Si9H12 e Si15H16. Apenas a metodologia do Funcional da Densidade indicou a existência de uma estrutura distorcida para o Si9H12. Os cálculos CASSCF mostraram a necessidade do uso de funções multideterminantais. Com relação ao aglomerado Si15H16, obteve-se uma estrutura distorcida com os dímeros alternados como a forma mais estável. Entretanto, o uso do CASSCF mostrou ser a forma simétrica a mais estável. Com base no estudo dos aglomerados Si9H12 e Si15H16, partimos para a investigação dos mecanismos envolvidos no processo de interação e incorporação de átomos de nitrogênio na superfície de silício. Estruturas com simetria de spin quarteto e dupleto foram estudadas, sendo os mecanismos bastante semelhantes, com o nitrogênio interagindo inicialmente com um dos silícios dímeros para em seguida poder formar duas outras estruturas, uma com o nitrogênio ligado aos dois silícios dímeros e outra com o nitrogênio inserido no interior do aglomerado. Esta última estrutura corresponde à forma mais estável. Os resultados mostram claramente que a reação é bastante favorável do ponto de vista energético. Questionamos também o uso de vínculos durante a otimização das estruturas por impedir um devido relaxamento apropriado dos átomos de silício da primeira e segunda camadas. / In this thesis, quantum chemical techniques were used to study molecular systems containing silicon and nitrogen atoms. ln this context, the work was concentrated on three main topics: silicon-nitrogen clusters, the Si(100) surface, and the interaction of a nitrogen atom with that surface. We studied initially the clusters SiNN and Si3N2, where we tried to characterize with rigour the stability and the nature of the chemical bonds of the various species. Concerning the system SiNN, using high level calculation, the results showed the necessity of new experimental data for the unequivocal characterization of this species. Moreover, the properties of four new structures were described for the first time in this study. The structures of various species with formula Si3N2 were studied, and the global minimum corresponds to a planar structure containing only Si-N bonds. For the simulation of Si(100) surface, the clusters Si9H12 and Si15H16 were used. Density Functional Theory predicted the existence of an asymmetric geometry for the Si9H12 cluster. The CASSCF calculations showed the necessity of using multideterminantal wave functions. Concerning the Si15H16 cluster, the most stable structure corresponds to a distorted one with alternated dimers. However, the CASSCF methodology showed that the symmetric structure is the most stable. Based on the results for the Si9H12 and Si15H16 clusters, we started to investigate the mechanism involved in the interaction and incorporation of a nitrogen atom into the silicon surface. Structures with quartet and doublet spin symmetry were studied, being their mechanisms very similar, with the nitrogen atom interacting initially with one silicon dimer; after that two other structures can be formed, one with the nitrogen bonded to two silicon dimers, and the other with the nitrogen bonded to one silicon dimer and two internal silicons. This last structure corresponds to the most stable species. The results show clearly that the reaction is very favorable energetically. The use of geometrical constraints during the optimization was questioned since it prevents the proper relaxation of the silicon atoms in the first and second layers.