In the last decade, ultracold plasmas have been created in the laboratory by photo-ionizing laser-cooled atoms. To understand the overall dynamics of ultracold plasmas, one needs to understand Rydberg collisional processes at ultracold temperatures. The two kinds of problems addressed in this thesis are: study of Rydberg atomic processes at ultracold temperatures, and a study of the overall dynamics of the ultracold plasmas. Theoretical methods based on quantal-classical correspondence is used to understand Rydberg atomic processes such as radiative cascade, and radiative recombination. A simulation method suitable for ultracold collisions is developed and tested. This method is then applied to study collisional-Stark mixing in Rydberg atoms. To study the dynamics of the ultracold plasmas, a King model for the electrons in plasmas is proposed. The King model is a stationary, finite sized electron distribution for the electrons in a cloud of fixed ions with a Gaussian distribution. A Monte-Carlo method is developed to simulate the overall dynamics of the King distribution.

Electric field studies, electrochromism, were used to obtain excited-state data for analogous divalent sulfur compounds. The sulfides investigated were dimethyl sulfide and small cyclic sulfides including the three to six member ring compounds. The excited-state dipole moments and polarizabilities are reported for the first s, p, and d Rydberg absorption bands which occur in the near vacuum ultraviolet region from 230 to 170 nm. The excited-state data are interpreted in terms of the particular excited-state (s, p, or d) for the molecules and the bending differences due to the presence of the ring and the number of atoms in the ring. The next section describes the use of electrochromism to investigate the pressure effect of argon, carbon tetrafluoride and sulfur hexafluoride on the spectra for molecular Rydberg states.

Made available in DSpace on 2016-12-12T20:15:49Z (GMT). No. of bitstreams: 1 Claudia Schmidt.pdf: 341536 bytes, checksum: e474da47db5271f6ff692ba3d3603ea9 (MD5) Previous issue date: 2012-07-31 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Rydberg atoms have as one of their characteristic the high principal quantum number. One consequence is the dimension of the order n2 implying in a large dipole moment, which allows one to use them for studies of atomic interactions with electromagnetic fields, including processes of photoionization. The increasing attention given to the investigation of photoionization cross sections of these highly excited atoms, is due to its importance to several areas like Atomic and Molecular Physics, Astrophysics, Plasma Physics, among others. Based on the model proposed by Aymar and co-workers, we studied the photoionization cross sections of some alkali atoms, namely, Sodium, Potassium, Rubidium and Cesium, expanding the previous analysis to n < 44. Since the photoionization processes can be enormously affected by the photoelectron energy, the we performed an analysis of the behaviour of radial wave functions depending on the photoelectron energies. And, this analysis, also studied the possible evidence of the Cooper minima. / Átomos de Rydberg têm como uma de suas principais características o alto numero quântico principal. Uma das conseqüências é a sua dimensão da ordem n2, implicando em um grande momento de dipolo, o que permite usá-los para estudos de interações atômicas com campos eletromagnéticos, incluindo processos de fotoionização. A atenção crescente dada a investigação de seções de choque de fotoionização desses átomos altamente excitados, é devido a sua importância para diversas áreas como Física Atômica e Molecular, Astrof&#305;sica, Física de Plasma, entre outros. Com base no modelo proposto por Aymar e colaboradores, estudamos as seções de choque de fotoionização de alguns átomos alcalinos, ou seja, Sódio, Potássio, Rubídio e Césio, ampliando a analise anterior para n < 44. Uma vez que os processos de fotoionização podem ser enormemente afetados pela energia dos fotoelétrons, realizou-se uma analise do comportamento das funções de onda radiais em função das energias do fotoelétron. E, desta analise, também estudamos a possível evidencia dos mínimos Cooper.

Neste trabalho estudamos a interação entre átomos de Rydberg no estado nP e sua dependência com o campo elétrico dc. Estes estados apresentam ressonâncias Föster para um número quântico principal n menor que 37. Nestes processos de ressonância um par atômico no estado nP muda para um par nS+(n+1)S. Realizamos dois experimentos de evolução temporal para 32&le;n&le;36. No primeiro investigamos a dependência da taxa de transferência de população NnS em função do número quântico principal n. E no segundo estudamos a transferência de população para um estado fixo de n=33 em função do campo elétrico. Além disso, estudamos a dependência da população no estado 33S em função da densidade de átomos de Rydberg no estado 33P. Estes resultados nos permitem observar duas contribuições distintas, uma linear relacionada a radiação de corpo negro e uma quadrática ligada a interação de dois corpos. Estes resultados confirmam o modelo de taxa para o efeito da radiação de corpo negro. / In this work we studied the role of Rydberg atoms interactions in the nP state and the dc electric field dependency of this process. The nP state shows Föster resonances for principal quantum number less than 37. In this resonance process, an atomic pair in nP state changes to a pair nS+(n+1)S. We have performed two time evolution experiments for 32&le;n&le;36. In the first one we have investigated the NnS population transfer rate for a variable principal quantum number n. In the second we have study the population transfer for a fixed n=33, by varying an electric field. Moreover, we observed the density dependency of the population in the 33S state by varying the nP state atomic density. The results allow us to observe two distinct contributions, a linear contribution related with the black body radiation and a quadratic one connected with two body process. The results agree well with the rate model used to treat the black body radiation.

We investigate chaotic ionization of highly excited hydrogen atom in crossed electric and magnetic fields (Rydberg atom) and intra-molecular relaxation in planar carbonyl sulfide (OCS) molecule. The underlying theoretical framework of our studies is dynamical systems theory and periodic orbit theory. These theories offer formulae to compute expectation values of observables in chaotic systems with best accuracy available in given circumstances, however they require to have a good control and reliable numerical tools to compute unstable periodic orbits. We have developed such methods of computation and partitioning of the phase space of hydrogen atom in crossed at right angles electric and magnetic fields, represented by a two degree of freedom (dof) Hamiltonian system. We discuss extensions to a 3-dof setting by developing the methodology to compute unstable invariant tori, and applying it to the planar OCS, represented by a 3-dof Hamiltonian. We find such tori important in explaining anomalous relaxation rates in chemical reactions. Their potential application in Transition State Theory is discussed.

Neste trabalho estudamos a interação entre átomos de Rydberg no estado nP e sua dependência com o campo elétrico dc. Estes estados apresentam ressonâncias Föster para um número quântico principal n menor que 37. Nestes processos de ressonância um par atômico no estado nP muda para um par nS+(n+1)S. Realizamos dois experimentos de evolução temporal para 32&le;n&le;36. No primeiro investigamos a dependência da taxa de transferência de população NnS em função do número quântico principal n. E no segundo estudamos a transferência de população para um estado fixo de n=33 em função do campo elétrico. Além disso, estudamos a dependência da população no estado 33S em função da densidade de átomos de Rydberg no estado 33P. Estes resultados nos permitem observar duas contribuições distintas, uma linear relacionada a radiação de corpo negro e uma quadrática ligada a interação de dois corpos. Estes resultados confirmam o modelo de taxa para o efeito da radiação de corpo negro. / In this work we studied the role of Rydberg atoms interactions in the nP state and the dc electric field dependency of this process. The nP state shows Föster resonances for principal quantum number less than 37. In this resonance process, an atomic pair in nP state changes to a pair nS+(n+1)S. We have performed two time evolution experiments for 32&le;n&le;36. In the first one we have investigated the NnS population transfer rate for a variable principal quantum number n. In the second we have study the population transfer for a fixed n=33, by varying an electric field. Moreover, we observed the density dependency of the population in the 33S state by varying the nP state atomic density. The results allow us to observe two distinct contributions, a linear contribution related with the black body radiation and a quadratic one connected with two body process. The results agree well with the rate model used to treat the black body radiation.

Les faisceaux d’électrons et d’ions sont au cœur de nombreuses techniques instrumentales servant à explorer, analyser et agir sur des matériaux à l’échelle du micromètre au nanomètre (Microscopie électronique, spectrométrie d’électrons, techniques de « FIB »). Les limites de résolution spatiale et énergétique de ces techniques dépendent en grande partie des propriétés des sources qu’elles utilisent et en particulier de leur température de fonctionnement. De fait, depuis plus de 10 ans, le potentiel des atomes froids ionisés comme nouveau type de source d’électrons ou d’ions est intensivement exploré.Le projet expérimental réalisé au LAC et décrit dans cette thèse utilise un jet d’atomes de césium issu d’un piège magnéto-optique à deux dimensions. La température transverse du jet est de l’ordre de 100 µK. Malgré cela, le jet est encore trop divergent après la sortie de la zone de refroidissement pour notre expérience. Afin guider le jet d’atomes jusqu’à la zone d’ionisation, nous avons étudié une méthode particulière de guidage dipolaire. L’utilisation d’un seul laser convenablement réglé nous a permis de guider et pousser les atomes du jet en même temps tout en limitant le chauffage. Nous avons ainsi pu compresser avec ce laser pousseur-guideur le jet d’atomes sur un diamètre de 400 µm à 60 cm de la zone de refroidissement du PMO-2D.Le jet est ensuite ionisé par la méthode d’ionisation en champ électrique statique d’atomes de Rydberg. Les atomes sont tout d’abord excités par laser sur un état de Rydberg (n~30) en présence d’un champ électrique uniforme et homogène. Les atomes du jet ainsi excités voyagent vers une zone présentant un fort gradient de champ où ils vont alors s’ioniser autour de la même valeur de potentiel, réduisant ainsi la taille de la zone d’ionisation et donc de la dispersion en énergie potentielle initiale du faisceau d’électron. La probabilité d’ionisation des atomes dans le champ dépend grandement de l’état de Rydberg préalablement excité. Le choix de l’état de Rydberg optimal, i.e. qui a une probabilité d’ionisation la plus grande possible, nécessite une étude de l’ionisation des états de Rydberg du césium. Un modèle à deux niveaux est présenté dans cette thèse qui permet de retrouver le comportement d’ionisation d’état de Rydberg observé expérimentalement. Ce modèle simple nous a permis de comprendre quel type d’état nous devions exciter. Enfin une étude expérimentale est également présentée. / Electron and Ion beams are at the base of many instrumental techniques used to explore, to analyse and to modify materials from the micrometer to the manometer scale (Electronic Microscopy, Electron Spectrometry, Focused Ion beams techniques…). Spatial and Energetic resolutions of these techniques are strongly dependent on its source‘s properties and particularly their working temperature. In fact, for more than ten years, the potential of ionised cold atoms have been intensively studied. Our experiment at LAC, described in this thesis, uses a 2 dimensional magneto-optical trap (2D-MOT) to create a caesium atomic beam. The transverse temperature of the beam is around 100 µK. Despite this, the beam is still too divergent after exiting the cooling area. To guide the atomic beam up to the ionisation area, we have studied and implemented a particular method of dipolar guiding. The use of a unique laser properly set allowed us to push and guide altogether the atoms of the beam while limiting the heating effect. Thus, we have managed to compress the atomic beam’s size to 400 µm at 60 cm from the output of the MOT.Afterward, the atomic beam is ionised by the method of Rydberg (static) field ionisation. The atoms are firstly excited by laser on a Rydberg state (n~30) as a static homogeneous and uniform electric field is applied. The excited atoms of beam travel therefore to a high-gradient field area where they ionise around the same electric potential value, therefore reducing the ionisation area’s size and the initial potential energy spread of the electron beam. The ionisation probability of the atoms in the field depends greatly on the excited Rydberg state. The choice of an optimal Rydberg state , i.e. with the highest probability of ionisation, needs better knowledge of the ionisation of cesium Rydberg states. A two levels model us to describe the ionisation behaviour of some Ryberg. This simple models helps to understand what kind of states we want to excite in order to optimise the ionisation area‘s size. An experimental study of cesium Rydberg states is also presented.