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Differential and momentum-transfer cross sections for elastic e−–CH2O scattering in the 16–80 eV as well as differential and integral cross sections for the X1A1→a3A2 excitation by electron impact in the 20–80 eV range are reported. The Schwinger variational iterative method is applied to solve the elastic scattering equations, whereas the distorted-wave approximation combined with that method is applied to calculate cross sections for the inelastic process. There is a good agreement between our excitation cross sections with other theoretical data available in the literature. This comes in support of the reliability of our results for the elastic process.

Elastic differential, integral and momentum transfer cross-sections for electron-H2S collisions are reported at impact energies ranging from 2 to 50 eV. The Schwinger variational iterative method in a fixed-nuclei static-exchange approximation is used to calculate the low partial-wave scattering amplitudes. Higher partial-wave contributions were taken into account through the Born approximation with a point-dipole potential. Comparison of our calculated cross-sections with recent experimental and theoretical results is encouraging.

Elastic differential and momentum transfer cross sections are reported for electron scattering by H2O at impact energies ranging from 4 to 50 eV. The iterative Schwinger variational method in the fixed-nuclei, static-exchange approximation is used to calculate the low partial wave scattering amplitudes and the higher partial wave contributions are included via closure using the Born approximation for a point-dipole. Comparison of our calculated cross sections with recent experimental and other theoretical results is encouraging.

The authors report rotationally unresolved vibro-electronic cross sections and branching ratios associated with the X 1 Sigma +g to d 3 Pi u electronic transition in H2, using the distorted-wave approximation within the adiabatic-nuclei framework. The present study shows that the Franck-Condon approximation (FCA) can provide reliable cross sections. The calculated branching ratios associated with the vibro-electronic transitions exhibit some oscillations at low energies; however, their magnitudes agree well with the FCA predictions.

The authors report calculated differential and integral cross sections for e-C2H2 collisions in the 10-200 eV energy range. These cross sections were derived from fixed-nuclei scattering amplitudes in such a way that the low angular momentum components are obtained using the Schwinger iterative method and the large ones are treated by a Born closure. Using this combined theory, it can be shown that the elastic differential cross sections obtained at the static-exchange level are nearly exact, even at higher energies. The inclusion of a semi-empirical polarization potential is discussed as well.

The validity of the distorted-wave approximation for e--H2 scattering is tested through a comparative study with the two-channel Schwinger multichannel method. Calculated differential cross sections are reported for the excitations to b 3 Sigma u+, a 3 Sigma g+ and c 3 Pi u states in the energy range covering from near threshold up to 30 eV. General good agreement is observed between both theories for incident energies above 20 eV.

The authors report differential cross sections for the X 1 Sigma +g to b 3 Sigma +u, a 3 Sigma +g and c 3 Pi u excitations of H2 by electron impact at 20 eV. They are particularly interested in studying correlation effects of the target on these cross sections. Comparisons are made with available theoretical and experimental results.

We report a theoretical study on the photoionization of phosphine in the static-exchange level and frozen core approximation, using the method of continued fractions. The main subject of the present study is to investigate in which extent the Hartree-Fock description of the target applied to molecular photoionization is valid. Also, the role played by multichannel coupling is analysed. Our study shows that single-channel Hartree-Fock calculations can provide reliable results except for photon energies near the photoionization threshold. Apresentamos um trabalho teórico sobre a fotoionização da fosfina ao nível estático-troca e da aproximação de caroço congelado, usando o método de frações continuadas. O objetivo principal do presente estudo é investigar até que ponto a descrição Hartree-Fock do alvo é válida, quando aplicada em estudos de fotoionização molecular. Também analisamos o papel desempenhado pelos acoplamentos multicanais. Nosso estudo mostra que cálculos Hartree-Fock monocanais podem fornecer resultados confiáveis, exceto para energias do fóton próximas ao limiar da fotoionização.

The coherent renormalised multicentre potential model including intramolecular multiple scattering has been extended, for the first time to study the vibrationally elastic and inelastic scattering of electrons by linear molecules in the intermediate energy range. The authors' calculated vibrationally elastic cross sections for e--H2 scattering agree very well with experiments and the vibrational excitation cross sections agree qualitatively with the measurements. Their results also show that the inclusion of the multiple-scattering effect in the calculation is important at lower incident energies.

The authors report experimental elastic differential, integral and momentum transfer cross sections for electrons scattered by Ar and O2 in the impact energy range 300-1000 eV and in the angular range 5-140 degrees . In their experiments, the intensities of the elastically scattered electrons were obtained in the crossed-electron-beam-molecular-beam geometry and the absolute values of the cross section were derived through the relative-flow technique using N2 as the secondary standard. For both gases, the cross sections are compared with the available experimental and theoretical results.

Ratios of elastic differential cross sections of CO2 to those of N2 have been measured at electron impact energies of 500, 800 and 1000 eV in the angular range of 5 degrees to 120 degrees , using a crossed electron-beam-molecular-beam geometry and the relative flow technique. These ratios have been multiplied by previously known N2 elastic differential cross sections to obtain elastic differential cross sections for CO2. At 500 eV, the authors' experimental results agree quite well with the absolute values of Bromberg (1974). Comparison has also been made with the theoretical results obtained in the renormalised multicentre potential model approach of Botelho et al. and shows an agreement within 20% in the entire angular range. The elastic integral and momentum transfer cross sections have also been determined from the differential cross sections.

Absolute total cross sections for electron scattering on Ar have been measured over the energy range of 0.5-3.0 keV. The correction due to the small-angle scattering was carried out using the theoretical differential elastic and inelastic cross sections. The corrected total cross sections agree well with the earlier experimental and theoretical results. The comparison with the high-energy Born theory is discussed.

The electron-impact excitation and dissociation cross sections for the C 1Πu, c 3Πu, B′ 1Σu+, and E(F) 1Σg+ states of H2 have been calculated within the distorted-wave approximation. The distorted waves are obtained as solutions of the static-exchange potential field of the ground electronic state. Both differential and integral inelastic cross sections are reported and compared with other calculated results and available experimental data.

Distorted-wave cross sections for the excitation of eight low-lying excited states (1,3 Sigma u+, 1,3 Pi g, 1,3 Pi u and 1,3 Delta u) of CO2 by electrons in the 25 to 60 eV energy range are presented. The authors report both differential and integral cross sections.

The Stieltjes-Tchebycheff momentum theory (STMT) has been applied to study the non-Franck-Condon effect in the photoionisation of 3 sigma g in N2. The cross sections for the photoionisation process which leads to the production of the N2+(X2 Sigma g+) ion in the final vibrational states v'=0, 1 and 2 are calculated. The obtained branching ratios show good agreement with the experimental data. Comparison is also made with the results from other theoretical studies.

The elastic scattering of intermediate and high energy electrons with N2 and CO molecules is investigated in a two-potential coherent approach in which the intramolecular scattering effects are also considered. The short-range static potentials centred at both the atoms are derived from target molecular wavefunctions. Results are compared with recent measurements and the corresponding calculations using Yukawa-type atomic scattering factors. The new results are in better accord with experiment.

The elastic scattering of intermediate and high energy electrons by the CO2 molecule is investigated in the two-potential coherent approach of Hayashi and Kuchitsu (1976a). The short-range static potential is derived directly from the target molecule wavefunction within the renormalised multicentre potential formalism of Lee and Freitas (1981a). Comparison of the calculated differential, integral and momentum transfer cross sections with the experimentally measured data is made and shows a good agreement.

Distorted-wave cross sections are presented for the excitations of the w /sup 1/..delta../sub u/, W /sup 3/..delta../sub u/, A /sup 3/..sigma../sub u//sup +/, b' /sup 1/..sigma../sub u//sup +/, and c' /sup 1/..sigma../sub u//sup +/ states of N/sub 2/ by low-energy electron impact. The distorted waves are obtained in the static-exchange field of the ground electronic state. Differential and integral cross sections are presented from near threshold up to 60-eV impact. Comparison is made with available experimental data and with other calculations.

Discusses a variational functional for scattering theory which has been recently proposed by Takatsuka and McKoy (1980). It is shown that this functional can provide results with a purely discrete set of functions which are approximately equivalent to those obtained by Lucchese et al. (1980) from the first iteration of the iterative Schwinger method. Applications to the scattering of electrons by systems including CO+ and LiH illustrate this relationship and other features of the method.

The authors present distorted-wave cross sections for excitation of the A 1 Pi , a 3 Pi , a' 3 Sigma +, D 1 Delta and d 3 Delta states of CO by electrons in the 20 to 50 eV energy range. In these studies both the initial and final distorted waves are obtained in the static-exchange field of the ground electronic state. Differential and integral cross sections are presented and compared with available experimental data and with other calculations. The calculated differential cross sections for the A 1 Pi and a 3 Pi states agree poorly in magnitude, but better in shape, with available experimental data. In general the distorted-wave cross sections differ substantially from those of plane-wave-type theories such as the Born-Ochkur-Rudge approximation.

Neste trabalho, realizamos um estudo teórico do espalhamento elástico e inelástico de elétrons por moléculas de H2. A aproximação de ondas distorcidas em segunda ordem e o método de frações continuadas multicanal foram aplicados no cálculo de seções de choque diferencial e integral (SCD e SCI) destes processos. Na aproximação de ondas distorcidas em segunda ordem, os elementos da matriz-T em primeira ordem bem como as funções de onda distorcidas foram geradas em um potencial estático-troca do estado fundamental do alvo e o método de frações continuadas ao nível monocanal foi usado para calcular a função de Green completa de ondas distorcidas. Para o espalhamento elástico, SCD e SCI na faixa de energia do elétron incidente de 1-40eV foram calculadas. Para o espalhamento inelástico, estudamos transições a partir do estado fundamental da molécula de H2 para os três primeiros estados tripletos b3 Σ+u, a3 Σ+g e c3πu e para o estado singleto B1 Σ+u para energias do elétron incidente iguais a 20 e 30eV. Nos estudos com o método de frações continuadas multicanal, foram calculadas SCD e SCI a 2 e 5 canais para transições a partir do estado fundamental da molécula de H2 para os estados tripletos tripletos b3 Σ+u, a3 Σ+g e c3πu e para os estados singletos B1Σ+u, E1Σ+g e C1>πucom energias do elétron incidente na faixa de 15-40eV. A comparação entre os resultados obtidos com a aproximação de ondas distorcidas em segunda ordem e o método de frações continuadas multicanal nos fornecerá informações sobre a convergência da série de ondas distorcidas em cálculos de seção de choque para o espalhamento inelástico e- - H2 In this work, we report a theoretical study on elastic and inelastic electron scattering by H2 molecules. The second-order distorted-wave approximation and the multichannel formulation of the method of continued fractions were applied on the evaluation of differential and integral cross sections (DCS and ICS) of these processes. In the second-order distorted-wave approach, first-order T-matrix elements as well as distorted-wave functions were generated from the static-exchange potential of the ground state of the target and the method of continued fractions in a single-channel formulation was applied to the evaluation of the full distorted-wave Green\'s function. For elastic scattering, we have calculated DCS and ICS in the 1-40 eV incident energy range. For inelastic scattering, electronic excitation cross sections for the X1 Σ+g,→ b3 Σ+u, a3 Σ+g, c3πu and B1 Σ+u transitions in H2 by electron impact were obtained at 20 and 30 eV. In the studies with the multichannel formulation of the method of continued fractions, we have calculated DCS and ICS in both two- and five-channel formulation for transitions from the ground state of H2 to the triplet states b3Σ+u, a3Σ+g and c3πu, and to the singlet states B lEu+, E1 Eg9 and CI IIu in the 15 40 eV energy range. The comparision of the results from both methods can provide significant informations about the convergence of the distorted-wave series in inelastic e- H2 scattering calculations.

Nesse trabalho investigamos as propriedades de sistemas Coulombianos de três corpos usando coordenadas hiperesféricas. A partir da quasi-separabilidade da equação de Schrodinger nessas coordenadas desenvolvemos um procedimento que permite uma descrição unificada de sistemas atômicos, moleculares e íons exóticos. O método, denominado procedimento hiperesférico adiabático, permite o cálculo das propriedades do estado fundamental com grande precisão bem como, é adequado a análise de estados excitados, ressonâncias no continuo e funções de espalharnento. Novas técnicas de solução das equações diferenciais foram desenvolvidas de forma a incorporar efeitos de centro de massa a sistemas de massas díspares. Conseqüentemente foi possível a aplicação desta metodologia ao problema de excitons aprisionados em semicondutores, a mesomoléculas e a íons exóticos. Finalmente desenvolvemos uma nova técnica para o cálculo das curvas de potencial baseadas na construção de uma família de funções trancedentais ortogonais. O átomo de hélio é usado para o teste da eficiência do método Coulombic three-body systems are investigated using the hyperspherical adiabatic approach. By using a suitable z = tg(&#945/2) angular differential equation for the determination of the potencial curves, we are able to obtain stable series expansion solutions, valid for small and large values of the hyperspherical radius. The analysis of the mathematical of the differential equations in the variable z, offers an insight into the physics of the problem and into the determinaion of stable converging solutions as well. In order to illustrate our investigation we apply this study to several carefully chosen systems: He, dd&#956, d2+, and éxcitons boun Coulomb Center in different semiconductors. Eigenenergies are obtained and compared with other methods. In this work we make use of a recently proved set of basic inequalities which provide, for the first time, a lower bound/upper bound for the ground state energies of the selected systems

Neste trabalho, estudamos a fotoassociação de átomos de rubídio no regime de curto alcance. Realizamos espectroscopia por perda de átomos em uma armadilha magneto-óptica de 85Rb usando um laser de fibra de alta potência, o qual possuia largura de linha da ordem de 1MHz e 50W de potência no intervalo de 1060nm a 1070nm. Dois níveis vibracionais do potencial excitado 0u+ foram observados (v=137 e v=138). Além disso, medimos o tempo de vida de uma armadilha óptica de dipolo cruzada. Como esperado, o tempo de vida é menor quando o laser está sintonizado na ressonância. Um modelo teórico prevê a distribuição dessas moléculas nos níveis vibracionais do estado eletrônico fundamental após o processo de fotoassociação. Os resultados, nos sugerem perspectivas para a produção de moléculas de Rb2 no estado vibracional fundamental. In this work, we studied short-range photoassociation of rubidium atoms. We realize trap-loss spectroscopy in a magneto-optical trap of 85Rb using high power fiber laser, which had around 1MHz linewidth and 50W power at 1060nm to 1070nm interval. Two vibrational levels of the 0u+ excited potential were observed (v=137 e v=138). Besides that, we measured the lifetime of a crossed optical dipole trapped. As expected, the lifetime is shorter when the laser is tuned on resonance. A theoretical model predicts the molecular distribution in the vibrational levels of electronic ground state. The results suggest us perspectives to produce Rb2 molecules in the ground vibrational state.

Esta tese é composta por duas partes. Na primeira parte, os efeitos da interação elétron-elétron nas configurações do estado fundamental de dois anéis quânticos acoplados (CQRs) são estudados. Os CQRs podem formar um novo tipo de molécula artificial, onde o raio dos anéis juntamente com a distância entre os anéis, são novos parâmetros ajustáveis que fornecem novos graus de liberdade para controlar a estrutura eletrônica destas moléculas. Através da bem estabelecida teoria do funcional densidade dependente de spin, as configurações ou fases do estado fundamental dos CQRs com alguns elétrons são determinadas. Uma rica variedade de fases para o estado fundamental destas novas moléculas artificiais é encontrada para sistemas contendo até N=13 elétrons. Para CQRs com N menor ou igual a 8 são obtidas qualitativamente configurações para o estado fundamental similares às dos pontos quânticos acoplados (CQDs). As novas configurações eletrônicas aparecem para N maior ou igual a 9. Na segunda parte desta tese é desenvolvido um método numérico para estudar o espalhamento eletrônico através de um ponto quântico com N-elétrons confinados. Considera-se que o ponto quântico está imerso num sistema bidimensional ou confinado em um canal unidimensional. As taxas de espalhamento são obtidas resolvendo iterativamente a equação de Lippmann-Schwinger incluindo a interação elétron-elétron entre o elétron incidente e os N-elétrons confinados dentro do QD. Para exemplificar, este método é aplicado para um elétron externamente injetado sobre um QD contendo um único elétron. As taxas de espalhamento elástico, inelástico e de spin-flip são obtidas. Os efeitos da interação de troca no espalhamento eletrônico e transporte através do QD são analisados.Também são considerados os processos do espalhamento multi-canal neste sistema e suas influências nas propriedades de transporte. This thesis is composed of two parts. In the first part, we study the effects of electron-electron interactions on the ground state configurations of two vertically coupled quantum rings (CQRs). The CQRs can form a new type of artificial molecule (AM) where the ring radius together with the inter-ring distance are new tunable parameters providing new degrees of freedom to modulate and control the electronic structure of the artificial ring shaped molecules. In this work, we apply the well established spin-density functional theory to study the ground state configurations or phases of few-electron CQRs. A rich range of ground state phases of these new quantum ring AMs is uncovered for systems containing up to N = 13 electrons. For CQRs with N less or equal to 8 we found qualitatively similar ground state phases as for coupled quantum dots (CQDs). Novel phases appear for N greater or equal to 9. In the second part of this thesis, we develop a numerical method to study the electron scattering through an occupied quantum dot (QD) with a few electrons. The QD is considered embedded in a two-dimensional system or confined in a one-dimemsional channel. An external electron is injected and scattered through the QD. The scattering rates are obtained by solving iteratively the Lippmann-Schwinger equation including the electron-electron interactions between the incident electron and the N-electrons confined in the QD. As an example, we apply this model for an externally injected electron through a QD with one electron inside. The elastic, inelastic, and spin-flip scattering rates are obtained. The effects of electron exchange interaction on the electron scattering and transport through the QD are analyzed. We also show the multi-channel scattering processes in such systems and their influences on the electron transport properties.

O método adiabático hiperesférico (HAA) é usado para a determinação das energias e funções de onda dos estados ligados de um íon negativamente carregado (D-), na presença de um campo magnético, em semicondutores. Experimentalmente, a energia de ligação desse sistema é medida com o íon confinado em poços de potencial, ou seja, o sistema é quasi-bidimensional, mas, nesse trabalho é usada a aproximação teórica na qual o sistema é considerado como sendo estritamente bidimensional. Usando uma variável angular hiperesférica modificada, curvas de potencial são obtidas analiticamente, permitindo um cálculo preciso dos níveis de energia deste sistema. O método permite a determinação de um limite superior e um inferior para as energias, cujos valores têm sido obtidos por um procedimento não-adiabático. Os resultados têm precisão comparável aos resultados variacionais encontrados na literatura. The adiabatic hyperspherical approach (HAA) is used to determine the energies and wave functions of the bound states of a negatively charged ion (D-), in the presence of a magnetic field, in semiconductors. Experimentally, the bound energy of this system is measured with the ion confined in quantum wells, or either, the system is quasi-two dimensional, but, in this work is used the theoretical approach in which the system is considered as being strict1y two dimensional. U sing a modified hyperspherical angular variable, potential curves are analytically obtained, allowing an accurate calculation of the energy levels of this system. The method allows to the determination of an upper and lower limit for the energies, whose values have been gotten by a nonadiabatic procedure. The results have comparable accuracy to the variational results found in literature.

Embora o fenômeno da fotodissociação (dissociação de moléculas devido à interação com um campo externo dependente do tempo) venha já há muito tempo atraindo a atenção dos pesquisadores, esta ainda longe de ser completamente compreendido. Este problema é de difícil tratamento teórico por se tratar não apenas de um problema quântico de vários corpos, mas que apresenta também dependência temporal. Este trabalho tem como alvo de estudo a evolução temporal da função de onda de uma molécula diatômica sujeita a um campo externo dependente do tempo, servindo-se para isso do potencial de Morse. Este potencial unidimensional descreve razoavelmente bem os níveis vibracionais de moléculas diatômicas e pode ter seus parâmetros ajustados de forma a representar varias moléculas. O estado fundamental do oscilador é perturbado pelo campo e a função de onda é propagada através de diferentes métodos, que são comparados entre si. É interessante notar que as partes real e imaginária da função de onda começam a oscilar, mostrando que o pacote esta ganhando energia cinética, enquanto que a densidade de probabilidade permanece inicialmente quase inalterada. E discutido um efeito similar ao Stark, devido ao fato da variação temporal do campo externo ter sido assumida proporcional a um co-seno. O princípio do processo dissociativo, com as funções de onda se estendendo para maiores valores da coordenada espacial, é observado com o prosseguimento da propagação a tempos maiores Although the phenomenon of photodissociation (dissociation of molecules due to the interaction with an external time-dependent field) has been for a long time attracting scientists\' attention, it is yet far from being completely understood. Theoretical approach to this process is difficult not only because it is a many-body quantum problem, but also due to the time dependence of the external field. The main goal of this work is to study the time evolution of a diatomic molecule in the presence of an external time-dependent field, using the Morse potential. This unidimensional potential describes reasonably well the vibrational levels of diatomic molecules and may have its parameters adjusted in order to represent several molecules. The ground state is perturbed by the field and the wavefunction propagated through a few methods, which are compared among them. It is interesting to notice that the real and imaginary parts of the wavefunction start to oscillate, showing that the packet is gaining kinetic energy, while the probability density initially remains practically still. An effect similar to the Stark one, due to the fact that the time oscillation of the external field has been assumed proportional to a co-sine, is discussed. The beginning of the dissociation process, with the wavefunctions extending to greater values of the spatial coordinate, is observed as the propagation is continued through greater times

A energia não adiabática do estado fundamental para o átomo de hélio é obtida com o formalismo adiabático hiperesférico (HAA). Curvas de potencial, acoplamentos não adiabáticos e funções de canal são calculados por um procedimento numericamente exato baseado em uma expansão analítica das funções de canal. As equações radiais acopladas são resolvidas por técnicas usuais. A convergência do procedimento é investigada conforme os acoplamentos não adiabáticos são sistematicamente introduzidos. Com a inclusão, pela primeira vez, de onze curvas de potencial e funções de canal obtém-se uma energia para o estado fundamental que difere do melhor cálculo variacional por 0.1 partes por milhão. As forças de oscilador para as transições discretas do hélio na \"length-form\" e \"acceleration-form\" também são calculadas. Concluímos que o HAA não está mais obstruído pela falta de uma prescrição para se obter funções de onda de precisão arbitrária para sistemas coulombianos. The nonadiabatic ground state for the helium atom is obtained with the hyperspherical adiabatic approach (HAA). Potential curves, nonadiabatic couplings, and channel functions are calculated by a numerically exact procedure based on the analytical expansion of the channel functions. The coupled radial equations are solved by standard techniques. The convergence of the procedure is investigated as nonadiabatic couplings are systematically introduced. The inclusion, for the first time, of eleven potential curves and channel functions gives a groundstate energy that differs from the best variational calculation by 0.1 parts per million. The oscillator strength for the discrete helium transitions in the length-form and acceleration-form are also presented. We conclude that the HAA is no longer hampered by the lack of prescription for the obtainment of arbitrary precision wave functions for Coulombic systems.

A primeira parte deste trabalho refere-se ao estudo do processo de fotoionização do orbital 3&#963g da molécula de N2. Utilizando a Técnica de Stieltjes-Tchebycheff (TST) calculamos a seção de choque de fotoionização para a formação do íon N+2(X2&#931+g) nos níveis vibracionais &#957\' = O, 1 e 2. Os resultados das seções de choque relativas, definidas como Rx = (&#963v\"=0&#8594v\'/&#963v\"=0&#8594\'=0)o100, obtidos neste trabalho, são comparados com os resultados experimentais disponíveis, bem como com outros teóricos. A boa concordância com a experiência mostra a potencialidade da TST para estudos desta natureza. Na segunda parte calculamos seções de choque e parâmetros de assimetria para a fotoionização da molécula de CO, utilizando o Método Variacional de Schwinger e a Aproximação de Hartree-Fock de Caroço Congelado (FCHF). Diferentemente da TST a função de onda do elétron contínuo (ejetado) é calculada explicitamente. A vantagem de explicitar a função de onda do elétron ejetado reside na possibilidade de se obter a seção de choque diferencial de foto ionização (parâmetro de assimetria), o que não é possível utilizando-se a TST. Os resultados obtidos para a fotoionização dos orbitais de valência externos (5&#963, 1&#960, 4&#963) apresentaram uma boa concordância com os resultados experimentais disponíveis. Entretanto, para o orbital de valência interno (3&#963) e para os orbitais da camada-K (2&#963 e 1&963), a posição do máximo da curva de seção de choque, o qual corresponde à ressonância de forma (shape resonance), apresentou-se deslocada para a região de menor energia em alguns eV. Este deslocamento da posição de máximo da curva deveu-se, provavelmente, à não consideração do efeito de relaxação da densidade de carga eletrônica do íon molecular. As estruturas observadas nos valores experimentais das seçoes de choque de fotoionização, nas regiões próximas do limiar de ionização, são devidas aos elétrons resultantes do processo de autoionização. Todavia tais processos não foram incluídos no presente estudo In the first part of this thesis we report the study of photoionization for the orbital 3&#963g of the N2 molecule. Stieltjes-Tchebycheff Momentum Theory (STMT), we have Using the calculated the cross sections for the ionization process which leads to the formation of the N+2 (X2&#931+g) molecular ion in the vibrational levels &#957\' =0, 1 and 2. The calculated vibrational branching ratios, which are defined as Rx=(&#963v\"=0&#8594v\'/&#963v\"=0&#8594\'=0)o100, are compared with the experimental data and also with the results obtained by other theoretical studies. The good agreement between our results and experimental data shows the potentiality of the STMT for this sort of studies. In the second part we report the study of photoionization of the CO moleculein the Frozen-Core Hartree-Foek Approximation (FCHF) combined with the Sehwinger Variational Method. In this study the scattering wave function of the ejected electron in the CO+ molecular ion potential field is explicitly calculated, so that the differential photoionization cross sections (asymmetry parameters) can be straightforwardly obtained. These parameters cannot be calculated in the STMT. Our results of cross sections for photo ionization of the external valence shells (5&#963, 1&#960, 4&#963) agree quite well with the experimental data. However the maximum of the photo ionization cross section curve (shape resonance) for the inner-valence shell (3&#963) and the K-shell (2&#963 e 1&963) is shifted by a few eV in relation to the experimental data. The shift of the resonance peak is probably caused by the negligence of the relaxation of electronic charge density in the molecular ion. At the photon energies closed to the threshold, there has been observed structures in the measured photoionization cross sections. Nevertheless these structures are attributed to the autoionization processes, which are not included in this study.