Tese (doutorado)—Universidade de Brasília, Instituto de Física, 2016. / Submitted by Fernanda Percia França (fernandafranca@bce.unb.br) on 2017-02-13T14:20:53Z No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Approved for entry into archive by Raquel Viana(raquelviana@bce.unb.br) on 2017-03-22T16:49:32Z (GMT) No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Made available in DSpace on 2017-03-22T16:49:32Z (GMT). No. of bitstreams: 1 2016_DanielDouradodeAragãoSantos.pdf: 11726662 bytes, checksum: 466b96f2b7d83ab56c89e06f864925ea (MD5) / Nesse trabalho investigamos os efeitos da interação feixe-plasma por meio de um modelo hamiltoniano para um sistema composto de N partículas eletrostaticamente acopladas a uma onda. A discretização do sistema em feixes monocinéticos permite fazer uma abordagem perturbativa do problema e destacar importantes conexões entre as formulações hamiltoniana e vlasoviana. Por meio de resultados numéricos, mostramos que tanto o amortecimento como a amplificação de Landau surgem como consequência de um processo de phasemixing dos vários modos normais do sistema linearizado. Observamos que esse mecanismo possui características distintas dependendo se o sistema é estável ou instável. Os resultados obtidos para sistemas de muitos graus de liberdade foram possíveis graças ao desenvolvimento de uma nova técnica para obtenção de raízes complexas. Implementamos simulações de Vlasov via método semi-lagrangiano e descrevemos o processo de relaxação não colisional da distribuição de velocidades das partículas ressonantes e a evolução da intensidade da onda. Analisamos o espectro dos modos não amortecidos em plasmas livres quando se assume uma distribuição de velocidades estacionária com um pequeno (e suave) plateau nas vizinhanças da velocidade de fase da onda. Verificamos que essas oscilações são bastante sensíveis à forma do plateau principalmente quando possuem velocidades de fase próximas à velocidade térmica das partículas. Investigamos também, por meio do cálculo da função dielétrica, as modificações na relação de dispersão dos modos eletrostáticos em plasmas livres e plasmas magnetizados sujeitos a campo externo de radiação. Os cálculos são feitos com base em um modelo semiclássico, em que elétrons são descritos quanticamente e os campos classicamente. / In this work we discuss the effects of the beam-plasma interaction by means of a hamiltonian model for a system composed of N particles electrostatically coupled to a single wave. The discretization of the system in monokinetic beams allows us to use a perturbative approach of the problem and highlight important connections between the hamiltonian and vlasovian formulations. By means of numerical calculations, we show that both Landau damping and growth emerge due to a phase mixing mechanism that involves many normal modes of the linearized system. We observe that this mechanism exhibits a distinct behavior depending on whether the system is stable or unstable. For systems with many degrees of freedom, the results obtained were possible due to the development of a new root-finding technique in the complex plane. Using vlasovian simulations via semi-Lagrangian scheme we describe the process of collisionless relaxation of the velocity distribution for the resonant particles and the evolution of the wave intensity. We analyse the spectrum of undamped modes in free plasmas when one assumes a stationary velocity distribution with a small (and smooth) plateau centred on the wave phase speed.We find that these oscillations are quite sensitive to the shape of the plateau, mainly for those that have velocities close to the thermal particle velocity. We also investigate, using a semiclassical approach, the modifications in the dispersion relation of the electrostatic modes in free and magnetized plasmas subjected to external radiation field.

Cette étude, expérimentale et théorique, de physique des surfaces développe une méthode basée sur la spectroscopie de perte d'énergie des photoélectrons, pour déterminer les propriétés électroniques d'un matériau à partir de mesures XPS. Sur la base de la physique de la photoémission dans un solide homogène, la technique XPS-PEELS donne accès à la fonction de perte d'énergie ELF(E, q) liée à la partie imaginaire et la fonction diélectrique sur une large plage d’énergie (environ 50 eV) avec une sensibilité typique de ≈ 5 nm en profondeur. Dans les métaux ou les semi-conducteurs à faible gap, qui présentent un chevauchement important entre le pic quasi-élastique ZLP(E) et la région des pertes d'énergie, la technique est rendue applicable par la méthode de transformée de Fourier développée dans cette thèse. Les distributions en énergie de la source de rayons X et de la fonction d’appareil sont directement mesurées. La forme asymétrique du pic quasi-élastique ZLP(E) est obtenue à partir de la densité d’états électroniques (calcul par une méthode DFT), à l’aide du modèle de Hopfield-Wertheim-Citrin décrivant la réponse des électrons de valence à la création du trou (théorie multi-corps). L'algorithme XPS-PEELS utilise l'ensemble du spectre sans soustraction empirique d’une ligne de base. Il considère deux types d'excitations de plasmon - intrinsèques et extrinsèques - avec des taux de création différents mais impose la même distribution en énergie. Cette méthode originale permet d’accéder à des excitations électroniques de faible énergie (pertes proches du pic XPS) ; dans le cas de l'aluminium métallique, la transition inter-bandes est observée à 1.80 eV. En tant que technique de spectroscopie électronique, les effets de dispersion de la fonction de perte ELF(E, q) ont été considérés en utilisant un code TD-DFT (Exciting); quel que soit le niveau d'approximation (RPA, ALDA, LRC), les calculs ne décrivent pas correctement la largeur du spectre ELF expérimental (2.3 eV). À la fin du travail, nous comparons la méthode de la transformée de Fourier avec une méthode empirique d'élimination du pic élastique, valable pour les isolants, dans le cas de l'oxyde d'aluminium Al₂O₃. / This surface physics study, experimental and theoretical, develops a technique based on the energy loss spectroscopy of photoelectrons, to determine electronic properties of a material from XPS measurements. Based on the physics of photoemission in a homogeneous solid, the XPS-PEELS technique provides the energy loss function ELF(E, q) related to the imaginary part and the dielectric function with energy extension up to 50 eV and a typical sensitivity of ≈ 5 nm in depth. In metals or low-gap semiconductors, with important overlap between the elastic peak and the energy loss region, the technique became applicable by the Fourier transform method developed in this thesis. In addition to the distribution of the X-ray source and analyzer apparatus function, the asymmetric shape of the no-loss peak ZLP(E) is based on the calculation of the Density of Electronic States (DFT method) and the Hopfield-Wertheim-Citrin model describing the response of valence electrons to the creation of the hole (many body theory). The XPS-PEELS algorithm uses the entire spectrum without any empirical background subtraction. It considers two types of plasmon excitations - intrinsic and extrinsic - with different rates of creation but imposing the same distribution in energy. This original method was applied to the aluminum metal, allowing the analysis of the loss function and revealing interband excitations (1.80 eV) at low energy, very close to the no-loss peak. As an electron spectroscopy technique, dispersion effects of the loss function ELF(E, q) were considered using a TD-DFT code (Exciting); for any level of approximation (RPA, ALDA, LRC), the calculations do not describe properly the high width (2.3 eV) of the experimental ELF. At the end of the work, we compare the Fourier transform method with an empirical method of elastic peak elimination, valid for insulators, in the case of aluminum oxide Al₂O₃.