Orientador: Marco Aurelio Pinheiro Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica "Gleb Wataghin" / Made available in DSpace on 2018-08-06T16:52:29Z (GMT). No. of bitstreams: 1 Hisi_AndreiaNaluSoares_M.pdf: 1775574 bytes, checksum: 25943a839a6bab51223d4a915fca5faf (MD5) Previous issue date: 2006 / Resumo: Neste trabalho realizamos cálculos comparativos para seções de choque de excitações vibracionais integrais e diferenciais para o espalhamento de pósitrons e elétrons de baixa energia contra a molécula de Hidrogênio (H2). Para estes cálculos utilizamos o Método Multicanal de Schwinger (SMC) [1] para a colisão de pósitrons [2] e colisão de elétrons [3]. Usando um conjunto de amplitudes de espalhamento oriundas do SMC, calculadas segundo uma quadratura específica (Hermite [4]) de pontos correspondendo à distâncias internucleares, obtivemos as seções de choque vibracionalmente resolvidas segundo a Aproximação Adiabática [5]. Para poder comparar o processo de espalhamento de elétrons com o de pósitrons, as amplitudes de espalhamento foram calculadas com o mesmo conjunto de base representando o alvo molecular e a função de espalhamento da partícula. Para a função de onda vibracional utilizamos as autofunções de um oscilador harmônico simples (com freqiiência experimental) para ambos os casos de espalhamento de elétron e pósitron / Abstract: Here we report a comparative calculation of both integral and differential vibrational excitation cross sections of low energy positron and electron scattering against Hydrogen molecules (H2). For these calculations we have used the Schwinger Multichannel (SMC) Method [1] for positron collision [2] and electron collision [3]. Using a set of SMC scattering amplitudes, calculated at specific quadrature points (Hermite [4]) of internuclear distances, we have obtained vibrational resolved cross sections in the Adiabatic Aproximation [5]. In order to compare the electron scattering process with the positron one, the scattering amplitudes were calculated with the same basis set for representing the molecular target and scattering particle wave functions. For the vibrational wave functions we have used the eigenfunctions of a simple harmonic oscillator (with the experimental frequency) for both cases, the electron and positron scattering / Mestrado / Física Atômica e Molecular / Mestre em Física

Orientador: Marco Aurelio Pinheiro Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-06T20:07:28Z (GMT). No. of bitstreams: 1 Oliveira_ElianeMarquesde_M.pdf: 1735401 bytes, checksum: 96cad8d360043e7a2e9b4a6b65936027 (MD5) Previous issue date: 2006 / Resumo: A relação entre ressonâncias de Feshbach e excitações eletrônicas é bem conhecida no caso de espalhamento de elétrons por moléculas de H2, mas ainda pouco explorada no espalhamento de pósitrons, tornando interessante, portanto, um estudo comparativo. Porém, antes de pensarmos em comparações, nos preocupamos em tentar entender se, de fato, uma ressonância de Feshbach se manifesta no espalhamento de pósitrons por moléculas de H2. Esta preocupação foi motivada devido à não observação experimental desta ressonância em investigação realizada logo após nosso grupo de pesquisa ter publicado um estudo no qual havia evidências para a existência de uma ressonância de Feshbach em colisões pósitron-H2, obtida num cálculo realizado na aproximação de núcleos fixos para a distância internuclear de equilíbrio Ro=1.4ao. Nossa estratégia foi calcular seções de choque, ainda na aproximação de núcleos fixos, para diversas distâncias internucleares em torno de Ro. Os resultados obtidos sugerem que a ausência experimental da ressonância esteja ligada à abertura de um novo canal eletrônico para R > Ro. Para esses cálculos utilizamos o Método Multicanal de Schwinger (SMC), para obtenção dos parâmetros de colisão (posição e largura da ressonância), em combinação com a aproximação do Potencial Complexo Local (também conhecido por Boomemng Model), para inclusão do acoplamento dos graus de liberdade vibracionais. A ausência da ressonância para algumas distâncias internucleares no espalhamento de pósitrons impede o cálculo de seções de choque vibracionalmente resolvidas através da aproximação local. Por outro lado, para o espalhamento de elétrons, que é um desafio como problema teórico, realizamos os cálculos e encontramos acordo razoável com os dados experimentais disponíveis / Abstract: The relationship between Feshbach resonances and electronic excitation is well known in electron scattering by Hydrogen molecules, but still little exploited in the positron scattering case, making interesting, a comparative study. However, before thinking about comparisons, we have to address the question if, in fact, a Feshbach resonance manifests itself in the positron scattering by Hydrogen molecules. This issue is motivated by the fact that recent measured cross sections have not shown the same features presented by our earlier calculated Feshbach resonance in positron-Hydrogen collisions, carried out in the fixed-nuclei approach for the equilibrium internuclear distance, Ro=1.4ao. Our strategy was to calculate cross sections, still in the fixed nuclei approach, for several internuclear distances around Ro. The obtained results suggest that the experimental absence of the resonance is related to the opening of a new electronic channel for R > Ro. For these calculations we have used the Schwinger Multichannel Method (SMC) to obtain the collision parameters (position and width of the resonances) in combination with the Local Complex Potential approach (also known for Boomerang Model), for inclusion of the coupling of the vibrational degrees of freedom. The absence of the resonance for some internuclear distantes prevents the calculation of vibrationally resolved cross sections by positron impact using the local approach. On the other hand, for the electron scattering case, which is also a challange as a theoretical problem, we have carried out the calculation and have found good agreement with available experimental data / Mestrado / Física Atômica e Molecular / Mestre em Física

Nowadays, reactions on surfaces are attaining great scientific interest because of their diverse applications. Some well known examples are production of ammonia on metal surfaces for fertilizers and reduction of poisonous gases from automobiles using catalytic converters. More recently, also photoinduced reactions at surfaces, useful, textit{e.g.}, for photocatalysis, were studied in detail. Often, very short laser pulses are used for this purpose. Some of these reactions are occurring on femtosecond (1 fs=$10^{-15}$ s) time scales since the motion of atoms (which leads to bond breaking and new bond formation) belongs to this time range. This thesis investigates the femtosecond laser induced associative photodesorption of hydrogen, H$_2$, and deuterium, D$_2$, from a ruthenium metal surface. Many interesting features of this reaction were explored by experimentalists: (i) a huge isotope effect in the desorption probability of H$_2$ and D$_2$, (ii) the desorption yield increases non-linearly with the applied visible (vis) laser fluence, and (iii) unequal energy partitioning to different degrees of freedom. These peculiarities are due to the fact that an ultrashort vis pulse creates hot electrons in the metal. These hot electrons then transfer energy to adsorbate vibrations which leads to desorption. In fact, adsorbate vibrations are strongly coupled to metal electrons, textit{i.e.}, through non-adiabatic couplings. This means that, surfaces introduce additional channels for energy exchange which makes the control of surface reactions more difficult than the control of reactions in the gas phase. In fact, the quantum yield of surface photochemical reactions is often notoriously small. One of the goals of the present thesis is to suggest, on the basis of theoretical simulations, strategies to control/enhance the photodesorption yield of H$_2$ and D$_2$ from Ru(0001). For this purpose, we suggest a textit{hybrid scheme} to control the reaction, where the adsorbate vibrations are initially excited by an infrared (IR) pulse, prior to the vis pulse. Both textit{adiabatic} and textit{non-adiabatic} representations for photoinduced desorption problems are employed here. The textit{adiabatic} representation is realized within the classical picture using Molecular Dynamics (MD) with electronic frictions. In a quantum mechanical description, textit{non-adiabatic} representations are employed within open-system density matrix theory. The time evolution of the desorption process is studied using a two-mode reduced dimensionality model with one vibrational coordinate and one translational coordinate of the adsorbate. The ground and excited electronic state potentials, and dipole function for the IR excitation are taken from first principles. The IR driven vibrational excitation of adsorbate modes with moderate efficiency is achieved by (modified) $pi$-pulses or/and optimal control theory. The fluence dependence of the desorption reaction is computed by including the electronic temperature of the metal calculated from the two-temperature model. Here, our theoretical results show a good agreement with experimental and previous theoretical findings. We then employed the IR+vis strategy in both models. Here, we found that vibrational excitation indeed promotes the desorption of hydrogen and deuterium. To summarize, we conclude that photocontrol of this surface reaction can be achieved by our IR+vis scheme. / Heutzutage werden Reaktionen auf Oberflächen wegen ihrer vielfältigen Anwendungen intensiv untersucht. Einige der bekannten Beispiele sind die Herstellung von Ammoniak auf Metalloberflächen für die Kunstdüngerproduktion und die Reduktion giftiger Abgase in Autokatalysatoren. In letzter Zeit wurden auch photoinduzierte Reaktionen an Oberflächen eingehender untersucht, die z.B. für die Photokatalyse verwandt werden können. Häufig werden in diesen Untersuchungen sehr kurze Laserpulse benutzt. Einige der Reaktionen finden auf einer Femtosekunden-Zeitskala mbox{(1 fs =10$^{-15}$ s)} statt, da die Bewegungen einzelner Atome in derart kurzen Zeitspannen ablaufen (durch die der Bindungsbruch und das Knüpfen neuer Bindungen verursacht wird). Diese Arbeit untersucht die femtosekunden-laserinduzierte assoziative Photodesorption von Wasserstoff, H$_2$, und Deuterium, D$_2$, von einer Rutheniumoberfläche. Viele interessante Eigenschaften dieser Reaktion wurden in Experimenten entdeckt: (i) ein großer Isotopeneffekt in der Desorptionswahrscheinlichkeit von H$_2$ und D$_2$, (ii) die Desorptionsausbeute steigt nicht-linear mit der (vis) Laserfluenz an und (iii) eine Nicht-Gleichverteilung der Energie auf die einzelnen Freiheitsgrade. Diese Auffälligkeiten sind durch den Umstand verursacht, dass der ultrakurze vis-Laserpuls heiße Elektronen im Metall erzeugt. Die heißen Elektronen transferieren dann Energie in die Schwingungen des Adsorbats, was zur Desorption führt. Tatsächlich sind die Adsorbatschwingungen stark an die Elektronen gekoppelt, nämlich durch nicht-adiabatische Kopplungen. Dies bedeutet, dass durch Oberflächen neue Kanäle für den Energietransfer geöffnet werden, was die Kontrolle von Oberflächenreaktionen im Vergleich zu solchen in der Gasphase erschwert. In der Tat sind die Quantenausbeuten von photochemischen Oberflächenreaktionen bekannterweise klein. Eines der Ziele in der vorliegenden Arbeit ist es auf der Basis von theoretischen Simulationen Strategien vorzuschlagen, um die Photodesorptionsausbeute von H$_2$ und D$_2$ von Ru(0001) zu kontrollieren bzw. zu verbessern. Zu diesem Zweck schlagen wir ein gemischtes Kontrollschema für die Reaktion vor, bei dem zunächst die Adsorbatschwingungen vor dem vis-Puls durch einen infraroten (IR) Puls angeregt werden. Sowohl adiabatische als auch nicht-adiabatische Repräsentationen für photoinduzierte Desorptionsprozesse werden dabei benutzt. Die adiabatische Repräsentation ist in klassischen Molekulardynamik-Simulationen mit elektronischer Reibung verwirklicht. In einer quantenmechanischen Beschreibung werden nicht-adiabatische Repräsentationen innerhalb der Dichtematrixtheorie für offene Quantensysteme verwandt. Die zeitliche Entwicklung des Desorptionsprozesses wird in einem Zwei-Modenmodell reduzierter Dimensionalität mit einer Schwingungs- und einer Translationskoordinate des Adsorbats beschrieben. Die Potentiale für den elektronische Grundzustand und den angeregten Zustand sind abgeleitet aus quantenchemischen Rechnungen (textsl{first principles}). Die IR-getriebene Schwingungsanregung der Adsorbatmoden mit moderatem Wirkungsgrad wird mit (modifizierten) $pi$-Pulsen und/oder der Theorie der optimalen Kontrolle erreicht. Die Abhängigkeit der Desorption von der Fluenz wird mit Hilfe der elektronischen Temperatur des Metalls berechnet, welche im Rahmen des Zwei-Temperatur-Modells bestimmt wird. Dabei weisen unsere Ergebnisse eine gute Übereinstimmung mit experimentellen und früheren theoretischen Arbeiten auf. Daraufhin wandten wir die IR+vis Strategie in beiden Modellen an. Dadurch konnten wir zeigen, dass Schwingungsanregung in der Tat die Desorption von Wasserstoff und Deuterium begünstigt. Zusammenfassend stellen wir fest, dass die Photokontrolle dieser Oberflächenreaktion durch unser IR+vis Schema erreichbar ist.

A two-dimensional model of the resonant electron-molecule collision processes with one nuclear and one electronic degree of freedom introduced by Houfek, Rescigno and McCurdy [Phys. Rev. A 73, 032721 (2006)] and a similar two- dimensional model of the dissociative recombination with potential proposed by Hamilton [Ph.D. thesis, University of Colorado, (2003)] are formulated within the time-dependent framework and solved numerically using the finite-element method with the discrete variable representation basis, the exterior complex scaling method and the generalized Crank-Nicolson method. On the model of electron-molecule collisions we illustrate how the time-dependent calculations can provide a deep insight into the origin of oscillatory structures in the vibrational excitation cross sections if one evaluates the cross sections not only at sufficiently large time to obtain the final cross sections, but rather at several characteristic times which are given by the evolution of the system. With use of the time- dependent calculations we demonstrate the complex nature of the dissociative recombination model dynamics and we propose the interpretation of the recom- bination process mechanism. We also propose few techniques for the explanation of the sharp structures in the dissociative recombination cross sections...

The purpose of this thesis is to construct a numerically solvable quantum mechanical model describing the dynamics of the indirect mechanism of the dissociative recombination process of a molecular cation by electron impact. The model also describes vibrational excitation of a molecular cation by electron impact. The solution of this model is carried out by implementing a combination of finite elements, discrete variable representation and exterior complex scaling methods. This is then specifically applied to the dissociative recombination and vibrational excitation of H$_2^+$ by an incoming electron. The results can be used to test the accuracy of approximative methods and the programs expanded to cover the cases of other diatomics. Powered by TCPDF (www.tcpdf.org)

This work aims at calculating the cross sections for vibrational excitation of the oxygen molecules by collisons with electrons. Potential energy curves are obtained with standard quantum chemistry methods and the R-matrix method with good agreement with measurable molecular properties, the cross sections are calculated within the local complex potential approximation. It was shown that the results obtained with different, but seemingly satisfactory settings can vary by a significant degree. Comparison with experimental data then point out the insufficiency of the local complex potential approximation. Powered by TCPDF (www.tcpdf.org)

La Spectroscopie par Effet Tunnel Inélastique (IETS) avec un Microscope à Effet Tunnel (STM) est une nouvelle technique de spectroscopie vibrationnelle, qui permet de caractériser des propriétés très fines de molécules adsorbées sur des surfaces métalliques. Des règles de selection d’excitation vibrationnelle basées sur la symétrie ont été proposées, cependant, elles ne semblent pas exhaustives pour expliquer la totalité du mécanisme et des facteurs en jeu; elles ne sont pas directement transposables pour les propriétés d'un adsorbat et sont lourdes d'utilisation. Le but de cette thèse est donc d'améliorer ces règles de selection par une étude théorique. Un protocole de simulation de l'IETS a été développé, paramétré, et évalué, puis appliqué pour calculer des spectres IETS pour différentes petites molécules, qui sont systématiquement liées, sur une surface de cuivre. Des principes additifs de l'IETS ont été developpés, notamment concernant l’extension dans le vide de l’état de tunnel, l'activation/ quench sélectif de certains modes du aux propriétés électroniques de certains fragments moléculaires, et l'application de certaines règles d'addition de signaux IETS. De plus, des empreintes vibrationnelles par des signaux IETS ont été determinées pour permettre de différentier entre les orientations des adsorbats, la nature chimique des atomes et les isomères de structures. Une stratégie simple utilisant les propriétés de distribution de la densité électronique de la molécule isolée pour prédire les activités IETS sans des couts importants de calculs a aussi été développée. Cette expertise a été utilisée pour rationaliser et interpréter les mesures expérimentales des spectres IETS pour des métalloporphyrines et métallophtalocyanines adsorbées. Ces études sont les premières études IETS pour des molécules aussi larges et complexes. L'approche expérimentale a permis de déterminer les limitations actuelles des simulations IETS. Les défauts associés à l'identification ont été résolus en faisant des simulations d'images STM complémentaires. / Inelastic Electron Tunneling Spectroscopy (IETS) with the Scanning Tunneling Microscope (STM) is a novel vibrational spectroscopy technique that permits to characterize very subtle properties of molecules adsorbed on metallic surfaces. Its proposed symmetry-based propensity selection rules, however, fail to fully capture its exact mechanism and influencing factors; are not directly retraceable to an adsorbate property and are cumbersome. In this thesis, a theoretical approach was taken to improve them. An IETS simulation protocol has been developed, parameterized and benchmarked, and consequently used to calculate IETS spectra for a set of systematically related small molecules on copper surfaces. Extending IETS principles were deduced that refer to the tunneling state’s vacuum extension, the selective activating/quenching of certain types of modes due to the moieties’ electronic properties, and the applicability of a sum rule of IETS signals. Also, fingerprinting IETS-signals that enable discrimination between adsorbate orientations, the chemical nature of atoms and structural isomers were determined and a strategy using straightforward electronic density distribution properties of the isolated molecule to predict IETS activity without (large) computational cost was developed. This expertise was used to rationalize and interpret experimentally measured IETS spectra for adsorbed metalloporphyrins and metallophthalocyanines, being the first IETS studies of this large size. This experimental approach permitted to determine the current limitations of IETS-simulations. The associated identification shortcomings were resolved by conducting complementary STM-image simulations.