Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-06-08T13:19:18Z No. of bitstreams: 1 rafaelfelipecoelhoneves.pdf: 4428183 bytes, checksum: c285f713f0bc1b3a74476dcda5baab40 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-07-13T13:27:17Z (GMT) No. of bitstreams: 1 rafaelfelipecoelhoneves.pdf: 4428183 bytes, checksum: c285f713f0bc1b3a74476dcda5baab40 (MD5) / Made available in DSpace on 2016-07-13T13:27:17Z (GMT). No. of bitstreams: 1 rafaelfelipecoelhoneves.pdf: 4428183 bytes, checksum: c285f713f0bc1b3a74476dcda5baab40 (MD5) Previous issue date: 2015-10-01 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A substituição de combustíveis fósseis por biocombustíveis obtidos a partir de fontes renováveis consiste em uma importante estratégia para minimizar as emissões de gases do efeito estufa. Neste sentido, a conversão da biomassa lignocelulósica em biocombustíveis representa uma fonte sustentável que atende à demanda global de energia. Elétrons de baixa energia em plasmas têm potencial para superar a resistência físico-química da biomassa possibilitando a geração de produtos químicos de alto valor. Isto ocorre devido à quebra química induzida pela captura eletrônica dissociativa ou por processos de fragmentação através da excitação ou ionização por impacto de elétrons. Nesta tese foi investigado o espalhamento de elétrons pelas biomoléculas fenol e furfural, que são subunidades da biomassa lignocelulósica. Espectros experimentais de perda de energia de elétrons (EPEE) foram registrados para o fenol e furfural, para energias de impacto de 15, 20, 30, 40 e 250 eV, para ângulos entre 10◦ e 90◦. Estes espectros foram usados para obtenção de Seção de Choque (SC) e Força de Oscilador Generalizada (FOG) para as transições eletrônicas permitidas por dipolo, possibilitando também, a determinação de suas Forças de Oscilador Óptico. Seções de Choque Diferenciais (SCD) foram medidas para a excitação por impacto de elétrons de estados eletrônicos, bem como de modos vibracionais compostos não resolvidos do fenol. Estas investigações fornecem os primeiros resultados publicados na literatura para os estados eletrônicos excitados singleto e tripleto do fenol, até o primeiro potencial de ionização, e ainda, para as Seções de Choque de excitações vibracionais. Foram também obtidos dados de Seções de Choque Integrais (SCI) para excitações eletrônicas e vibracionais por impacto de elétrons em fenol. Dados de Seções de Choque Diferenciais Triplas (SCDT) para a ionização do fenol por impacto de elétrons (e, 2e) foram gerados através de uma cinemática coplanar assimétrica para energia eletrônica incidente de 250eV. Para o furfural, os dados experimentais foram utilizados em conjunto com cálculos teóricos na atribuição de seus estados excitados. A boa concordância entre os resultados teóricos e os experimentais permite fornecer a primeira atribuição quantitativa da espectroscopia dos estados eletrônicos do furfural na faixa de energia estudada. / Replacing fossil fuels with biofuels obtained from renewable sources is an important strategy to minimize greenhouse gas emissions. In this sense, the conversion of lignocellulosic biomass into biofuels is a sustainable source of energy to meet global energy demands. Low-energy electrons in plasmas have the potential to overcome the biomass physicochemical resistance enabling the production of high value chemicals. This happens due to chemical breakdown induced through dissociative electron attachment or electron impact excitation- or ionizationfragmentation processes. In this thesis was investigated the electron scattering by the biomolecules phenol and furfural, that are subunits of the lignocellulosic biomass. Experimental electron-energy loss spectra (EELS) were recorded to phenol and furfural, measured at impact energies of 15, 20, 30, 40 and 250 eV, for angles between 10◦ and 90◦. These spectra were used to derive Cross Sections (CS) and Generalized Oscillator Strengths (GOS) for the dipoleallowed electronic transitions, also allowing the determination of their Optical Oscillator Strengths. Differential cross sections (DCS’s) were measured for the electron-impact excitation of the electronic states as well as of a number of composite unresolved vibrational modes in phenol. These investigations provide the first results published on the literature to singlet and triplet excited electronic states of phenol, up to the first ionization potential and, to the vibrational excitation Cross Sections. In addition, it were obtained integral cross sections (ICS’s) data for electron impact vibrational and electronic excitations in phenol. Triple differential cross sections (TDCS) data for the electron-impact ionization of phenol (e,2e) were generated with coplanar asymmetrical kinematics for an incident electron energy of 250 eV. Regarding to furfural, the experimental data were used along with calculations in order to assign the excited states. The good agreement between the theoretical results and the measurements allows providing the first quantitative assignment of the electronic state spectroscopy of furfural over the energy range studied.

abstract: The origins of carrier mobility (μe) were thoroughly investigated in hydrogenated indium oxide (IO:H) and zinc-tin oxide (ZTO) transparent conducting oxide (TCO) thin films. A carrier transport model was developed for IO:H which studied the effects of ionized impurity scattering, polar optical phonon scattering, and grain boundary scattering. Ionized impurity scattering dominated at temperatures below ~240 K. A reduction in scattering charge Z from +2 to +1 as atomic %H increased from ~3 atomic %H to ~5 atomic %H allowed μe to attain >100 cm^2/Vs at ~5 atomic %H. In highly hydrogenated IO:H, ne significantly decreased as temperature increased from 5 K to 140 K. To probe this unusual behavior, samples were illuminated, then ne, surface work function (WF), and spatially resolved microscopic current mapping were measured and tracked. Large increases in ne and corresponding decreases in WF were observed---these both exhibited slow reversions toward pre-illumination values over 6-12 days. A hydrogen-related defect was proposed as source of the photoexcitation, while a lattice defect diffusion mechanism causes the extended decay. Both arise from an under-coordination of the In. An enhancement of μe was observed with increasing amorphous fraction in IO:H. An increase in population of corner- and edge-sharing polyhedra consisting of metal cations and oxygen anions is thought to be the origin. This indicates some measure of medium-range order in the amorphous structure, and gives rise to a general principle dictating μe in TCOs---even amorphous TCOs. Testing this principle resulted in observing an enhancement of μe up to 35 cm^2/Vs in amorphous ZTO (a-ZTO), one of the highest reported a-ZTO μe values (at ne > 10^19 cm^-3) to date. These results highlight the role of local distortions and cation coordination in determining the microscopic origins of carrier generation and transport. In addition, the strong likelihood of under-coordination of one cation species leading to high carrier concentrations is proposed. This diverges from the historical indictment of oxygen vacancies controlling carrier population in crystalline oxides, which by definition cannot occur in amorphous systems, and provides a framework to discuss key structural descriptors in these disordered phase materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

No description available.

In this thesis I have theoretically studied electron recombination processes with small molecular ions. In these kind of processes resonant states are involved. To calculate the potential energy for these states as a function of internuclear distance, structure calculations and scattering calculations have to be performed. So far I have been studying the ion-pair formation with in electron recombination with H3+. The cross section for this process has been calculated using different kind of models, both a time dependent quantum mechanical and a semiclassical. I have also studied the direct process of dissociative recombination of HF+. To calculate the total cross section for this process, we have performed wave packet propagation on thirty resonant states and summed up the individual cross sections for these states. The cross sections for both these processes have a similar appearance to those measured experimentally in the ion storage ring CRYRING in Stockholm. / QC 20101103

No description available.

No description available.

In this Thesis experimental studies of nano-clusters using synchrotron radiation based photoelectron (UPS and XPS) and Auger Electron Spectroscopy (AES) are presented. The investigations may be divided into two parts where the first reports on the structure of heterogeneous two component clusters, and the second concerns electronic decay processes. Using photoelectron spectroscopies as investigative tools the radial composition of heteroclusters of argon mixed with xenon, krypton or neon has been determined. Two methods of heterogeneous cluster production were employed: co-expansion and doping/pick-up. By analyzing the line shapes, energy positions, and widths of the spectral cluster features the radial composition of the clusters produced by co-expansion were found to form close-to-equilibrium structures, placing the component with larger cohesive energy in the cluster core while the second component was to varying degree segregated toward the surface. By instead using the doping/pick-up technique the opposite radial structures, i.e. far-from-equilibrium structures, may be formed. In the case of argon/krypton clusters a similar surface structure is formed regardless of production technique. The second part of the Thesis concerns post-ionization decay processes. Experimental evidence for the Interatomic Coulombic Decay process, a theoretically predicted decay channel, is presented in a study of homogeneous neon clusters. The time scale of the decay was determined to 6±1 fs for bulk atoms and >30 fs for surface atoms in the neon cluster, showing the connection between local geometry and dynamics of the decay. Another channel for electronic relaxation is Auger decay. This Thesis presents a method of disentangling the spectral surface and bulk responses from clusters in Auger spectra. Studies of argon clusters show that the AES technique is more surface sensitive than XPS, even at the same electron kinetic energy. Furthermore, the effect scattering of the photoelectron has on the Auger spectra was investigated. Special effort was put into explaining an experimentally observed photon energy dependent intensity appearing on the high-kinetic energy side on the Auger signal. We propose that this intensity is due to a solid state-specific photoelectron recapture process we name Pre-Auger Recapture (PAR), which affects the kinetic energy of the Auger electrons.

In this thesis, two selected topics in magnetism are studied using theoretical modelling and computational methods. The first of these is the magnetocrystalline anisotropy energy (MAE) of transition metal based magnets. In particular, ways of finding 3d transition metal based materials with large MAE are considered. This is motivated by the need for new permanent magnet materials, not containing rare-earth elements, but is also of interest for other technological applications, where the MAE is a key quantity. The mechanisms of the MAE in the relevant materials are reviewed and approaches to increasing this quantity are discussed. Computational methods, largely based on density functional theory (DFT), are applied to guide the search for relevant materials. The computational work suggests that the MAE of Fe1-xCox alloys can be significantly enhanced by introducing a tetragonality with interstitial B or C impurities. This is also experimentally corroborated. Alloying is considered as a method of tuning the electronic structure around the Fermi energy and thus also the MAE, for example in the tetragonal compound (Fe1-xCox)2B. Additionally, it is shown that small amounts (2.5-5 at.%) of various 5d dopants on the Fe/Co-site can enhance the MAE of this material with as much as 70%. The magnetic properties of several technologically interesting, chemically ordered, L10 structured binary compounds, tetragonal Fe5Si1-xPxB2 and Hexagonal Laves phase Fe2Ta1-xWx are also investigated. The second topic studied is that of magnetic effects on the elastic scattering of fast electrons, in the context of transmission electron microscopy (TEM). A multislice solution is implemented for a paraxial version of the Pauli equation. Simulations require the magnetic fields in the sample as input. A realistic description of magnetism in a solid, for this purpose, is derived in a scheme starting from a DFT calculation of the spin density or density matrix. Calculations are performed for electron vortex beams passing through magnetic solids and a magnetic signal, defined as a difference in intensity for opposite orbital angular momentum beams, integrated over a disk in the diffraction plane, is observed. For nanometer sized electron vortex beams carrying orbital angular momentum of a few tens of ħ, a relative magnetic signal of order 10-3 is found. This is considered realistic to be observed in experiments. In addition to electron vortex beams, spin polarised and phase aberrated electron beams are considered and also for these a magnetic signal, albeit weaker than that of the vortex beams, can be obtained. / <p>Felaktigt ISBN i den tryckta versionen: 9789155497149</p><p></p>

This thesis presents a set of measurements of spin asymmetries from the heavy atoms krypton and rubidium. These investigations allow examination of the spin orbit interaction for electron scattering from the target atoms. These measurements utilise spin polarized electrons in a crossed beam experiment to measure the Sherman function from krypton and the A2 parameter from the 52P state of rubidium. The measurements utilise a new spin polarized electron energy spectrometer which is designed to operate in the 20-200 eV range. The apparatus consists of a standard gallium arsenide polarized electron source, a 180 degrees hemispherical electron analyser to detect scattered electrons and a Mott detector to measure electron polarization. A series of measurements of the elastic Sherman function were performed on krypton at incident electron energies of 20, 50, 60, 65, 100, 150 and 200 eV. Scattered electrons are measured over an angular range of 30-130 degrees. These measurements are compared with calculations of the Sherman function which are obtained by solution of the Dirac-Fock equations. These calculations include potentials to account for dynamic polarization and loss of flux into inelastic channels. At the energies 50, 60 and 65 eV, experimental agreement with theory is seen to be extremely dependent on the theoretical model used. Measurement of the A2 parameter from the combined 52P1/2,3/2 state of rubidium are performed at an incident energy of 20 eV. The scattered electrons are measured over an angular range of 30-110 degrees. This measurement represents the first such measurement of this parameter for rubidium. Agreement with preliminary calculations performed using the R-matrix technique are good and are expected to improve with further theoretical development.

Die vorliegende Arbeit befasst sich mit Elektronenenergieverlustspektroskopie an unterdotierten Kupratsupraleitern und Übergangsmetalldichalcogeniden. Nach einem kurzen Abriss über die der experimentellen Methode zugrundeliegenden theoretischen Tatsachen folgen zwei experimentelle Kapitel. Für das prototypische Kupratsystem Ca2-xNaxCuO2Cl2 wird für verschiedene Dotierungskonzentrationen zunächst die Entwicklung der Ladungstransferanregungen untersucht. Man findet eine substanzielle Umverteilung des spektralen Gewichtes, verbunden mit einem starken Einbruch der Dispersion dieser Anregungen. Beides wird im Rahmen der Wechselwirkung mit Spinfreiheitsgraden innerhalb der Kupfer-Sauerstoff-Ebene diskutiert. Anschliessend erfolgt die Diskussion einer ausschließlich für zehnprozentige Dotierung auftretenden Symmetriebrechung der optischen Antwortfunktion, für die verschiedene mögliche Szenarien vorgeschlagen werden. Im Kapitel über die Dichalcogenide liegt der Fokus auf dem Verhalten des Ladungsträgerplasmons, das für alle Substanzen dieser Gruppe mit Ladungsordnung eine negative Dispersion aufweist. Dieses Verhalten läßt sich durch in-situ Interkalation zusätzlicher Ladungstraeger umkehren, so dass man eine dotierungsabhängige Plasmonendispersion erhält. Es werden verschiedene Szenarien für dieses Verhalten diskutiert. / The present thesis describes electron energy-loss spectroscopy on underdoped cuprate superconductors and transition-metal dichalcogenides. After a brief introduction into the experimental method there are two experimental chapters. For the prototype cuprate system Ca2-xNaxCuO2Cl2 the behavior of the charge-transfer excitations is investigated as a function of doping. The observed substantial redistribution of spectral weight and the accompanying breakdown of their dispersion is discussed in terms of a coupling to the spin degrees of freedom within the copper-oxygen plane. For x=0.1 there is a pronounced symmetry breaking in the optical response function which is discussed in terms of different possible scenarios. The chapter on the dichalcogenides focuses on the properties of the charge-carrier plasmon which shows a negative dispersion for all representatives of this family exhibiting a charge-density wave instability. This behavior can be influenced by in-situ intercalation of additional charges, the result being a doping dependent plasmon dispersion. Several approaches to reconcile these findings are considered.