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WALLDINEY PEDRA GURGEL
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Phase transitions in decanoic acid crystals at low temperatures.
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Data: 23/12/2022
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The present work aims to analyze the stability of crystals of capric acid (AC) in structural form C when submitted to low temperatures using the techniques of X-ray diffraction (XRD) and Raman spectroscopy. AC, also known as decanoic acid, is part of a class of organic compounds known as fatty acids, whose molecule is formed by a hydrocarbon chain with a methyl group at one end and a carboxylic group at the other. These compounds are abundant in nature as chemical components of various vegetable oils, thus attracting the attention of the food, cosmetics, chemical, pharmaceutical, medicine industries, among others. AC crystals were characterized under ambient conditions of temperature and pressure using powder X-ray diffraction (XRD) techniques, Rietveld refinement using Gaussian 9 software, Raman spectroscopy and infrared spectroscopy. The Raman and infrared spectra were theoretically determined through the density functional theory (DFT) as a comparison with the spectra obtained experimentally, as well as to make the classification of the respective vibrational modes. The crystals were subjected to low temperature conditions (300 to 8 K) in the Raman spectroscopy experiment and (293 to 83 K) in the XRD experiment. Regarding the data obtained via X-ray diffraction, it was possible to observe two regions of interest, where a series of changes were verified, both related to the diffraction pattern and the discontinuity in the lattice parameters. Using the Raman technique several spectral changes were observed throughout the spectrum, such as splitting of bands belonging to the dimer, which may be related to a process of thermal isomerization, which occurs when the crystal is cooled, in this process the molecules change from an all trans to a gauche conformation. In addition, the behavior of Gibbs energy as a function of temperature in the range between 300 and 08 K for the AC crystal was verified. In this process, three phase transitions were observed.
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TIAGO BALIEIRO TAVARES
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Study of the Opto-Electronic Properties of Composites of polyvinyl carbazole and Fe3O4, FeLiO2 and (La0,6Sr0,4)(Co0,2Fe0,8)O3
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Data: 30/11/2022
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Bandgap, Composites, Absorbance, Emission, Ferrites, Polymers, Raman, Infrared.
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GISLAYLLSON DIAS DOS SANTOS SOUZA
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Phase transition in C-form of dodecanoic acid crystal and doping with rare earth ions: a structural and vibrational investigation.
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Data: 28/11/2022
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Dodecanoic acid, also called lauric acid (LA), is part of a class of organic compounds known as fatty acids (FA), whose molecule is formed by a hydrocarbon chain with a methyl group at one end and a carboxylic group at the other. These compounds are chemical components of various vegetable oils, mainly from Amazonian flora. FA has attracted the attention of the food, cosmetic, chemical, and pharmaceutical industries, in addition to medicinal applications. In this work, crystals of LA in the C-form were obtained by heating until a temperature value above the melting point and, after this, the sample was cooled down to room temperature. Additionally, crystals of dysprosium- and europium-doped LA (LADy and LAEu, respectively) were obtained by the slow evaporation of the solvent method. Pure LA crystals were characterized under ambient conditions using powder X-ray diffraction (PXRD), and Raman and infrared techniques. Furthermore, the doped crystals were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Density functional theory (DFT) was used to calculate the Raman and infrared spectra to compare with the experimental spectra for a correct assignment of vibrational modes. Pure LA crystals were studied under low-temperature conditions (300 to 38 K) and their structural stability was evaluated by Raman spectroscopy and PXRD. Several spectral changes, such as the appearance and disappearance of modes in the regions associated with intra and intermolecular vibrations, discontinuities in wavenumbers, band splittings, and anomalous behavior of Raman modes revealed that the LA crystals underwent a reversible structural phase transition of the monoclinic C-phase to the triclinic A2-phase, within the temperature range between 230 and 160 K. This phase transition was supported by the results of DFT calculations and PXRD at low temperatures.
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MARCOS VINICIUS DE SOUSA SILVA
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Regular spacetimes in the general relativity and alternative theories of gravity
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Data: 28/11/2022
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In this work, we investigate the existence of regular solutions in general relativity and alternative theories of gravity. In the context of general relativity, we briefly review Bardeen’s solution. We analyzed some aspects of the interpretations of this solution for when we have a source with a magnetic or electric charge. By inserting the cosmological constant, we study thermodynamics for a regular black hole. By imposing the energy conditions, we build new regular solutions that violate only the strong energy condition. In addition to the standard regular solutions, we also study the existence of black bounces in general relativity. We also study the possibility of generalizing electrically charged regular black hole solutions, built in general relativity, for f(R) theory and f(G) theory, where R is the curvature scalar G is the Gauss-Bonnet term. We build the formalism in terms of a mass function and this results in different gravitational and electromagnetic theories for each mass function, in the cases of f(R) and f(G) theories. By imposing the limit of some constant, we get again the results of general relativity.
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FABIO FURTADO LEITE
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Studies of High Pressure Induced Phase Transitions on (DMA)PbBr3 and (MHy)Mn(H2POO)3 Perovskites
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Data: 09/11/2022
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In this work, the monocrystals of Halide perovskite Di-Methyl Lead Bromine (DMA)PbBr3 (DMA = (CH3)2NH2+) and Methyl-Hydrazine Manganese Hypophosphate (MHy)Mn(H2POO)3 (MHy = CH3N2H3) were synthesized and characterized using spectroscopic techniques. Structural, electronic and vibrational properties were studied using X-Ray Diffraction, Photoluminescence and Raman spectroscopy under high pressure conditions. For the halide-perovskite crystal, three structural phase transitions are observed in the range between 0.0 and 8.0 GPa around 0.9; 3.6 and 6.0 GPa, respectively. These transitions are induced by the off-center shift process of lead ions in the center of the octahedron structures. Initially, between 0,0 and 0.9 GPa, the crystal belongs to the hexagonal system with space group P63/mmc (phase I) and after the first phase transition it changes to the orthorhombic system with possible space group P212121 (phase II), between 1.3 and 3.1 GPa. In the second phase transition, our results indicate that the crystal belongs to the monoclinic crystalline system, exhibiting some changes in the Raman spectral profile between 3.6 and 5.6 GPa (phase III) and between 6.0 and 8.0 GPa (phase IV). Density Functional Theory (DFT) calculations were performed using the orthorhombic phase as the initial structure in the pressure range between 2.0 and 6.0 GPa. These results corroborate those observed experimentally, helping us to understand and describe the behavior of the crystal in this pressure range. For the (MHy)Mn(H2POO)3 crystal, single crystal diffraction experiments were carried out, in collaboration with Professor Dr.Miroslaw Maczka, and Raman spectroscopy under high pressures. Two phase transitions are identified in the range between 0.0 and 7.9 GPa, the first being around 1.3 GPa and the second at 4.7 GPa. These transitions have their mechanism in the distortion of the hypophosphate ligants molecules (H2POO−) that connect the MnO6 octahedral structures. In the first transition the crystal changes from the orthorhombic crystal system with space group Pnma (phase α) to the monoclinic system with space group P21/c (phase γ). The second phase transition (phase δ) was identified only in our Raman experiments, and it was not possible to know which crystal system and space group the material belongs to. Throughout the decompression process in our Raman experiment the sample remains stable until the pressure value around 1.3 GPa when we identify a phase transition that makes the spectral profile similar to that observed in the α phase during the sample compression. This result suggests the occurrence of the hysteresis phenomenon responsible for the suppression of the γ phase.
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JOHN EVERTON BATISTA BARBOSA
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THEORETICAL STUDY OF 3d NANOSTRUCTURES ADSORPIVED ON SURFACES Ta(001) AND W(001)
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Data: 21/10/2022
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In this research, we used the ab initio Real Space - Linear Muffin Tin Orbital - Atomic Sphere Approximation (RS-LMTO-ASA) method, based on the Density Functional Theory (Density Functional Theory - DFT) and implemented for the calculation of collinear and non-collinear magnetic structures, in order to analyze the magnetic properties of linear dimers and trimers consisting of 3d metals (V, Cr, Mn, Fe, Co and Ni ) adsorbed on 5d-type metallic surfaces of (W(001) and Ta(001)), in addition to nanostructures composed of 3d and 5d metals on these same 5d(001) substrates. This work aimed to understand the fundamental characteristics of the Dzyaloshinskii-Moriya interaction (iDM) for these systems in collinear and non-collinear magnetic configurations. We obtained the ground state magnetic configurations of all systems verifying that in cases where the configuration was approximately collinear, the iDM contributions are originated from the spin-orbit coupling (SOC). It was shown that, for the non-collinear case in some of these systems, the Dzyaloshinskii-Moriya-type interaction, arising from non-collinearity, presents a significant contribution. Furthermore, it was shown that for some trimers composed of Fe-W-Fe/W(001) and Mn-W-Mn/W(001) there is a competition between the Heisenberg exchange interactions and the iDM generating a complex non-collinear magnetic configuration, showing that the interdiffusion of a 5d metal between two 3d metals in these systems decreases the exchange interaction and raises the iDM, where the physical origin of the DM interaction between 3d-5d-type metal pairs is related to the charge current, while between pairs of 3d-3d-type metals, the iDM comes from the spin current. Other systems such as trimers V3/Ta(001) and Fe3/W(001) also showed a non-collinear magnetic configuration due to the high values of iDM, in which the physical origin of this interaction (iDM between 3d metals) is due to the spin current plus the SOC contribution.
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JORGE OTAVIO NOVAIS DE SOUZA FILHO
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Iron and Bismuth Based Polycrystals: Synthesis, Conventional DRX and in situ with synchrotron, MEV, MET and Magnetization radiation
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Data: 24/08/2022
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In this work, BiFeO3 nanocrystals and composites of this material with other crystalline phases (Bi2Fe4O9, Bi25FeO40 and Bi2O3) were obtained by the Ghosh solvent evaporation method. Changes in synthesis parameters, such as stoichiometric variations of the bismuth nitrate and iron nitrate reagents, were studied using X-ray diffraction and the Rietveld refinement method. It was concluded that these changes influence the formation of crystalline phases in the samples. The evolution and growth pattern of the crystalline phases were investigated by in situ X-ray diffraction using synchrotron radiation. The measurements were made for maximum temperatures of 400 and 500ºC. It can be seen that the formation of the BiFeO3 crystalline phase occurs at temperatures close to 385ºC. For samples with a maximum temperature of 400ºC, stabilization occurred soon after its formation, with crystallite size around 40 nm. For samples with a maximum temperature of 500ºC, it had a constant growth until reaching an average of 100 nm, stabilizing around 70 minutes after the appearance of the first peaks. The morphology of these structures was studied using both scanning and transmission electron microscopy, and it was possible to perceive the average size of the particles was compatible with the sizes identified in the XRD. Using the EDS mapping together with FFT measurements, it was possible to analyze the distribution of crystalline phases within the BiFeO3/Bi2Fe4O9 composite, where it was noticed that both Bi2Fe4O9 and the amorphous material tend to agglomerate on the surface of BiFeO3. Magnetic properties were studied through measurements of magnetization as a function of temperature, with these studies we could perceive compatible magnetic behaviors for the crystalline phases of BFO around 40~60 nm. We also noticed changes in the magnetic behavior for BiFeO3/Bi2Fe4O9 composites with higher rates of Bi2Fe4O9.
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JAIME ANTONIO URBAN
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Free energy calculation of enzymatic reactions at ab initio level applied to the investigation of RNA methylation by the nsp16-nsp10 complex of SARS-CoV-2
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Data: 19/08/2022
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In this thesis, initially, a reference potential method known as paradynamics is presented, the method is computationally implemented and tested in the reaction of the haloalkane dehalogenase enzyme with 1,2-dichloroethane (DCE). The second part of the work involves the application of a reference potential method to the study of RNA methylation of SARS-CoV-2. The inhibition of key enzymes that may contain the viral replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has assumed central importance in drug discovery projects. Nonstructural proteins (nsps) are essential for RNA capping and coronavirus replication since it protects the virus from host innate immune restriction. In particular, nonstructural protein 16 (nsp16) in complex with nsp10 is a Cap-0 binding enzyme. The heterodimer formed by nsp16-nsp10 methylates the 5'-end of virally encoded mRNAs to mimic cellular mRNAs and thus it is one of the enzymes that is a potential target for antiviral therapy. In this study, we have evaluated the mechanism of the 2'-O methylation of the viral mRNA cap using hybrid quantum mechanics/molecular mechanics (QM/MM) approach. It was found that the calculated free energy barriers obtained at M062X/6-31+G(d,p) is in agreement with experimental observations. Overall, we provide a detailed molecular analysis of the catalytic mechanism involving the 2'-O methylation of the viral mRNA cap and as expected the results demonstrate that the transition state stabilization is critical for the catalysis.
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ANTONIO JOSE BARROS DOS SANTOS
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Vibrational properties of the pyrochlore system KAWO6 (A = Ti,Fe) under extreme pressure conditions
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Data: 29/07/2022
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In this work, the crystalline materials and are studied, which belong to the family of pyrochlore materials, where we propose to investigate the vibrational properties of such crystals in ambient conditions and in extreme pressure conditions. The crystal had its vibrational properties investigated, under extreme hydrostatic pressure, in the range of 0.1 to 9.2 GPa, while the crystal was studied in the range of 0.3 to 9.0 GPa. We made the attribution of the vibrational modes observed under ambient conditions, for both systems, from the results of theoretical calculations based on Wilson's FG matrices, which allowed us to have a better understanding of the vibrational nature of these systems. In order to have a better understanding of the results, in addition to the calculations, we also analyzed the evolution of the wavenumber of the observed modes with the increase in pressure and performed principal component analysis (PCA). We observed for the pyrochlore , 14 active Raman modes, at ambient pressure, with only one of these modes being suppressed with increasing pressure, around 2.8 GPa, this mode centered at 146.8 cm-1, associated with the translation of the potassium. The analysis of the evolution of wave numbers with pressure, for all other modes, shows a significant discontinuity between 5.2 and 6.0 GPa, a discontinuity that was also verified for the PCA analysis, close to the same pressure range. For the pyrochlore , we observed 13 Raman modes and the suppression of one of them with increasing pressure, around 4.6 GPa, this mode centered at 486.2 cm-1, associated with librations of the octahedrons (TiW)O6. We observed significant changes in the behavior of the evolution of wave numbers, for the other modes, between 4.6 and 6.0 GPa, which is corroborated by the PCA analysis. These results lead us to suggest that both materials undergo a first-order phase transition, with such transition for the system taking place between 5.0 and 6.0 GPa and, for the system, the transition takes place between 4.6 and 6.0 GPa. We conclude that the observed phase transitions have as their dominant mechanism, instability of octahedral distortions, causing the collapse of structures close to 5.0 GPa. Our results show that the effect of the differences between iron and titanium ions are not preponderant to change the pressure point where the transition occurs, that is, the exchange of these ions does not seem to be relevant. We conclude that all changes undergone by both pyrochlores are reversible, given that in the decompression experiment the spectral signature of both crystals is recovered with the return of the system to the initial conditions.
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HENRIQUE ALVES VIEIRA
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Bardeen-Kiselev Black Hole with Cosmological Constant
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Data: 26/07/2022
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In this paper, we analyze a solution that mimics the Bardeen solution with a cosmological constant surrounded by quintessence. We obtained this by integrating the Einstein equations associated with nonlinear electrodynamics. We show what the conditions are for it to be regular throughout spacetime.
We also describe the thermodynamics associated with this solution. First by establishing the form of Smarr's formula and the first law of thermodynamics. Then we obtained relevant quantities such as the entropy, the thermodynamic potentials and coefficients, and the compressibility factor. We found that the Black Hole can undergo phase transitions, we named these as: small, small-large, and large. We define the Helmholtz free energy, and thus construct a graph showing the range of existence of each phase. Finally, we calculate the critical exponents of the solution, for a given set of parameter values, and conclude that they are the same as for the Van der Waals fluid.
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RAIMUNDO RONIS DA SILVA E SILVA
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MAGNETIC PROPERTIES OF Mn MONOLAYER ON THE Fe(001) SURFACE
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Data: 27/05/2022
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Here, we present a theoretical study of a Mn monolayer on a Fe(001) surface. The electronic structure and magnetic properties of this system are investigated via electronic structure calculations based on the Density Functional Theory (DFT). In particular, we used the first principle Real Space - Linear Muffin-Tin Orbital - Atomic Sphere Appproximation method (RS-LMTO-ASA), focusing on the investigation of the microscopic mechanisms responsible for the formation of non-collinear spin magnetic configuration. We investigate the behavior of the local magnetic properties of this system, performing calculations of the isotropic Heisenberg exchange interaction (Jij ) and the anisotropic Dzyaloshinskii-Moriya interaction (Dij ) starting from collinear and non -collinear magnetic configurations. These parameters Jij and Dij are analyzed as a function of the Fe-Fe, Mn-Mn and Fe-Mn distances in order to describe the role of these interactions in stabilizing the complex non-collinear structure inherent to this system. We show that, for the non-collinear case, the Dzyaloshinskii-Moriya-like interaction presents a high contribution even in the absence of spin-orbit coupling. Our results are compared with experimental data from the
literature.
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RODRIGO FARIAS OZELA
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PQED, MASS GENERATION AND FRACTAL DIMENSIONS
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Data: 23/02/2022
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In this thesis, we investigate generalizations of the Pseudo quantum electrodynamics (PQED) -- a nonlocal dimensional projection with pseudo operators developed to deal with mixed-dimensionality systems that has flourished after it was discovered it could properly describe 2d materials.
The description of projected photons have been extended to the case of massive photons of two different natures, one of them projected from the gauge-invariant Chern-Simons term and the other from Proca electrodynamics; For the Pseudo-Chern-Simons formulation, we use Schwinger-Dyson equation to characterize the dynamical mass generation for the fermions and the critical limits in which it produces a chiral symmetry breaking, while for the Pseudo-Proca, we calculate the quantum corrections that gives the Fermi velocity renormalization, interaction potential and -factor.
Mixed-dimensionality systems, however, covers essentially any material where one kind of particle is restricted to a smaller than the other particles dimensional subset. This motivates us to also extend PQED in the sense of projecting the interaction onto fractal geometries, but to do that, we first discuss the main ideas behind fractional calculus, giving a few examples, and then we start the discussion on the possibility of a Fractal-PQED in the perspectives.
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JEFFERSON MARCIO SANCHES LOPES
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Excited State Absorption-Based Photochemistry: Porphyrins as Mediators and Probes
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Data: 07/02/2022
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Playing important roles in life, science, and technology, molecular photochemistry is closely dependent on the population of reagent´s excited states. Although the usual one-photon light absorption process (OPA) is widely present in everyday examples, the adoption of high-intensity lasers opens up the opportunity to access nonlinear unusual excitation mechanisms capable of promoting reactions with several benefits, including increased selectivity, use of lowest energy photons (longer wavelengths), and less damage to adjacent components in the target system. This work is devoted to the investigation of photochemical reactions triggered by the nonlinear optical mechanism named excited state absorption (ESA). For such purpose, a nanosecond pulsed laser source is employed. In addition to allowing high-energy excited states to be populated by adopting longer wavelengths in comparison to what is practiced via OPA, ESA allows exploring the role of parities from reagents´ orbitals in the photochemical reaction pathways. Being well known ESA-active absorbers, porphyrins are adopted in this thesis simultaneously as mediators for the promotion of photochemical reactions and as molecular probes to track the formation of photoproducts. Several structures such as free base, zinc-substituted, and supramolecular porphyrins (tetraruthenated porphyrins) were investigated. Three types of ESA-based photochemical reactions are discussed in this work: (i) the photodissociation of a tetraruthenated porphyrin; (ii) the ESA-triggered porphyrin-mediated chloroform decomposition including its effects in the porphyrin structure; and (iii) the ESA-assisted photoprocessing of a zinc porphyrin. In all these cases, the initial solutions evolve to complex molecular systems in which several intermolecular interactions involving remnant reactants and photoproducts leads to phenomena such as exciplex, ground-state complexation, J-aggregation, and resonant energy transfer. The assignment of these phenomenologies is provided by steady-state and time-resolved spectroscopic methods. Systematic investigations addressing the optimal conditions (such as solvent and irradiation conditions) for ESA-based photochemistry in these systems are presented. The stability of photoproducts and the role of porphyrin´s structure are also discussed. Finally, ESA based photochemistry mediated by porphyrins is demonstrated to be a well-controlled, promising and, reliable approach to manipulating molecular systems.
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RUAN LUCAS SOUSA LIMA
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Stokes Spectropolarimetry: apparatus, artifacts and possibilities
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Data: 28/01/2022
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Polarimetry is the technique employed for determining the polarization state of a light beam; i.e., the time-averaged trajectory pattern of the wave’s electric field on a plane orthogonal to propagation. Among many versions, the rotating-waveplate polarimeter has been implemented in several single-wavelength applications to determine the Stokes parameters of light through the Stokes-Mueller formalism. The extension of such technique to spectroscopic measurements is of great interest to the materials research community, and attempts have already been made towards dispersions of the polarization state. However, even though spectral measurements have been demonstrated possible, the need for achromatic polarizing elements requires special attention to their modeling, especially the quarter-wave plate. This dissertation establishes an account of the necessary corrections of the established Fourier analysis approach to determining the Stokes parameters, which fully describe any partially polarized light beam. In this work, we visit the literature’s state-of-the-art in polarimetry and implement the rotating-waveplate polarimeter for spectral measurements with achromatic polarizing elements over the visible range (400-800 nm). Initially using the simplistic ideal model available for such items, we are able to probe strong signs of artifacts in the spectral measurements. After analyzing the broadband calibration data and revisiting the literature, we develop a better model for the waveplate, such that the real Stokes parameters may be determined apart from the intrinsic dispersive properties of the retarder; the equivalent rotatory angle, the fast axis tilt, and the retardance. Here we present the mathematical and experimental procedure to have them properly characterized and taken as correction terms in the polarimetric measurements. We also show the enhancement in precision on simulated states, as well as on application examples of solution samples, which corroborates that the remodeling is necessary, and the approach developed here is essential for reliable and trustworthy spectropolarimetric data generation and analysis.
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