L'observation et l'étude des milieux moléculaires tel que le milieu interstellaire requièrent une connaissance détaillée des vitesses des processus de peuplement radiatifs et collisionnels afin d'accéder aux caractéristiques physiques (températures, densités, abondances des molécules, ...) de ces milieux. Les hydrures d'azote sont très présents dans les milieux interstellaires où ils s'avèrent être des intermédiaires de réaction important dans la chimie de l'azote. L'interprétation des observations des hydrures d'azote réalisées grâce au satellite HERSCHEL et l'interféromètre millimétrique ALMA, nécessite de connaître avec suffisamment de précision les taux de collision de ces molécules. Nous nous sommes donc intéressés dans cette thèse à la détermination des taux de collisions inélastiques des hydrures NH, NH2 et NH3 avec Ne, H2 et H, respectivement. A partir de surfaces d’énergie potentielle, les sections efficaces ont été obtenues pour les trois systèmes par la méthode exacte des équations couplées en utilisant les codes de dynamique moléculaire MOLSCAT et HIBRIDON. Les taux de collision sont ensuite obtenus pour des températures allant de 5 à 200 K. Enfin, les nouveaux taux de collisions de NH2 avec H2 ont été utilisés dans un code de transfert radiatif afin de reproduire les observations de NH2 en direction d'un nuage de formation d’étoile massive W31C. Nous montrons que les nouveau taux de collisions calculés apportent des contraintes sur les conditions physiques du nuage moléculaire. / Observation and study of interstellar molecular clouds require the knowledge of molecular data to derive the physical conditions (temperature, gas density, molecular abundance) of these media. Nitrogen hydrides are highly abundant species in the interstellar medium and they are found to be important reaction intermediates in the nitrogen chemistry. The interpretation of nitrogen hydrides observations from the HERSCHEL spatial observatory and the ALMA interferometer, requires accurate collisional rate coefficients of these molecules. The present thesis focuses on the determination of new rate coefficients for NH, NH2 and NH3 molecules in collision with Ne, H2 et H, respectively. Inelastic cross sections for the rotational excitation of all the studied systems have been computed with a close coupling method using the molecular dynamic codes MOLSCAT and HIBRIDON. The cross sections are then used to calculate the collisional rate coefficients for temperatures ranging from 5 to 200 K. The new rate coefficients were included in radiative transfer calculations in order to model the observed transitions of NH2 towards high-mass star-forming region W31C. We show that using the previously published rate coefficients instead of these new ones have a significant impact on the modeling, leading to important differences on the density, abundance and on the OPR of NH2. The new data will allow to put interesting constraints on the physical condition of the molecular cloud.

No description available.

Les nanocristaux de CdSe font l'objet d'applications émergentes dans les domaines de la nano-électronique, des technologies laser ou du marquage fluorescent de biomolécules. Pour ces applications, la détermination de la structure fine de l'exciton de bord de bande et des mécanismes de relaxation entre sous-niveaux est d'un intérêt majeur. Cette thèse a été consacrée à l'étude spectroscopique à basse température et sous champ magnétique de nanocristaux individuels de CdSe/ZnS. La remarquable photostabilité des nanocristaux étudiés a permis de caractériser les propriétés optiques des deux états excitoniques de plus basse énergie: l'état excitonique fondamental "noir", et l'état excitonique "brillant" situé quelques meV plus haut en énergie. Ces études ont aussi permis d'identifier un état excitonique chargé (trion) et de caractériser ses propriétés photophysiques. La possibilité de générer une cascade radiative biexciton-exciton a également été démontrée dans ces systèmes. / CdSe nanocrystals are attractive for many applications such as nanoscale electronics, laser technology, and biological fluorescent labelling. A detailed understanding of the band-edge exciton fine structure and the relaxations pathways between sub-levels are crucial for these applications. During this project we have studied the optical properties of single CdSe/ZnS nanocrystals at cryogenical temperature and under magnetic field. The dramatic photostability of the nanocrystals' emission has allowed the optical study of the two lowest exciton states: the "dark" excitonic state and the "bright" excitonic state, lying few meV above. These studies have also enabled us to identify a charged excitonic state (negative trion) and to characterize its photophysical properties. Besides we have demonstrated that in these nanocrystals, radiative cascade biexciton-exciton might be generated.

Doctor of Philosophy / Department of Physics / B.D. DePaola / In recent years, coherent excitation techniques have focused on the ability to efficiently prepare atomic or molecular systems into a selected state. Such population control plays a key role in cutting-edge research taking place today, such as in the areas of quantum information and laser-controlled chemical reactions. Stimulated Raman adiabatic passage (STIRAP) is a widely-used coherent excitation technique that provides a relatively robust control mechanism for efficiently exciting a target population into a desired state. While the technique is well proven, current experimental techniques yield little information on the population dynamics taking place throughout the excitation process, and experimentalists rely solely on final excited-state measurements to determine the efficiency of population transfer. This dissertation presents a unique diagnostic tool to measure multilevel coherent population transfer on a short (nanosecond) timescale. The technique described here uses magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) as a noninvasive probe of a coherently-controlled system. It provides extremely detailed information about the excitation process, and highlights some important characteristics seen in excited populations that would otherwise be misleading or completely overlooked if one were to use more traditional diagnostic techniques. This dissertation discusses both the theoretical and experimental results applied to three-level coherently excited target populations of Rb-87.

Philosophiae Doctor - PhD / In the first experiment high spin states in 194Tl, excited through the 181Ta (18O, 5n) heavyion fusion evaporation reaction were studied using the AFRODITE array at iThemba LABS. The γ-γ coincidences, RAD ratios and linear polarization measurements were carried out and the previously known level scheme of 194Tl was significantly extended. A total of five rotational bands four of which are new were observed. A pair of rotational bands associated with the πh9/2 ⊗ νi−1 13/2 configuration at lower spins and with the πh9/2 ⊗ νi−3 13/2 configuration at higher spins was found and interpreted as the first possible chiral bands followed above the band crossing. The two 4-quasiparticle bands show exceptionally close near-degeneracy in the excitation energies. Furthermore close similarity is also found in their alignments and B(M1)/B(E2) reduced transition probability ratios. In the second experiment lifetimes in 194Tl were measured using the DSAM technique with the excited states in this nucleus populated through the 181Ta (18O, 5n) reaction. A total of 25 lifetimes and 30 reduced transition probabilities of magnetic dipole B(M1) and electric quadrupole B(E2) have been evaluated. Furthermore B(M1) and B(E2) reduced transition probabilities in Bands 1 and 4, which have been regarded as chiral candidates, were found to be close to each other and reveals strong splitting along spin values. This further supports the proposed chiral nature of these two bands.

Ce mémoire porte sur l'étude et l’optimisation du fonctionnement des extensomètres à corde vibrante pour la surveillance des constructions stratégiques. Ce travail se décompose en trois étapes.Premièrement, le fonctionnement du capteur en mode amorti et en mode entretenu est décrit. Un dispositif électronique de lecture à base de microcontrôleur est réalisé pour valider les développements. Deuxième, le comportement du capteur est étudié en établissant des modèles représentant les couplages magnéto-mécaniques entre la corde vibrante, la bobine d'excitation et la bobine de mesure. Les influences du mode d'excitation et des paramètres parasites sur la réponse du capteur sont analysées. Enfin, l'effet de la foudre est simulé en utilisant un simulateur de décharge électrostatique. La réponse du capteur est mesurée avant, pendant et après la décharge. Les résultats de ce travail peuvent se résumer en quatre points essentiels. Premièrement, le mode entretenu est préférable au mode amorti. Deuxièmement, la position d'excitation optimale est au centre de la corde. De plus, une excitation continue ou une excitation pulsée d'un grand nombre de cycles est préférable à une excitation pulsée d'un petit nombre de cycles. Troisièmement, la fréquence de résonance mesurée en mode entretenu diffère de celle mesurée en mode amorti à cause de deux paramètres : la force magnétique moyenne et le couplage mutuel entre les bobines. Quatrièmement, sous l'action des décharges, le champ magnétique résiduel est modifié, conduisant ainsi à une modification de la fréquence de résonance mesurée par le capteur. / This thesis focuses on the study and the optimization of the operation of vibrating wire sensors for strategic construction monitoring. This work is divided into three stages.Firstly, the sensor operation in damped mode and in sustained mode is described. To validate the developments, an electronic microcontroller-based reading device is realized. Secondly, the sensor behavior is investigated by establishing models representing the magneto-mechanical coupling between the vibrating wire, the excitation coil and the measurement coil. The influences of excitation mode and spurious parameters on the sensor response are analyzed. Finally, the effect of lightning is simulated using electrostatic discharge simulator. The sensor response is measured before, during and after discharge.The results of this work can be summarized in four main points. Firstly, the sustained mode is preferable to the damped mode. Secondly, the optimal excitation position is at the center of the wire. Furthermore, a continuous excitation or a pulsed excitation of a large number of cycles is preferable to a pulsed excitation of a small number of cycles. Thirdly, the resonant frequency measured in sustained mode differs from that measured in damped mode due to two parameters: average magnetic force and mutual coupling between coils. Fourthly, under the action of discharges, the remaining magnetic field is modified, thus leading to a variation in the resonant frequency measured by the sensor.

Les supraconducteurs à base d'oxyde de cuivre ont fait l'objet d'études approfondis mais ils restent toujours au centre de nombreux débats. Après 30 années de recherche, certaines questions scientifiques ont été éclairées, alors que d'autres restent controversées. L'évolution du paramètre d'ordre supraconducteur avec la température et le dopage en est un cas exemplaire. Dans cette thèse nous décrivons notre étude systématique de supraconducteurs à haute temperature critique par Angle Resolved PhotoEmission Spectroscopy (ARPES: Spectroscopie de photoémission résolue en angle) et par ARPES resolué en temps. On a mesuré du Bi2Sr2CaCu2O8+δ avec dopage optimale (c.a.d. montrant la Tc la plus élevé), afin d'explorer la possibilité que une photo-excitation très intense du supraconducteur crée un état avec des paires de Cooper incohérentes et sans aucune densité superfluide. La méthode expérimentale employée nous a permis de mesurer la dynamique des électrons hors équilibre et du gap supraconducteur, en fournissant des informations complémentaires à l'ARPES conventionnelle et aux mesures optiques. Nos données de ARPES résolue en temps sur Bi2Sr2CaCu2O8+δ montrent que une photo-excitation génère un effondrement du gap supraconducteur qui dépend du moment. La relaxation des quasi-particules acquiert une composante rapide à la fluence F de seuil ou le gap s'effondre complètement. La comparaison entre le F_pair et le F_phase spar spectroscopie optique THz résolue en temps suggère que il y aurait un régime de fluence ou les paires de Cooper survivent, mais ne portant plus un courant superfluide.Un autre défi majeur dans la physique des cuprates supraconducteurs à haute température critique (HTSCs) est la compréhension de l'état normale à haute température. Nous presentons aussi un étude ARPES du pseudo-gap proche du point nodale de la surface de Fermi dans le système La2-xBaxCuO4 (LBCO), qui montre un possible lien avec les modulations de densité de charge (stripes). Nos données suggerent que le gap s'ouvre en dessous de la transition LTT-LTO associé à l'apparition des modulations de charge tandis que la fonction spectrale n'est pas affecté par le modulation de la densité de spin. On observe aussi que la structure de bande dans LBCO est rénormalisé avant le "kink" à environs 70 meV. Nous corrélons cette rénormalisation en énergie et moment à l'amollissement du mode d'étirement de la liaison Cu-O. / The superconductors of the copper-oxide family have been matter of extensive investigations and are still subject of fierce debates. After 30 years of research, some issues have been settled, whereas others remain controversial. The evolution of the superconducting order parameter with temperature and doping level is an exemplary case. In this thesis, we report a systematic Time resolved Angle Resolved PhotoEmission Spectroscopy (ARPES) study of the optimally doped Bi2Sr2CaCu2O8+δ to explore the possibility that an intense photoexcitation of the superconductor can generate a state with incoherent copper pairs and no superfluid density. The employed experimental methods allow us to measure the dynamics of non-equilibrium electrons and of the superconducting gap, providing complementary information to conventional ARPES and optical measurement. Our time resolved ARPES data of Bi2Sr2CaCu2O8+δ, report a momentum-dependent collapse of the superconducting gap upon photoexcitation. Interestingly, the QP relaxation develops a faster component at the threshold fluence F_pair where the gap has fully collapsed. The comparison between the F_pair and the F_phase extracted by tr-THz suggested the existence a fluence regime when the Cooper pairs have survived, but without holding superfluid current. A second major challenge in the physics of HTSCs is the poor understanding of the normal phase at high temperature. We also present the ARPES study of the near nodal pseudo-gap in La2-xBaxCuO4 (LBCO) to show a possible link with charge modulation (stripes). Our data show that the near nodal gap open below the LTT-LTO transition, which is linked to the formation of such modulations, instead of the one where spin modulations appear. The data show that the band structure of LBCO is affected by a renormalization setting in prior to the 70 meV kink. We were able to correlate this renormalization of the ARPES data to the region where the Cu-O bond-stretching mode soften, both in energy and momentum space.

During the past two decades, many researchers have developed methods for the detection of structural defects at the early stages to operate the aerospace vehicles safely and to reduce the operating costs. The Surface Response to Excitation (SuRE) method is one of these approaches developed at FIU to reduce the cost and size of the equipment. The SuRE method excites the surface at a series of frequencies and monitors the propagation characteristics of the generated waves. The amplitude of the waves reaching to any point on the surface varies with frequency; however, it remains consistent as long as the integrity and strain distribution on the part is consistent. These spectral characteristics change when cracks develop or the strain distribution changes. The SHM methods may be used for many applications, from the detection of loose screws to the monitoring of manufacturing operations. A scanning laser vibrometer was used in this study to investigate the characteristics of the spectral changes at different points on the parts. The study started with detecting a load on a plate and estimating its location. The modifications on the part with manufacturing operations were detected and the Part-Based Manufacturing Process Performance Monitoring (PbPPM) method was developed. Hardware was prepared to demonstrate the feasibility of the proposed methods in real time. Using low-cost piezoelectric elements and the non-contact scanning laser vibrometer successfully, the data was collected for the SuRE and PbPPM methods. Locational force, loose bolts and material loss could be easily detected by comparing the spectral characteristics of the arriving waves. On-line methods used fast computational methods for estimating the spectrum and detecting the changing operational conditions from sum of the squares of the variations. Neural networks classified the spectrums when the desktop – DSP combination was used. The results demonstrated the feasibility of the SuRE and PbPPM methods.

During the past two decades, many researchers have developed methods for the detection of structural defects at the early stages to operate the aerospace vehicles safely and to reduce the operating costs. The Surface Response to Excitation (SuRE) method is one of these approaches developed at FIU to reduce the cost and size of the equipment. The SuRE method excites the surface at a series of frequencies and monitors the propagation characteristics of the generated waves. The amplitude of the waves reaching to any point on the surface varies with frequency; however, it remains consistent as long as the integrity and strain distribution on the part is consistent. These spectral characteristics change when cracks develop or the strain distribution changes. The SHM methods may be used for many applications, from the detection of loose screws to the monitoring of manufacturing operations. A scanning laser vibrometer was used in this study to investigate the characteristics of the spectral changes at different points on the parts. The study started with detecting a load on a plate and estimating its location. The modifications on the part with manufacturing operations were detected and the Part-Based Manufacturing Process Performance Monitoring (PbPPM) method was developed. Hardware was prepared to demonstrate the feasibility of the proposed methods in real time. Using low-cost piezoelectric elements and the non-contact scanning laser vibrometer successfully, the data was collected for the SuRE and PbPPM methods. Locational force, loose bolts and material loss could be easily detected by comparing the spectral characteristics of the arriving waves. On-line methods used fast computational methods for estimating the spectrum and detecting the changing operational conditions from sum of the squares of the variations. Neural networks classified the spectrums when the desktop – DSP combination was used. The results demonstrated the feasibility of the SuRE and PbPPM methods.

No description available.