No description available.

<p> We study a many particle system contained within a harmonic oscillator potential, with the single particle spacings equispaced and nondegenerate. We consider either fermions or bosons within this harmonic oscillator potential and derive the density of states for this system, with low excitation energy corrections. We extend our method for more than one species of fermions or bosons within this harmonic oscillator potential and compare our results with number-theoretic techniques. We move on to the degenerate harmonic oscillator potential and show that, for a fermion system, shell effects are contained within low excitation energy terms. We attempt to arrive at the density of states for an infinite number of bosons within the same potential. </p> / Thesis / Master of Science (MSc)

No description available.

Desenvolve uma pesquisa em dinâmica estrutural não-linear com enfoque teórico e experimental direcionada para uma importante classe de estruturas flexíveis. É motivada pelos novos requerimentos das indústrias em termos de inovações, das agências certificadoras em termos de segurança e conforto e, por restrições relativas ao meio ambiente cada vez mais severas. Como conseqüência, o cenário atual e os desafios da engenharia moderna são bem diferentes daqueles encontrados antigamente. Atualmente as estruturas são mais flexíveis e operam sob condições cada vez mais severas. O aumento da flexibilidade torna as não-linearidades mais ativas e, juntamente com a ação de diferentes formas de excitação, produzem um cenário dinâmico complexo. Neste cenário, diversos fenômenos dinâmicos intrinsicamente não-lineares podem se desenvolver e conseqüêntemente comprometer a integridade estrutural, prejudicar a operação e incrementar os problemas de ruído. Tais fenômenos são altamente perigosos, principalmente por não serem previstos e nem ao menos conhecidos pela teoria dinâmica linear. Dentre estes fenômenos, a pesquisa se propõe a abordar dois: vibrações ressonantes paramétricas e autoparamétricas. Especificamente, a pesquisa investiga a influência da viscosidade do meio de operação e da presença de excitações combinadas nos fenômenos de ressonância paramétrica. No caso das ressonâncias autoparamétricas o objetivo específico é avaliar técnicas experimentais na caracterização do fenômeno, assim como, promover entendimentos mais profundos sobre suas características. Para atingir os objetivos propostos, são construídas duas estruturas de laboratório com características aeronáuticas. A primeira faz alusão a um estabilizador vertical. Nesta estrutura foram desenvolvidos os trabalhos relativos à vibração ressonante paramétrica. A segunda é uma simplificação de um sistema estrutural asa-pilone-turbina. Nesta segunda estrutura foram avaliadas técnicas experimentais para a identificação, caracterização e análise da vibração ressonante autoparamétrica. Os resultados teóricos e experimentais demonstram que a viscosidade do meio de operação age positivamente na dinâmica da estrutura, reduzindo níveis máximos de vibração em regime permanente, simplificando a dinâmica em respostas transientes e facilitando as relações de estabilidade/instabilidade. Por fim, apresenta resultados experimentais demonstrando que a energia vibratória da asa pode ser transferida por intermédio de uma ressonância autoparamétrica principal para a sub-estrutura pilone-turbina resultando em níveis extremamente elevados de vibração. / This document presents results of theoretical and experimental investigations on the non-linear vibration characteristics of an important class of flexible structures. The motivation for such a study arises mainly from the increasing need for lightweight structural members. The weight reduction associated to the use of novel materials contribute to the increase of flexibility what can cause the appearance of nonlinear effects not previously observed. These nonlinear phenomena associated to the fact that, in field conditions the structure is frequently subjected to complex dynamic loads of different nature, results in a complex dynamic environment when estimation of the structure's dynamic response is concerned. Moreover, these nonlinear effects potentially may cause undesired vibration level, in some cases causing bad function and failure of the entire structure. The research is focused on studying the effects of medium viscosity as well as combined excitations on parametrically resonant vibrating structures. It is speciffically aimed characterize the phenomenons either analytically and experimentally by constructing laboratory test specimens that resemble aircraft structures. For that purpose a vertical fin is built in order to conduct experiments on the principal parametric resonance phenomenon. An analytical single degree of freedom model of this structure including nonlinear terms is derived and the results of numerically simulated results through perturbation technique are compared to experimental results obtained in the laboratory. A second structure is built that resembles a typical wing-pylon-engine substructure and it is used to study autoparametric resonance vibrations. In this case the structure is considered with multiple degrees of freedom and the results of a finite element model is correlated with experimentally obtained results. Theoretical and experimental results show that the environment viscosity affects in a significant manner the dynamic response of the structures under test, decreasing the maximum vibration levels in steady-state regime, simplifying the dynamics in transient responses and facilitating the relationship between instability/stability. At the end, it is shown experimental results demonstrating that vibratory energy from the wing substructure can be transferred by an autoparametric resonance to the substructure pylon-engine. All the experimental results do not found linear theory counterparts.

Desenvolve uma pesquisa em dinâmica estrutural não-linear com enfoque teórico e experimental direcionada para uma importante classe de estruturas flexíveis. É motivada pelos novos requerimentos das indústrias em termos de inovações, das agências certificadoras em termos de segurança e conforto e, por restrições relativas ao meio ambiente cada vez mais severas. Como conseqüência, o cenário atual e os desafios da engenharia moderna são bem diferentes daqueles encontrados antigamente. Atualmente as estruturas são mais flexíveis e operam sob condições cada vez mais severas. O aumento da flexibilidade torna as não-linearidades mais ativas e, juntamente com a ação de diferentes formas de excitação, produzem um cenário dinâmico complexo. Neste cenário, diversos fenômenos dinâmicos intrinsicamente não-lineares podem se desenvolver e conseqüêntemente comprometer a integridade estrutural, prejudicar a operação e incrementar os problemas de ruído. Tais fenômenos são altamente perigosos, principalmente por não serem previstos e nem ao menos conhecidos pela teoria dinâmica linear. Dentre estes fenômenos, a pesquisa se propõe a abordar dois: vibrações ressonantes paramétricas e autoparamétricas. Especificamente, a pesquisa investiga a influência da viscosidade do meio de operação e da presença de excitações combinadas nos fenômenos de ressonância paramétrica. No caso das ressonâncias autoparamétricas o objetivo específico é avaliar técnicas experimentais na caracterização do fenômeno, assim como, promover entendimentos mais profundos sobre suas características. Para atingir os objetivos propostos, são construídas duas estruturas de laboratório com características aeronáuticas. A primeira faz alusão a um estabilizador vertical. Nesta estrutura foram desenvolvidos os trabalhos relativos à vibração ressonante paramétrica. A segunda é uma simplificação de um sistema estrutural asa-pilone-turbina. Nesta segunda estrutura foram avaliadas técnicas experimentais para a identificação, caracterização e análise da vibração ressonante autoparamétrica. Os resultados teóricos e experimentais demonstram que a viscosidade do meio de operação age positivamente na dinâmica da estrutura, reduzindo níveis máximos de vibração em regime permanente, simplificando a dinâmica em respostas transientes e facilitando as relações de estabilidade/instabilidade. Por fim, apresenta resultados experimentais demonstrando que a energia vibratória da asa pode ser transferida por intermédio de uma ressonância autoparamétrica principal para a sub-estrutura pilone-turbina resultando em níveis extremamente elevados de vibração. / This document presents results of theoretical and experimental investigations on the non-linear vibration characteristics of an important class of flexible structures. The motivation for such a study arises mainly from the increasing need for lightweight structural members. The weight reduction associated to the use of novel materials contribute to the increase of flexibility what can cause the appearance of nonlinear effects not previously observed. These nonlinear phenomena associated to the fact that, in field conditions the structure is frequently subjected to complex dynamic loads of different nature, results in a complex dynamic environment when estimation of the structure's dynamic response is concerned. Moreover, these nonlinear effects potentially may cause undesired vibration level, in some cases causing bad function and failure of the entire structure. The research is focused on studying the effects of medium viscosity as well as combined excitations on parametrically resonant vibrating structures. It is speciffically aimed characterize the phenomenons either analytically and experimentally by constructing laboratory test specimens that resemble aircraft structures. For that purpose a vertical fin is built in order to conduct experiments on the principal parametric resonance phenomenon. An analytical single degree of freedom model of this structure including nonlinear terms is derived and the results of numerically simulated results through perturbation technique are compared to experimental results obtained in the laboratory. A second structure is built that resembles a typical wing-pylon-engine substructure and it is used to study autoparametric resonance vibrations. In this case the structure is considered with multiple degrees of freedom and the results of a finite element model is correlated with experimentally obtained results. Theoretical and experimental results show that the environment viscosity affects in a significant manner the dynamic response of the structures under test, decreasing the maximum vibration levels in steady-state regime, simplifying the dynamics in transient responses and facilitating the relationship between instability/stability. At the end, it is shown experimental results demonstrating that vibratory energy from the wing substructure can be transferred by an autoparametric resonance to the substructure pylon-engine. All the experimental results do not found linear theory counterparts.

Electronic Excitation Energy Transfer is an important intermolecular photophysical process that can affect the excited state lifetime of a chromophore. A molecule in an electronically excited state can return to the ground state by radiative as well as non-radiative processes. During the excited state lifetime, if the chromophore (energy donor) finds a suitable species (energy acceptor) nearby with resonant energy levels, it can transfer the excitation energy to that species and return to the ground state. This process is called Electronic Excitation Energy Transfer. When the energy donor is fluorescent, the process is called Fluorescence Resonance Energy Transfer (FRET) [1]. FRET is a non-radiative process that affects the fluorescence intensity as well as the excited state lifetime of the donor. It occurs due to the electrostatic coulombic interaction between the transition charge densities of the donor and the acceptor. The rate of energy transfer can be evaluated using the Fermi golden rule of quantum mechanics [2]. When the donor and the acceptor are separated by distances that are much larger in comparison with the sizes of the donor and the acceptor, the interaction between them can be thought of as that between their transition dipoles. In such a case, the interaction between the donor and the acceptor is dipolar and the rate of energy transfer has an R−6 dependence, where R is the distance between the donor and the acceptor [3]. This dependence has first been suggested theoretically by Forster in 1947 [4] followed by the experimental verification by Stryer and Haugland [5]. Since then the process has been used as a spectroscopic ruler to study the conformational dynamics of biopolymers like DNA, RNA, proteins etc [6]. A variety of dye molecules have been explored for donors and acceptors in FRET and the range of distances that can be measured using FRET involving dyes is in the range 1 − 10 nm. When the distances between the donor and the acceptor are not much larger in comparison with their sizes, the dipolar approximation to the interaction is not a very good approximation, thereby leading to deviations from the traditional R-6 dependence. Such non-R-6dependencies are found for polymers, quantum wells, quantum wires etc [7–9]. The interest in such dependencies is due to the need for developing nanoscopic rulers that can measure distances well beyond 10 nm. The objective of our work has been to study energy transfer from fluorophores to various kinds of acceptors that have extended charge densities and understand the distance dependence of the rate of energy transfer [10]. We use the Fermi golden rule as the starting point and develop analytical models for evaluating the rate as a function of the distance between the donor and the acceptor. We study the process of energy transfer from fluorescent dye molecules that serve as energy donors to a variety of energy acceptors namely, graphene, doped graphene, single-walled carbon nanotubes and metal nanoparticles. We also study transfer from fluorophores to a semiconducting sheet and a semiconducting tube of electronic charge density. There have been experimental studies in the literature of the fluorescence quenching of dyes near single-walled carbon nanotubes [11–13]. But, there are no studies of the distance dependence of rate. Single-walled carbon nanotubes can be thought of as rolled up sheets of graphene. However, interestingly, there were no reports of fluorescence quenching by graphene at the time when we thought of this possibility. Therefore, we first study the process of energy transfer from a fluorophore, which is kept at a distance z above a layer of graphene to the electronic energy levels of graphene. We find that the long range behavior of the rate has an z -4 dependence on the distance [14, 15]. From our study of transfer from pyrene to graphene, we find that fluorescence quenching can be experimentally observed up to a distance of ~ 30 nm, which is quite large in comparison with the traditional FRET limit (10 nm). Recent experiments that have been performed after our theory was reported have in fact observed the fluorescence quenching of dyes near graphene. Further, the process has been found to be very useful in fabricating devices based on graphene [16], in eliminating fluorescence signals in resonance Raman spectroscopy [17] and in visualizing graphene based sheets using fluorescence quenching microscopy [18]. The process has also been found to be useful in quantitative DNA analysis [19, 20]. We study the transfer of an amount of energy hΩ from a dye molecule to doped graphene [21]. We consider the shift of the Fermi level from the K-point into the conduction band of graphene as a result of doping and evaluate the rate of transfer. We find a crossover of the distance dependence of the rate from z -4 to exponential as the Fermi level is increasingly shifted into the conduction band, with the crossover occurring at a shift of the Fermi level by an amount hΩ/2. We study the process of transfer of excitation energy from a fluorophore kept at a distance d away from the surface of a carbon nanotube to the electronic energy levels of the nanotube. We find both exponential and d−5 behavior of the rate [22]. For the case of metallic nanotubes, when the emission energy of the fluorophore is less than a threshold, the dependence is exponential. Otherwise, it is d−5 . For the case of semiconducting nanotubes, we find that the rate follows an exponential dependence if the amount of energy that is transferred can cause only the excitonic transition of the tube. However, if any other band gap transition is allowed, the rate follows a d−5 dependence. For the case of transfer from pyrene to a (6, 4) nanotube, we find that energy transfer is appreciable up to a distance of ~ 17 nm. We then study the process of energy transfer from a fluorophore to a semiconducting sheet of electronic charge density [10]. We find that the rate has an z-4 dependence. For the case of transfer to a semiconducting tube, we find that the rate has a d -5dependence. The dependencies are in agreement with those obtained for graphene and carbon nanotubes respectively. This shows that the asymptotic distance dependencies are a consequence of the dimensionality of the transition charge densities and are robust. Strouse et al. [23, 24] have studied the process of energy transfer from the dye fluorescein to a 1.4 nm diameter gold nanoparticle. Double-stranded DNA molecules of various lengths were used to fix the distances between the donor and the acceptor. The rate was found to have a d-4distance dependence. They refer to this process as Nanoparticle Surface Energy Transfer (NSET) and the range of distances that can be measured using NSET is more than double that of the traditional FRET experiments. However, theoretical studies that consider the transfer to the plasmonic modes of the nanoparticle find a predominant R-6 dependence [25]. We study the process of energy transfer from the dye fluorescein to a 1.4 nm diameter gold nanoparticle considering the excitation of plasmons as well as electron-hole pairs of the nanoparticle [26]. We find that the rate follows the usual Forster type R−6 distance dependence at large distances. But, at short distances, there are contributions of the form R−-n with n > 6. This is due to the quadrupolar and octupolar modes of excitation of the nanoparticle, the rates corresponding to which have R-8 and R−-10 dependencies respectively. Recent calculations using DFT also find similar deviations at short distances [27].

Typescript (photocopy). / Digitized by Kansas Correctional Industries

International Telemetering Conference Proceedings / October 18-21, 2004 / Town & Country Resort, San Diego, California / This paper presents the design and development of a fluorometry sensor network with LED excitation. The design of the electronics in junction with the capability of LED excitation will significantly reduce the size and costs of the flrorometer units. The coverage and effectiveness of the sensing and monitoring capability will also be greatly enhanced by the addition of the wireless networks.

No description available.

Temporal features in the vocalizations of animals and insects play an important role in a diverse range of species-specific activities such as mate selection, territoriality, and hunting. The neural mechanisms underlying the response to such stimuli remain largely unknown. Two species of anuran amphibian provide a starting point for the investigation of the neurological response to species-specific advertisement calls. Neurons in the anuran midbrain of Rana pipiens and Hyla regilla exhibit an atypical response when presented with a fixed number of advertisement calls. The general response to these calls is mostly inhibitory; only when the correct number of calls is presented at the correct repetition rate will this inhibition be overcome and the neurons reach a spiking threshold. In addition to rate-dependent call-counting, these neurons are sensitive to missed calls: a pause of sufficient duration—the equivalent of two missed calls—effectively resets a neuron to its initial condition. These neurons thus provide a model system for investigating the neural mechanisms underlying call-counting and interval specificity in audition. We present a minimal computational model in which competition between finely-tuned excitatory and inhibitory synaptic currents, combined with a small propagation delay between the two, broadly explains the three key features observed: rate dependence, call counting, and resetting. While limitations in the available data prevent the determination of a single set of parameters, a detailed analysis indicates that these parameters should fall within a certain range of values. Furthermore, while network effects are counter-indicated by the data, the model suggests that recruitment of neurons plays a necessary role in facilitating the excitatory response of counting neurons—although this hypothesis remains untested. Despite these limitations, the model sheds light on the mechanisms underlying the biophysics of counting, and thus provides insight into the neuroethology of amphibians in general.