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Concept# Diffusion Rayleigh

Résumé

La diffusion Rayleigh est un mode de diffusion des ondes, par exemple électromagnétiques ou sonores. Elle opère lorsque la longueur d'onde est beaucoup plus grande que la taille des particules diffusantes. On parle de diffusion élastique, car cela se fait sans variation d'énergie, autrement dit l'onde conserve la même longueur d'onde. Elle est nommée d'après John William Strutt Rayleigh, qui en a fait la découverte.
Lorsque les particules ont une taille suffisamment grande devant la longueur d'onde incidente, il faut utiliser d'autres théories comme la théorie de Mie qui fournit une solution exacte à la diffusion par des particules sphériques de taille quelconque (la diffusion Rayleigh est un cas limite de la théorie de Mie).
Diffusion Rayleigh des ondes électromagnétiques
L'onde électromagnétique peut être décrite comme un champ électrique oscillant couplé à un champ magnétique oscillant à la même fréquence.
Ce champ électrique va déformer le nuage électronique des atomes

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Lumière

vignette|Rayons de lumière sortant des nuages.
Dans son sens le plus habituel, la lumière est le phénomène à l'origine d'une sensation visuelle. La physique montre qu'il s'agit d'ondes électromagnétiq

Sunlight

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's at

Indice de réfraction

vignette|Image des fronts d'onde émis par une source ponctuelle mettant en évidence le phénomène de réfraction. La zone inférieure située sous la ligne grise a un plus grand indice de réfraction et do

State-to-state molecule/surface scattering experiments prepare the incident molecules in a specific quantum state and measure the quantum state distribution of the scattered molecules. The comparison of state resolved experiments with theory can serve as stringent tests of the molecule/surface interaction potential and of the scattering dynamics. The overall motivation is to develop a predictive understanding of the molecule/surface interactions and reactions needed for the understanding and optimization of heterogeneous catalysis. In this thesis, I describe the design characterization, and first applications of a new apparatus, dedicated to performing state-to-state surfaces scattering using the bolometric infrared laser tagging detection (BILT). An important advantage of the BILT detection method over other state-resolved detection techniques such as resonant multi-photon ionization (REMPI), is its applicability to any molecule with an infrared active vibrational mode and a rotationally resolved vibrational gas phase spectrum. For example, BILT detection can be used to detect important molecules such as methane and carbon dioxide for which REMPI detection is not possible. Our new state-to-state scattering machine features a liquid helium-cooled bolometer detector installed on a rotatable lid allowing independent variation of the incident and the scattering angle. With this capability, one can study energy transfer such as the conversion of translational to rotational or vibrational energy as well as vibrational energy redistribution for molecules colliding with a well-defined single crystal surface at a wide variety of scattering geometry. To demonstrate the capabilities of the BILT machine, I studied the rotationally inelastic scattering of CH4 from Ni(111). The results show that rotational excitation depends not only on the kinetic energy along the surface normal but also on the parallel component although with lower efficiency. The extent of rotational excitation is found to increase with increasing surface temperature. The conversion efficiency appears to be higher for low velocity component normal to the surface. The observations indicate a mechanism of rotational excitation by phonon annihilation with the probability related to the relative velocity of the incoming molecules and surface atoms. Using the experimentally determined formula which takes into account the conversion efficiency of the normal, the parallel component of incident kinetic energy, and the surface thermal energy to the rotational energy, the mechanism of rotational excitation of CH4 scattering from Ni(111) is quantitatively unraveled. Besides rotational inelastic scattering, I report very distinct behavior of vibrationally inelastic scattering of CH4 from clean Ni(111) and Gr-Ni(111). Vibrational energy transfer to the surface dramatically changes when a clean Ni(111) surface is covered with a single layer of graphene. Theoretical calculations based upon reaction path Hamiltonian suggest that the probability of the vibrational energy transfer is related to the catalytic activity of the surface impact sites. Therefore, by monitoring the fate of the initially prepared vibrational energy, the state-to-state surface scattering technique can potentially serve as a probe of the catalytic activity of surfaces.

In this thesis, a new method has been developed, which determines the optical coefficients spectra (absorption and scattering) of small volumes of tissue (1 mm3) by the measurement of spatially resolved reflectance between 480 and 950 nm. Firstly, an algorithm has been developed to determine the optical coefficients form reflectance measurements: It is based on the optical propagation model introduced by Bevilacqua [Bevilacqua 99 JOSA]. This model takes the first two moments of the phase function into account through the γ parameter, allowing to accurately simulate the propagation of photons close to the source by the Monte Carlo method. These simulations are then fitted to the reflectance to obtain the optical coefficients. To obtain any value of absorption, scattering or γ parameter, the simulations are interpolated by 3D B-splines. A complete robustness to noise study of the algorithm has been presented. A theoretical study of the γ parameter in tissue over broad wavelengths has been reported. Tissue was simulated with Mie theory for a fractal distribution of sphere diameters. For such a distribution, the γ spectrum is almost flat, instead of being wavy like in the case of mono-disperse sphere diameter. The correlation of the mean value of γ with the fractal power (or dimension) of the distribution was made and showed an analytical form of hyperbolic tangent. It allowed deepening the interpretation of the γ parameter as being an indicator of the relative number of small scatterers (much smaller than the wavelength, e.g. of Rayleigh type) in comparison of bigger ones (their size being comparable to the wavelength). The assessment of the set-up and the algorithm was made by measuring the reflectance of a solution containing 5 different diameters of polystyrene spheres simulating the scattering and Nigrosin colorant as absorber. It showed a good agreement between measurements and prediction. In order to built a better comprehension of absorption and scattering at the tissue scale, a first part of in vivo investigations has been made on an animal model. The skin of the mouse back was altered with Freund's adjuvant and TPA and the optical coefficients were measured on treated and control sites. The effect of alteration (epidermis and dermis thickening, oil vesicle appearing, inflammatory cells infiltration, etc.) was clearly correlated with optical coefficients variations. The scattering spectrum always showed the better contrast. The absorption spectrum, especially in the haemoglobin peak region (between 550 and 600 nm) also showed significant information, in terms of oxy and deoxy-haemoglobin concentration and saturation. Sensitivity and specificity of the "diagnosis" was evaluated. The study of stomach epithelium in vivo on 29 human subjects was made using a small optical fibre probe (

This thesis introduces a novel approach for in vivo separation and quantification of spatio-temporal dynamics of optical coefficients (absorption, scattering), oxygen saturation (SO2) and haemoglobin concentrations within rat brain somatosensory cortex under baseline and neuronal stimulation conditions. The quantification of mentioned parameters became possible due to design of a novel optical setup for localized spatio-temporally-resolved (within ∼ 1 mm3 at 10 Hz) reflectance spectroscopy (STRReS) measurements at small source-detector separations. The possibility to automatically take into account the light propagation path lengths in tissue using Monte Carlo modeling, defines the novelty of this approach and its perspectives for brain research. This kind of corrections is currently limited or inaccurate with existing techniques like optical intrinsic signal imaging. In particular, the main aim of this thesis is to explore and better understand the local spatio-temporal characteristics of neurovascular and neurometabolic coupling mechanisms within in vivo rat cerebral cortex at initial and subsequent stages of neuronal stimulation with using STRReS technique. It has been proven theoretically (with Monte Carlo simulations) and experimentally (from in vivo rat brain cortex), that STRReS measurements are suitable from in vivo somatosensory cortex under neuronal stimulation conditions. As a part of this research, we have validated theoretically and experimentally the biological importance of new optical parameter ”gamma” (parameter of the phase function), which is nessesary to take into account for accurate optical coefficients extraction. On the other side, the STRReS technique was not found to be helpful to determine differential factors corrections for improvement of optical intrinsic signal imaging analysis due to differences in experiment’s geometries. Finally, it has been shown that temporal dynamics of reflectance is strongly influenced by light scattering variations within 500-900 nm range. Specifically, the shape of reflectance dynamics at 600 and 610 nm differs significantly from deoxy-haemoglobin concentration dynamics at initial stage of neural activation. For this reason, it is important always to take into account the variations of both components of reflectance signal: absorption and scattering. Additionally, an initial delay of 1 s was found in SO2 and haemoglobin concentrations dynamics. The interpretation of this delay, however, is strongly dependent on the origin of scattering signal. Our results are well correlated with ”metabolic” hypothesis of underlying neurovascular and neurometabolic coupling mechanisms, assuming that the scattering signal has a non-vascular origin at 500-599 nm. Nevertheless, the ”neurogenic” hypothesis is not completely excluded as far as the scattering signal has a vascular origin at 500-599 nm.