Quantum coherence and interference(QCI) effects have been studied for decades and are widely exploited in many areas. For media with QCI effect, the optical properties can change drastically, which leads to many interesting effects, such as coherent population trapping, electromagnetically induced transparency(EIT), lasing without population inversion(LWI) and so on. We have theoretically studied the pulsed regime of EIT. In particular, simulations of propagation of gaussian and 0 - pi co-propagating laser pulses in a medium consisting of 3-level Lambda-atoms have been performed. It has been found that, even at the two-photon resonance, the length of propagation for the 0 - pi pulses is much smaller than that for the Gaussian probe pulses. We explained such a behavior using the dark and bright basis and the dressed state basis. Some possible applications are discussed. We also investigated the collision-induced coherence of two decay channels along two optical transitions. Quantum interference will suppress the spontaneous emission. The degree of this suppression is measured by the branch ratio of these two transitions. Our preliminary calculations show that a significant decrease of the branching ratio with increase of electron densities is reproduced in the theory. We have developed a new variant of Raman spectroscopy with shaped femtosecond pulses. It has several advantages to be applied in multiscatterd media. It is based on change of the spectra of femtopulses due to Raman scattering (stimulated or coherent). The technique can be used for a broad range of applications from atomic and molecular optical and IR spectroscopy to spore detection and tissue microscopy. Finally, we have shown that Fano interference in the decay channels of three levels system can lead to considerably different absorption and emission profiles. We found that a coherence can be built up in the ground state doublet whose strength depends on a coupling parameter that arises from Fano interference. This can in principle lead to breaking of the detail balance between the absorption and emission processes in atomic systems.

High-precision half-life and branching-ratio measurements for the superallowed beta+ emitter 26Alm were performed at the TRIUMF-ISAC radioactive ion beam facility located in Vancouver, Canada. The branching ratio measurement was performed with the 8pi Spectrometer, an array of 20 high-purity germanium detectors, in conjunction with SCEPTAR, a plastic scintillator array used to detect the emitted beta particles. An upper limit of <= 12 ppm at 90% confidence level was found for the second forbidden beta + decay of 26Alm to the first 2+ state at 1809 keV in 26Mg. An inclusive upper limit of <= 15 ppm at 90% confidence level was found when considering all possible non-analogue beta +/EC decay branches of 26Alm, resulting in a superallowed branching ratio of 100.0000+0−0.0015%. The half-life measurement was performed using a 4pi continuous-flow gas proportional counter and fast tape transport system. The resulting value for the 26Alm half-life, T1/2 = 6.34654(76) s, is consistent with, but 2.5 times more precise than, the previous world average, and represents the single most precisely measured half-life of any superallowed emitting nucleus to date. Combining these results with world-average Q-value measurements yields a superallowed beta -decay ft value of 3037.58(60) s, the most precisely determined ft value for any superallowed emitting nucleus to date. Combined with the small, and precisely quoted, theoretical isospin-symmetry-breaking corrections for this nucleus, the corrected Ft value for 26Alm of 3073.1(12) s is also the most precisely determined for any superallowed emitter by nearly a factor of two and now rivals the precision of all the other 12 precisely measured superallowed beta decays combined. The high-precision experimental ft value for 26Alm superallowed decay reported here provides a new benchmark to refine theoretical models of isospin-symmetry-breaking effects in superallowed Fermi beta decays.

Dans le milieu interstellaire, parmi les 200 molécules observées, les hydrocarbures sont présents en abondance. La formation des hydrocarbures CHy(+) se fait entre un C+ et un atome d'hydrogène ou une molécule. Par la suite, ces hydrocarbures réagissent entre eux pour former des hydrocarbures plus gros. Ils sont donc les précurseurs de tous les hydrocarbures présents dans le milieu interstellaire ; il est par conséquent nécessaire de les étudier en détail. Ces hydrocarbures CHy(+) sont soumis à divers processus physiques dans le milieu interstellaire notamment la collision avec un électron, l'absorption d'un photon ultraviolet ou d'un rayon cosmique. Ils vont être excités par ces processus et gagner un excès d'énergie qu'ils vont libérer par fragmentation, ce qui a pour conséquence de redistribuer les espèces. Des simulations précises dans la chimie du milieu interstellaire nécessitent donc une connaissance précise des taux de réaction et des rapports de branchement de fragmentation. Afin de documenter tous ces rapports de branchement, peu importe le processus physique ou chimique mis en jeu, nous avons construit expérimentalement des « breakdown curves » semi-empiriques qui sont les rapports de branchement des voies de fragmentation en fonction de l'énergie interne déposée lors du processus.L'expérience a été réalisée en utilisant le multi détecteur silicium AGAT et l'accélérateur ANDROMEDE. Les molécules CHy(+) produites à haute vitesse (3 u.a) collisionnent avec des atomes d’Hélium au repos dans le référentiel du laboratoire. Le dispositif expérimental permet de détecter tous les fragments, même les neutres, et de résoudre toutes les voies de fragmentation. Nous avons mesuré les rapports de branchement de fragmentation des CHyq+ (y=2-4, q=0-3) et les distributions d'énergie cinétique des fragments neutres.A partir des rapports de branchement, des distributions d'énergie cinétique et des énergies de dissociation théorique, nous avons construit les breakdown curves qui se sont révélés en bon accord avec des rapports de branchement expérimentaux déjà existant dans la littérature pour la photodissociation, la recombinaison dissociative et les collisions électroniques. Enfin, un modèle a été développé pour prédire les rapports de branchement de réactions chimiques ainsi que leur évolution avec la température. / In the interstellar medium, among the 200 molecules observed, the hydrocarbons are in abundance. The formation of hydrocarbons CHy(+) is done between a C+ and a hydrocarbon atom or molecule. Thereafter, these hydrocarbons are reacting between them to form bigger hydrocarbons. Therefore, they are the precursor of all the hydrocarbons present in the interstellar medium, so it is necessary to study them in details. These hydrocarbons CHy(+) are under a lot of different physical processes in the interstellar medium including the collision with an electron, the absorption of an ultra violet photon or a cosmic ray. They will be excited by these processes and gain excess energy they will liberate by fragmentation which leads to a redistribution of species. Therefore, a precise knowledge of the rate of reaction and of the branching ratios of the fragmentation is needed to do specific simulations in the chemistry of the interstellar medium. In order to document all these branching ratios, no matter the physical or chemical process at stake, we experimentally built semi-empirical breakdown curves which are the branching ratios of the paths of fragmentation as a function of the internal energy of the molecule.The experiment was done using the AGAT silicon multi-detector and the ANDROMEDE accelerator. CHy(+) molecules produced at high velocity (3 u.a.) are collided with He atom at rest in the lab. Thanks to the experimental developments, all fragments, neutral or charged, are separately identified, allowing to resolve all fragmentation channels. Therefore, we have been able to measure fragmentation branching ratios for CHyq+ (y=2-4, q=0-3) and the kinetic energy distributions of the neutral fragments.From the branching ratios, the kinetic energy distributions and the theoretical dissociation energies we built BDCs that revealed to be in accordance with the experimental branching ratios which already exists in the literature concerning the photo dissociation, the dissociative recombination and the electronic collisions. Finally, a model has been developed to predict the chemical reactions of the branching ratios as well as to predict their evolution with the temperature.

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As a part of our program to test the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, the decay of the superallowed 0⁺ --> 0⁺ beta emitters ³⁸Ca and ⁴⁶V has been studied in this dissertation. For ³⁸Ca, the half-life, 443.88(36) ms, and superallowed branching ratio, 0.7738(41), have been measured. In our half-life experiment, pure sources of ³⁸Ca were produced and the decay positrons detected in a high-efficiency 4[pi] proportional gas counter. Since the beta⁺ decay of ³⁸Ca feeds ³⁸K^m, which is itself a superallowed beta⁺ emitter, the data were analyzed as a linked parent-daughter decay. Our result for the half-life of ³⁸Ca, with a precision of 0.08%, is a factor of five improvement on the best previous result. The branching-ratio of ³⁸Ca depended on beta-delayed gamma-ray intensities being measured with a high-purity germanium detector calibrated for absolute efficiency to 0.2% precision. This branching-ratio result represents our first step in bringing the ft value for the superallowed ³⁸Ca transition into the desired range of 0.1%. With our half-life and superallowed branching ratio results for ³⁸Ca, we obtain the Ft to be 3072(17) s, in good agreement with the conserved vector current expectation. The half-life of ⁴⁶V has been measured to be 422.66(6) ms, a factor of two more precise than the best previous measurement. Our present result determines the corresponding Ft value to be 3074.5(26) s, which is consistent with the average $\overline{\mathcal{F}t}$ value of 3072.08(79) s established from the 13 best-known superallowed transitions. This demonstrates that previously accepted half-lives of ⁴⁶V were correct in their contribution to a precision test of the conserved vector current hypothesis.

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Using the general, model indepenent form of the effective Hamiltonian, the general expressions of the longitudinal, normal and transversal polarization asymmetries for l+ and l- for the effective B -&gt / K(K*)l+l- decays has been calculated. Existence of regions of Wilson coefficients for which the branching ratio coincides with the Standard Model result, while the lepton polarizations differ from the standard model prediction is expected. Hence, studying lepton polarizations in these regions of Wilson coefficients may be helpful in establishing new physics beyond standard model.

Master of Science / Department of Physics / Brett D. Esry / The two-state field-aligned (1-D) model has been employed to investigate the dissociation dynamics of a hydrogen molecular ion and its isotopes under the Born-Oppenheimer approximation without rotation. The emphasis of this work was on the role of mass during the dynamical dissociation processes and on the laser-induced branching ratios between different photon pathways. Firstly, we have found that scaling the pulse duration of the laser pulse, applied to H[subscript]2+ and D[subscript]2+ , by the square root of the mass ratio of these isotopes will produce similar structure in the nuclear kinetic energy release (KER) spectra. In fact, the similarity of the spectra is enhanced by including some averaging that is necessary for comparison with experiment. For this to occur, the same broad initial vibrational distribution and a short pulse are preferred. Using this scaling idea, it is possible to produce effectively shorter laser pulses by studying heavier isotopes, like D[subscript]2+. Secondly, we have demonstrated analytically and numerically that there is a carrier-envelope phase effect in the total dissociation probability (TDP) of H[subscript]2+, and this effect grows with nuclear mass. We further show that under the same laser conditions, the CEP effect in the asymmetry between breakup channels decreases with mass. Our analytic expressions enhance the idea that CEP effects can be understood as an interference between different n-photon processes. Thirdly, the trends in the dissociation dynamics of H[subscript]2+ and D[subscript]2+ in a 800nm ultra short intense laser field were demonstrated by studying the dissociation branching ratios of multiphoton processes as a function of the laser peak intensity (from 8[times]10[superscript]9 to 10[superscript]14 W/cm[superscript]2) or pulse length (5fs-7.5fs). Based on the two-state approximation, an energy-analysis method (EAM) was employed to separate multiphoton processes. The results show that the one-photon dissociation process dominates over all other photon processes under all the laser conditions applied in the calculations and that the zero-photon process contributes to a surprisingly large fraction of the total dissociation. Two- and three- photon dissociation are weaker processes, but become more and more important as the laser peak intensity and pulse length increases. A two-state Floquet method was used to check the accuracy of the EAM, and good agreement between the two methods was found, demonstrating the reliability of the EAM. In comparison with H[subscript]2+, D[subscript]2+ displays stronger two and three photon branching ratios (above-threshold dissociation - ATD), which can be attributed to the late arrival of D[subscript]2+ to the critical distance for ATD to occur due to its heavier mass. Therefore, this "mass" effect can be used to steer the molecular dissociation pathways.

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