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Photoassociation and Feshbach resonance studies in ultra-cold gases of ⁶Li and Rb atoms Gunton, Will


This thesis presents an experimental apparatus capable of producing and studying ultra-cold mixtures of ⁶Li and Rb, and progress towards the creation of ultra-cold ground state ⁶Li₂ and LiRb molecules. Ultra-cold experiments with molecules have applications in many quantum computation and simulation experiments. We discuss elements of the apparatus which are important to these experiments, including electric field plates in air capable of producing fields up to 18 kV/cm, and a laser system for photoassociation spectroscopy based on two Ti:Sapphire lasers phase locked to the same optical frequency comb. With respect to ⁶Li and ⁸⁵Rb mixtures, we report on the observation of six Feshbach resonances, which represent an important step towards molecule production and future experiments in this heteronuclear mixture. In addition, we demonstrate the production of a BEC of ⁶Li₂ Feshbach molecules, a degenerate Fermi gas of ⁶Li and the formation of BCS pairs. In the ⁶Li system, we report on the high resolution spectroscopy of the v'=20-26 vibrational levels of the c(1³∑⁺g) potential and the v'=29-35 vibrational levels of the A(1¹∑⁺u) potential. In the A(1¹∑⁺u) potential, we find that the v'=31 and v'=35 levels have the largest transition strength and are therefore good candidates to use as intermediate states in molecule formation. We demonstrate atom-molecule dark states in the BEC-BCS crossover regime and additionally use dark-state spectroscopy to make extremely high resolution measurements of the least bound N''=0 ro-vibrational levels in the X(1¹∑⁺g) and a(1³∑⁺u) potential. In addition, we present spectroscopy of all ten N''=0 and N''=2 ro-vibrational levels in the a(1³∑⁺u) potential and furnish a preliminary interpretation of the observed energy structure. Finally, we report on the observation of anomalous Autler-Townes and dark-state spectrum. Using an extension to the standard three level model, we show that these anomalous profiles are due to degeneracies that exist in the bound molecular states, and to the choice of polarization of the coupling fields. These results have a direct impact on molecule formation, and provide a clear guide to future experiments.

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