Browsing by Author "McAlexander, William Ian"
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Item Collisional interactions in an ultracold lithium gas(2000) McAlexander, William Ian; Hulet, Randall G.Laser cooling and atom trapping techniques have enabled atomic physicists to attain temperatures on the order of 100 nK. In this regime, a trapped gas must be described within the framework of quantum statistical mechanics. In 1995, Bose-Einstein condensation (BEC) was achieved for the first time in such a system by three independent groups. The success of attaining BEC has encouraged research in quantum degenerate Fermi gases (DFG). One of the most exciting opportunities for a DFG is the prospect for Cooper pairing in a two-component gas. The experimental techniques for obtaining degenerate gases, as well as the mechanism for Cooper pairing, hinges on understanding and characterizing ultracold collisional interactions. This thesis is a twofold study in this exciting field. The first section is an in-depth treatment of collisions in atomic lithium. Lithium has two isotopes, 6Li which is a fermion and 7Li which is a boson. This makes it an ideal candidate for studying quantum degenerate gases. In a series of experiments using a magneto-optical trap, we have spectroscopically measured the ground and excited state molecular potentials for both isotopes. From this work we have been able to obtain the most accurate potentials for lithium to date. These precise measurements have enabled us to extract the most accurate value to date for the 2P atomic radiative lifetime of lithium. Furthermore, we have developed a coupled-channel treatment of two-body collisions which has enabled us to calculate the scattering amplitude, elastic cross section, and rate of spin-exchange decay for any ground state collision in lithium as a function of temperature and magnetic field. Armed with this knowledge, we have made progress on the design and construction of an electromagnetic trap for lithium. This trap will enable the study of a degenerate Fermi gas of 6Li as well as the possibility of observing a Cooper pairing transition. The trap performance has been characterized for each isotope and current work is focussed on cooling lithium down to degeneracy. Measurements of spin-exchange decay have been made and compare well with the collisional theory that has been developed.Item Precise determination of the 2P radiative atomic lifetime of lithium using photoassociative spectroscopy(1995) McAlexander, William Ian; Hulet, Randall G.Spectroscopy of the high-lying vibrational levels of the $1\sp3\Sigma\sbsp{g}{+}$ and $1\sp1\Sigma\sbsp{u}{+}$ states of both $\rm\sp6Li\sb2$ and $\rm\sp7Li\sb2$ has been accomplished via photoassociation of ultracold atomic lithium confined in a magneto-optical trap. Both these molecular states correlate to $2S\sb{1/2}$ plus $2P\sb{1/2}$ free atomic states. The long range part of these molecular potentials depends on the 2P atomic radiative lifetime. Accurate models were constructed for both the $1\sp3\Sigma\sbsp{g}{+}$ and $1\sp1\Sigma\sbsp{u}{+}$ potentials. By fitting these models to the experimentally measured vibrational levels observed in $\rm\sp6Li\sb2$ and $\rm\sp7Li\sb2$, we have been able to extract a value for the 2P lifetime of lithium. Fitting to the absolute values of the data gives a value of 26.99 $\pm$ 0.16 ns whereas fitting to differences in the spectral features gives a value of 27.04 $\pm$ 0.18 ns. While both values agree with previous work, improvements made in the potential models would enable this analysis to resolve the current discrepancy between experiment and theory.