Browsing by Author "McAlexander, W.I."
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Item Elastic and inelastic collisions of 6Li atoms in magnetic and optical traps(American Physical Society, 1998) Houbiers, M.; Stoof, H.T.C.; McAlexander, W.I.; Hulet, R.G.; Rice Quantum InstituteWe use a full coupled-channels method to calculate collisional properties of magnetically or optically trapped ultracold 6Li. The magnetic-field dependence of the s-wave scattering lengths of several mixtures of hyperfine states are determined, as are the decay rates due to exchange collisions. In one case, we find Feshbach resonances at B=0.08T and B=1.98T. We show that the exact coupled-channels calculation is well approximated over the entire range of magnetic fields by a simple analytical calculation.Item Hyperfine structure in photoassociative spectra of Li26 and Li2(American Physical Society, 1996) Abraham, E.R.I.; McAlexander, W.I.; Stoof, H.T.C.; Hulet, R.G.; Rice Quantum InstituteWe present spectra of hyperfine resolved vibrational levels of the A1Σ+uand 13Σ+g states of Li26 and Li27 obtained via photoassociation of colliding ultracold atoms in a magneto-optical trap. A simple first-order perturbation theory analysis accurately accounts for the frequency splittings and relative transition strengths of all observed hyperfine features. Assignment of the hyperfine structure allows accurate determination of a vibrational level center of gravity, which significantly decreases the experimental uncertainty of vibrational energies. Differences in the spectra of Li26 and Li27 are attributed to quantum statistics. The 13Σ+g series obeys Hund’s case bβS coupling and the hyperfine constant is extracted for both isotopes.Item Laser-free slow atom source(American Physical Society, 1999) Ghaffari, B.; Gerton, J.M.; McAlexander, W.I.; Strecker, K.E.; Homan, D.M.; Hulet, R.G.; Rice Quantum InstituteA slow atom source, which does not rely on lasers, has been developed and characterized. The device, acting as an atomic low-pass velocity filter, utilizes permanent magnets to passively select the slow atoms present in a thermal atomic beam. Slow atoms are guided along a curved, conduction-limited tube by an octupole magnetic field, while fast atoms, unable to follow the curved trajectory, strike the tube wall and are removed from the beam. The performance of the device is demonstrated by loading a magneto-optical trap. Approximately 2×108 lithium atoms are loaded with a rate of ∼6×106 atoms/s, while maintaining a background gas pressure of ∼10−11 torr. This loading technique provides an exceptionally simple, economical, and robust alternative to laser cooling methods.Item Precise atomic radiative lifetime via photoassociative spectroscopy of ultracold lithium(American Physical Society, 1995) McAlexander, W.I.; Abraham, E.R.I.; Ritchie, N.W.M.; Williams, C.J.; Stoof, H.T.C.; Hulet, R.G.; Rice Quantum InstituteWe have obtained spectra of the high-lying vibrational levels of the 13Σ+g state of Li26 via photoassociation of ultracold Li6 atoms confined in a magneto-optical trap. The 13Σ+g state of the diatomic molecule correlates to a 2S1/2 state atom plus a 2P1/2 state atom. The long-range part of the molecular interaction potential for this state depends on the 2P atomic radiative lifetime. By calculating the energy eigenvalues of a model potential for the 13Σ+g state and fitting them to the experimentally measured vibrational levels, we have extracted a value for the 2P lifetime of 26.99±0.16 ns. The precision is currently limited by the accuracy of a region of the model potential provided by ab initio calculations.Item Radiative lifetime of the 2P state of lithium(American Physical Society, 1996) McAlexander, W.I.; Abraham, E.R.I.; Hulet, R.G.; Rice Quantum InstituteWe determine the radial dipole moment between the 2S and 2P states of atomic lithium by analyzing the long-range vibrational eigenenergies of the singly excited diatomic molecule. The result can be expressed in terms of the 2P12 radiative lifetime of Li7, which is found to be 27.102(2)(7) ns. This result agrees with most current atomic-structure calculations and resolves the long-standing disagreement with previous experiment. The current level of precision is sensitive to relativistic effects in the atomic-structure calculation and to non-Born-Oppenheimer and radiation retardation effects in the molecule.Item Singlet s-wave scattering lengths of Li6 and Li(American Physical Society, 1996) Abraham, E.R.I.; McAlexander, W.I.; Gerton, J.M.; Hulet, R.G.; Côté, R.; Dalgarno, A.; Rice Quantum InstitutePhotoassociation of ultracold lithium atoms into bound vibrational levels of the AΣu+1 excited state is used to probe the XΣg+1 ground-state interaction potential of Li26 and Li27. It had been predicted that the s-wave photoassociation signal strength would pass through a minimum as a function of vibrational level for positive s-wave scattering length. We report the observation of this novel effect, and use the location of the minimum to precisely determine the singlet s-wave scattering length for both isotopes. The sensitivity of this technique is demonstrated by distinguishing the minima for collisions involving Li7 atoms in different hyperfine states.Item Spectroscopic Determination of the s-Wave Scattering Length of Lithium(American Physical Society, 1995) Abraham, E.R.I.; McAlexander, W.I.; Sackett, C.A.; Hulet, Randall G.; Rice Quantum InstituteTwo-photon photoassociation of colliding ultracold 7Li atoms is used to probe the Σu+3(a) ground state of 7Li2. The binding energy of the least-bound state of this triplet potential, υ=10, is found to be 12.47±0.04 GHz. This spectroscopic information establishes that the s-wave scattering length for 7Li atoms in the F=2, mF=2 state is (−27.3±0.8)a0. The negative sign of the scattering length has important implications as to whether atoms at this doubly spin-polarized state can undergo a Bose-Einstein condensation.Item Superfluid state of atomic 6Li in a magnetic trap(American Physical Society, 1997) Houbiers, M.; Ferwerda, R.; Stoof, H.T.C.; McAlexander, W.I.; Sackett, C.A.; Hulet, R.G.; Rice Quantum InstituteWe report on a study of the superfluid state of spin-polarized atomic 6Li confined in a magnetic trap. Density profiles of this degenerate Fermi gas and the spatial distribution of the BCS order parameter are calculated in the local-density approximation. The critical temperature is determined as a function of the number of particles in the trap. Furthermore, we consider the mechanical stability of an interacting two-component Fermi gas, in the case of both attractive and repulsive interatomic interactions. For spin-polarized 6Li we also calculate the decay rate of the gas and show that within the mechanically stable regime of phase space, the lifetime is long enough to perform experiments on the gas below and above the critical temperature if a bias magnetic field of about 5 T is applied. Moreover, we propose that a measurement of the decay rate of the system might signal the presence of the superfluid state.Item Triplet s-wave resonance in Li6 collisions and scattering lengths of Li6 and Li7(American Physical Society, 1997) Abraham, E.R.I.; McAlexander, W.I.; Gerton, J.M.; Hulet, R.G.; Côté, R.; Dalgarno, A.; Rice Quantum InstituteThe triplet s-wave scattering length of Li6 is determined using two-photon photoassociative spectroscopy of the diatomic a3Σ+u state of Li26. The measured binding energy of the highest-lying bound state, combined with knowledge of the potential, determines the s-wave scattering length to be (-2160±250)a0, where a0 is the Bohr radius. This extraordinarily large scattering length signifies a near-threshold resonance. A complete table of singlet and triplet scattering lengths for collisions involving Li6 and Li7 determined from this and our previous spectroscopic investigations is given.