Spin relaxation due to atom–atom collisions is measured for magnetically trapped erbium and thulium atoms at a temperature near 500 mK. The rate constants for Er–Er and Tm–Tm collisions are 3.0×10(^{-10}) and 1.1×10(^{-10}) cm(^3) s(^{-1}), respectively, 2–3 orders of magnitude larger than those observed for highly magnetic S-state atoms. This is strong evidence for an additional, dominant, spin relaxation mechanism, electronic interaction anisotropy, in collisions between these “submerged-shell,” L≠0 atoms. These large spin relaxation rates imply that evaporative cooling of these atoms in a magnetic trap will be highly inefficient.

The collisional magnetic reorientation rate constant (\vartheta_{\mathbb{R}}) is measured for magnetically trapped atomic dysprosium (Dy), an atom with large magnetic dipole moments. Using buffer gas cooling with cold helium, large numbers (>10(^{11})) of Dy are loaded into a magnetic trap and the buffer gas is subsequently removed. The decay of the trapped sample is governed by collisional reorientation of the atomic magnetic moments. We find (\vartheta_{\mathbb{R}} = 1.9 \pm 0.5 \times 10^{-11} , \text{cm}^{3} , \text{s}^{-1}) at 390 mK. We also measure the magnetic reorientation rate constant of holmium (Ho), another highly magnetic atom, and find (\vartheta_{\mathbb{R}} = 5 \pm 2 \times 10^{-12} , \text{cm}^3, \text{s}^{-1}) at 690 mK. The Zeeman relaxation rates of these atoms are greater than expected for the magnetic dipole-dipole interaction, suggesting that another mechanism, such as an anisotropic electrostatic interaction, is responsible. Comparison with estimated elastic collision rates suggests that Dy is a poor candidate for evaporative cooling in a magnetic trap.

We report the creation of a Bose-Einstein condensate using buffer-gas cooling, the first realization of Bose-Einstein condensation using a broadly general method which relies neither on laser cooling nor unique atom-surface properties. Metastable helium ((^4)He*) is buffer-gas cooled, magnetically trapped, and evaporatively cooled to quantum degeneracy. 10(^{11}) atoms are initially trapped, leading to Bose-Einstein condensation at a critical temperature of (5 \mu K) and threshold atom number of 1.1×10(^6). This method is applicable to a wide array of paramagnetic atoms and molecules, many of which are impractical to laser cool and impossible to surface cool.

Noble gas permeabilities and diffusivities of Kapton, butyl, nylon, and "Silver Shield" are measured at temperatures between 22 and 115 degrees C. The breakthrough times and solubilities at 22 degrees C are also determined. The relationship of the room temperature permeabilities to the noble gas atomic radii is used to estimate radon permeability for each material studied. For the noble gases tested, Kapton and Silver Shield have the lowest permeabilities and diffusivities, followed by nylon and butyl, respectively.

Inelastic collision processes driven by anistropic interactions are investigated below 1 K. Three distinct experiments are presented. First, for the atomic species antimony (Sb), rapid relaxation is observed in collisions with (^4He). We identify the relatively large spin-orbit coupling as the primary mechanism which distorts the electrostatic potential to introduce significant anisotropy to the ground (^4S_{3/2}) state. The collisions are too rapid for the experiment to fix a specific value, but an upper bound is determined, with the elastic-to-inelastic collision ratio (\gamma \leq 9.1 x 10^2). In the second experiment, inelastic (\mathcal{m}_J)-changing and (J)-changing transition rates of aluminum (Al) are measured for collisions with (^3He). The experiment employs a clean method using a single pump/probe laser to measure the steady-state magnetic sublevel population resulting from the competition of optical pumping and inelastic collisions. The collision ratio (\gamma) is measured for both (\mathcal{m}J)- and (J)-changing processes as a function of magnetic field and found to be in agreement with the theoretically calculated dependence, giving support to the theory of suppressed Zeeman relaxation in spherical (^2P{1/2}) states [1]. In the third experiment, very rapid atom-atom relaxation is observed for the trapped lanthanide rare-earth atoms erbium (Er) and thulium (Tm). Both are nominally nonspherical ((L \neq 0)) atoms that were previously observed to have strongly suppressed electronic interaction anisotropy in collisions with helium ((\gamma > 10^4-10^5, [2,3])). No suppression is observed in collisions between these atoms ((\gamma \lesssim 10)), which likely implies that evaporative cooling them in a magnetic trap will be impossible. Taken together, these studies reveal more of the role of electrostatic anisotropy in cold atomic collisions.

We present a quantitative study of suppression of cold inelastic collisions by the spin-orbit interaction. We prepare cold ensembles of (>10^{11} Al(^2P_{1/2})) atoms via cryogenic buffer-gas cooling and use a single-beam optical pumping method to measure their magnetic (mJ-changing) and fine-structure (J-changing) collisions with (^3He) atoms at millikelvin temperatures over a range of magnetic fields from 0.5 to 6 T. The experimentally determined rates are in good agreement with the functional form predicted by quantum scattering calculations using ab initio potentials. This comparison provides direct experimental evidence for a proposed model of suppressed inelasticity in collisions of atoms in (^2P_{1/2}) states [T. V. Tscherbul et al., Phys. Rev. A 80, 040701(R) (2009)], which may allow for sympathetic cooling of other (^2P_{1/2}) atoms (e.g., In, Tl and metastable halogens).

We measure the ratio γ of the momentum transfer–to–vibrational quenching cross section for molecular thorium monoxide (ThO) [ X ( 1 Σ + ) , v

1 , J

0 ] in collisions with atomic helium between 800 mK and 2.4 K. We find γ ∼ 10 4 . We also observe indirect evidence for ThO-He van der Waals complex formation, which has been predicted by theory [Tscherbul, Sayfutyarova, Buchachenko, and Dalgarno, J. Chem. Phys. 134, 144301 (2011)], and in conjunction, we determine the three-body recombination rate constant at 2.4 K, Γ 3

8 ± 2 × 10 − 33

cm 6

s − 1 .

Metastable helium is buffer gas cooled, magnetically trapped, and evaporatively cooled in large numbers. 1011 4 He* atoms are trapped at an initial temperature of 400 mK and evaporatively cooled into the ultracold regime, resulting in a cloud of 2± 0.5 109 atoms at 1.4± 0.2 mK. Efficient evaporation indicates low collisional loss for 4 He* in both the ultracold and multi-partial-wave regime, in agreement with theory.