Person:

Au, Yat Shan

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 present a joint experimental and theoretical study of spin depolarization in collisions of alkali-metal atoms with (^3)He in a magnetic field. A rigorous quantum theory for spin-changing transitions is developed and applied to calculate the spin exchange and spin relaxation rates of Li and K atoms in cryogenic (^3)He gas. Magnetic trapping experiments provide upper bounds to the spin exchange rates for Li-(^3)He and K-(^3)He, which are in agreement with the present theory. Our calculations demonstrate that the alkali-metal atoms have extremely slow spin depolarization rates, suggesting a number of potential applications in precision spectroscopy and quantum optics.

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 inelastic cross sections for atomic thorium (Th) and molecular thorium monoxide (ThO) in collisions with $^3$He at temperatures near 1 K. We determine the Zeeman relaxation cross section for Th ($^3$F$_2$) to be $\sim 2 \times 10^{-17}$~cm$^{-2}$ at 800~mK. We study electronic inelastic processes in Th ($^3$P$_0$) and find no quenching even after $10^6$ collisions at 800~mK. We measure the vibrational quenching cross section for ThO~(X,~$\nu=1$) to be $(7.9 \pm 2.7) \times 10^{-19}$~cm$^{-2}$ at 800~mK. Finally, we observe indirect evidence for ThO (X, $\nu=0$)--$^3$He van der Waals complex formation, and measure the 3-body recombination rate constant to be $\Gamma_3 = (8 \pm 2) \times 10^{-33}$~cm$^6$s$^{-1}$ at 2.4~K. The stability of the ground Th ($^3$F$_2$) state, metastable Th ($^3$P$_0$) state, and vibrational excited ThO (X, $\nu=1$) state provides data on anisotropic interactions in new systems and opens up the possibility for further studies and experiments, including trapping.