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Physical Review Letters

Suppression of Angular Momentum Transfer in Cold Collisions of Transition Metal Atoms in Ground States with Nonzero Orbital Angular Momentum

Cindy Hancox, S. Charles Doret, Matthew T. Hummon, Roman V. Krems, John M. Doyle

Published Jan 3, 2005• 50 citations
Publisher

Abstract

The Zeeman relaxation rate in cold collisions of Ti(3d(2)4s(2) 3F2) with He is measured. We find that collisional transfer of angular momentum is dramatically suppressed due to the presence of the filled 4s(2) shell. The degree of electronic interaction anisotropy, which is responsible for Zeeman relaxation, is estimated to be about 200 times smaller in the Ti-He complex than in He complexes with typical non-S-state atoms.

DOI
10.1103/physrevlett.94.013201

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Angular momentum transfer is expected to occur rapidly in collisions of atoms in states of nonzero angular momenta due to the large torque of angular forces. We show that despite the presence of internal angular momenta transition metal atoms interact in collisions with helium effectively as spherical atoms and angular momentum transfer is slow. Thus, magnetic trapping and sympathetic cooling of transition metal atoms to ultracold temperatures should be readily achievable. Our results open up new avenues of research with a broad class of ultracold atoms.
Based on measurements of the Zeeman relaxation in a cold gas of (3)He [C. I. Hancox, S. C. Doret, M. I. Hummon, L. Luo, and J. M. Doyle, Nature (London) 431, 281 (2004)], we show that the electronic interaction anisotropy between rare-earth atoms with nonzero electronic orbital angular momenta and helium is extremely small. The interaction of the rare-earth atoms with He gives rise to several adiabatic potentials with different electronic symmetries. It is demonstrated that the energy splitting between these potentials does not exceed 0.09 cm(-1) at interatomic distances larger than the turning point for collisions at 0.8 K, including the region of the van der Waals interaction minima.
We report on the results of a multichannel calculation of fine-structure transition cross sections for collisions of ground-state Ti with He. We find significant suppression of fine-structure level quenching over a wide range of collision energies. We show that the suppression is due to a mechanism proposed by [Hancox et al., Nature (London) 431, 281 (2004)] to explain similar suppression of Zeeman relaxation in collisions of magnetically trapped rare-earth-metal atoms with cold He atoms. We find that the quenching rates are sensitive to the value of the Born-Oppenheimer adiabatic potential surfaces of the Ti-He system in the region of the inner repulsive wall. Adjusting the ab initio potentials for TiHe, available in the literature, we find good agreement with laboratory measurements.
We investigate Zeeman relaxation in cold Sb(${}^{4}{S}_{3/2}^{\ensuremath{\circ}}$)--He collisions in a magnetic field. Ensembles of >>>1013{10}^{13}1013 laser-ablated Sb atoms are cooled in cryogenic 4{}^{4}4He buffer gas to 800 mK and inelastic collisions are observed to equilibrate the mJ{m}_{J}mJ​-state distribution to the translational temperature. The ratio $\ensuremath{\gamma}$ of momentum transfer to inelastic collision rates is measured to be $\ensuremath{\leqslant}9.1\ifmmode\times\else\texttimes\fi{}{10}^{2}$. We also perform quantum scattering calculations of Sb--4{}^{4}4He collisions, based on ab initio interaction potentials, that demonstrate significant anisotropy of the ground state induced by the spin-orbit interaction. Agreement is obtained between theory and experiment with a $\ensuremath{\approx}101010%$ increase in the ab initio potential depth. This work suggests that buffer-gas-cooled pnictogen atoms lighter than Sb can be loaded into a magnetic trap.
Angular momentum changing collisions can be suppressed in atoms whose valence electrons are submerged beneath filled shells of higher principle quantum number. To determine whether spin-exchange collisions are suppressed in these "submerged shell" atoms, we measured collisional rates for six hyperfine states of Mn at T < 1 K. Although the 3d valence electrons in Mn are submerged beneath a filled 4s orbital, we find spin-exchange rate coefficients similar to Na and H (both nonsubmerged shell atoms).