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Physical Review A

Light-induced ultracold spin-exchange collisions

Renée C. Nesnidal, Thad G Walker

Published Aug 18, 2000• 8 citations
Publisher

Abstract

We study collisional loss rates due to spin-exchange collisions in very-low-intensity ${}^{87}\mathrm{Rb}$ magneto-optical traps. The loss rate exhibits an unpredicted intensity dependence for low intensities, accounting for a large disparity between loss rates observed in light-force traps and magnetically confined Bose condensates. Possible collision mechanisms considered include light-altered collision phase shifts, flux enhancement, and excited-state hyperfine changing collisions.

DOI
10.1103/physreva.62.030701

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Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^
We have measured the collisional loss rate for cold strontium atoms held in a magneto-optical trap as a function of light intensity in the regime of low intensity (2-6 mW/cm²). The results confirm our recently proposed model, where we showed that the sudden increase of loss rate at low intensities does not depend on hyperfine structure changing collision only. The model, which is based on radiative escape mechanism and a light intensity dependent escape velocity, is able to reproduce quite well the behavior of the experimental observations. The data here reported may be of importance for a recent proposed application for cold trapped 88Sr atoms, as well as other trappable alkaline earth metal atoms such as Ca.
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Abstract We study inelastic collisions between CaF molecules and 87Rb atoms in a dual-species magneto-optical trap. The presence of atoms increases the loss rate of molecules from the trap. By measuring the loss rates and density distributions, we determine a collisional loss rate coefficient k 2 = (1.43 ± 0.29) × 10−10 cm3 s−1 at a temperature of 2.4 mK. We show that this is not substantially changed by light-induced collisions or by varying the populations of excited-state atoms and molecules. The observed loss rate is close to the universal rate expected in the presence of fast loss at short range, and can be explained by rotation-changing collisions in the ground electronic state.