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Bulletin of the American Physical Society

Excited state trap loss collisions

P. L. Gould, Christopher D. Wallace, T. Dinneen, K. Y. N. Tan, Anantharaman Kumarakrishnan

Published Dec 1, 1993• 0 citations
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Abstract

The authors have measured rate constants for cold collisions which lead to ejection of rubidium atoms from a laser trap. Dramatic differences between {sup 85}Rb and {sup 87}Rb are seen which are attributed to the different hyperfine structures of both ground and excited states. The temperature dependence of the collisional rate constant can be investigated by exploiting the variation of temperature with trap laser intensity.

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Physical Review A•Dec 1, 1994•28 citations
The authors have measured rate constants for cold collisions which lead to ejection of rubidium atoms from a laser trap. Dramatic differences between {sup 85}Rb and {sup 87}Rb are seen which are attributed to the different hyperfine structures of both ground and excited states. The temperature dependence of the collisional rate constant can be investigated by exploiting the variation of temperature with trap laser intensity.
We report measurements of collisional rate constants for cold Rb atoms held in a magneto-optical trap. 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb display significantly different behavior. At low trap laser intensities, atoms which undergo a hyperfine-changing collision gain sufficient velocity to escape the trap. Since the hyperfine splittings are quite different for the isotopes, the loss rates display a large isotopic dependence in this regime. At higher trap laser intensities, where trap loss is due to inelastic collisions involving excited atoms, a dramatic and unexpected isotopic dependence is seen. Under identical conditions, the loss rate for 85Rb^{85}\mathrm{Rb}85Rb is 3.3 times that of 87Rb^{87}\mathrm{Rb}87Rb.
Summary form only given. Inelastic collisions between cold atoms can lead to their ejection from the laser trap that confines them. Understanding these trap loss collisions is important to efforts aimed at high densities of cold atoms. Previously we have observed pronounced differences in the collisional behavior of the two isotopes, /sup 85/Rb and /sup 87/Rb, for the trap laser tuned one natural linewidth below resonance. We have extended our measurements for both isotopes to smaller and larger detunings in an effort to separate the various contributions to the trap loss rate.
We report on measurements of trap-loss collisions between ultracold Rb atoms that are confined in a magneto-optical trap. Both isotopes, 85Rb{}^{85}\mathrm{Rb}85Rb and 87Rb,{}^{87}\mathrm{Rb},87Rb, have been studied over a wide range of trap laser intensities and detunings. The trap-loss collisional rate constant exhibits variations over three orders of magnitude. At low intensities and/or large detunings, the trap loss is dominated by ground-state hyperfine-changing collisions, while at high intensities and small detunings, collisions involving excited atoms are more important. We see significant differences between the isotopes in both regimes. At large detunings, the ability of the trap to recapture products of a hyperfine-changing collision 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. A surprisingly large increase in this rate is seen when the confining power of the trap is reduced.
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. ^
The authors have measured the rate of collisional trap loss for both isotopes of Rb over a wide range of trap laser parameters. They find that smaller detunings of the MOT laser allow recapture of products of ground-state hyperfine-changing collisions, while larger detunings allow these collisions to dominate the collisional loss. These findings are consistent with numerical simulations of the recapture process which indicate a greatly reduced trap depth at large detunings. The isotopic variation of collisional loss also points to the importance of these ground-state collisions.
We have measured the cold-collision loss rates of laser-trapped Na atoms in each ground-state hyperfine sublevel. The laser intensity dependence for the F=1 sublevel agrees with a simple theory and we establish experimentally that hyperfine-level-changing collision losses are absent as expected. We observed hyperfine-level-changing collisions in the F=2 sublevel but the dependence of the loss-rate constant on laser intensity disagrees with simple theories. Unexpected behavior observed for total trapping laser light intensity between 4 and 12 mW/cm2{\mathrm{cm}}^{2}cm2 may be a consequence of the sub-Doppler cooling in the F=2 trap.