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Physical Review Letters

Isotopic difference in trap loss collisions of laser cooled rubidium atoms

Christopher D. Wallace, Timothy P. Dinneen, Kit-Yan N. Tan, Timothy T. Grove, P. L. Gould

Published Aug 10, 1992• 122 citations
Publisher

Abstract

We report measurements of collisional rate constants for cold Rb atoms held in a magneto-optical trap. $^{85}\mathrm{Rb}$ and $^{87}\mathrm{Rb}$ 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 $^{85}\mathrm{Rb}$ is 3.3 times that of $^{87}\mathrm{Rb}$.

DOI
10.1103/physrevlett.69.897

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Brazilian Journal of Physics•Dec 1, 2004•11 citations
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.
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. ^
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 have realized a two-species mirror-magneto-optical trap containing a mixture of 87Rb(85Rb)^{87}\mathrm{Rb}\phantom{\rule{0.3em}{0ex}}(^{85}\mathrm{Rb})87Rb(85Rb) and 133Cs^{133}\mathrm{Cs}133Cs atoms. Using this trap, we have measured the heteronuclear collisional loss rate ${\ensuremath{\beta}}_{\mathrm{Rb-Cs}}^{\ensuremath{'}}$ due to intraspecies cold collisions. We find a distinct difference in the magnitude and intensity dependence of ${\ensuremath{\beta}}_{\text{Rb-Cs}}^{\ensuremath{'}}$ for the two isotopes 87Rb^{87}\mathrm{Rb}87Rb and 85Rb^{85}\mathrm{Rb}85Rb which we attribute to the different ground-state hyperfine splitting energies of the two isotopes.
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.