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Bulletin of the American Physical Society

The effects of hyperfine interactions on collisions between optically trapped atoms

Thad G Walker, P.Y. Feng, Dominikus Hoffmann

Published Dec 1, 1993• 0 citations
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Abstract

The authors report measurements of excited state collision rates of optically trapped {sup 85}Rb and {sup 87}Rb. The collision rates have been measured in the vicinity of the P{sub 3/2} and P{sub 1/2} states of both isotopes. By studying the collision rates as a function of the frequency of light used to cause the collisions, they obtain collision spectra that reveal the importance of hyperfine interactions on the collision dynamics. In certain frequency ranges the collision rates for the two isotopes are nearly identical, while in others they find substantially reduced rates for {sup 87}Rb (large hyperfine interaction) as compared to {sup 85}Rb (small hyperfine interaction). For the P{sub 1/2} states, where hyperfine splittings are large compared to the characteristic frequency scales for these collisions, the shapes of the spectra more closely correspond to those expected from simple models that neglect hyperfine interactions.

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We describe studies of excited-state/ground-state collisions between optically trapped Rb atoms. Collision rates are deduced from the transients observed as atoms are loaded into a vapor-cell optical trap. The rates are measured as a function of the frequency of the light used to excite the atom pairs. A comparison of the results for 85Rb and 87Rb shows that excited-state hyperfine interactions play a vital role in the dynamics of the collisions. In addition, the absolute rates are smaller than those expected from recent models.
Measurements of collisional loss of optically trapped Rb atoms are presented for excitation of the colliding atoms by light tuned near the 5S1/2{\mathit{S}}_{1/2}S1/2​\ensuremath{\rightarrow}5P1/2{\mathit{P}}_{1/2}P1/2​ transitions of 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb. Using the P1/2{\mathit{P}}_{1/2}P1/2​ state allows the effects of spontaneous emission on excited-state collisions to be studied with minimal complications arising from hyperfine structure. The shapes of the collision spectra are nearly identical for the two isotopes and are consistent with a simple model for the role of spontaneous emission during the collision.
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.
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