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Physical Review A

Optical suppression of hyperfine-changing collisions in a sample of ultracold sodium atoms

Sérgio Ricardo Muniz, Luís Gustavo Marcassa, Reginaldo de Jesus Napolitano, Gustavo Deczka Telles, J. Weiner, Sérgio Carlos Zílio, Vanderlei Salvador Bagnato

Published Jun 1, 1997• 12 citations
Publisher

Abstract

We report the observation of hyperfine-changing collisions and the suppression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ionization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppress the hyperfine-changing collisions, producing suppression up to 80%. The experimental results are compared with predictions of a simple Landau-Zener model and of a three-dimensional, multichannel close-coupling calculation. Both theories agree at low intensity, but at high intensity the experiment shows a saturation behavior that is qualitatively described only by the three-dimensional multichannel theory.

DOI
10.1103/physreva.55.4407

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We report the observation of hyperfine-changing collisions and the suppression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ionization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppress the hyperfine-changing collisions, producing suppression up to 80%. The experimental results are compared with predictions of a simple Landau-Zener model and of a three-dimensional, multichannel close-coupling calculation. Both theories agree at low intensity, but at high intensity the experiment shows a saturation behavior that is qualitatively described only by the three-dimensional multichannel theory.
Summary form only given. Collisions with cooled and trapped atoms have been the subject of intense investigation during the past years. Unprecedented details of the intermediate steps in the collision have been revealed. A remarkable feature in the ultracold system is the ability to control entrance-channel scattering flux and thereby control the exit channel of the collisional encounter. Here we present the continuation of our recent work where photoassociative ionizations (PAI) of sodium atoms were suppressed by application of an optical field. The new experiments show that suppression of almost 100% is possible when high intensity is used and a remarkable dependence with the polarization of the suppressor laser field is observed and calculated. This result shows a deviation from the simple picture based on the Landau-Zener avoided crossing model when high intensity is used.
Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine-changing collisions between ultracold 87Rb{}^{87}\mathrm{Rb}87Rb 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 $\ensuremath{\sim}50%$ and are in reasonable agreement with a simple Landau-Zener model. Our ability to control collisional trap loss processes may have important implications for the achievement of high densities in laser cooled samples.
\n We report the observation of polarization anisotropy in the optical\nsuppression of photoassociative ionization (PAI) collisions. Binary\ncollisions occur within a tightly collimated and optically cooled velocity\ngroup of a single sodium beam. A laser beam, blue-detuned with respect to\nthe atomic resonance transition, crosses the atomic beam and suppresses the\nPAI rate. We measure this suppression as a function of intensity and\nelectric field vector aligned parallel or perpendicular to the atomic beam\naxis. Quantum close-coupling and multichannel curve-crossing calculations of\nmodel systems yield insight into the nature of the anisotropy and are in\nreasonable agreement with the measurements.\n\n \n
We have developed a full three-dimensional quantum scattering approach to optical suppression of ultracold-collision rates. These calculations are carried out assuming colliding atoms without fine or hyperfine structure, which have a $^{1}\ensuremath{\rightarrow}^{1}$P transition. The three-dimensional model predicts that the optical suppression of ultracold-collision rates saturates with light intensity much more slowly than predicted by two-level curve-crossing models. Circularly polarized light is significantly more effective for optical suppression, and causes less increase in atomic kinetic energy due to excited-state production than linearly polarized light. The suppressor optical field can also cause orders of magnitude increases in ground-state elastic-scattering rates.
Three-dimensional quantum scattering calculations predict that the degree of optical shielding of cold collisions saturates with increasing intensity very differently from two-state models and is sensitive to the polarization of the shielding light. We report measurements showing how suppression of Na photoassociative ionization (PAI) rates is sensitive to the intensity and polarization of the shielding laser. At higher intensities, circular polarization suppresses PAI rates an order of magnitude more effectively than linear polarization, in qualitative agreement with our model calculations.
We present precise near-dissociation limit measurements of ultracold collision rate constants (trap loss, fine-structure changing, and photoassociative ionization) as a function of trap intensity from Isat{I}_{\mathrm{sat}}Isat​ to $I\ensuremath{\approx}{40I}_{\mathrm{sat}}$ for Cs and Na atoms cooled and confined in a magneto-optical trap. We find strong saturation for all processes at intensities approximately 10 times the free atom saturation level Isat.{I}_{\mathrm{sat}}.Isat​. Our results provide a precise reference point for testing and comparing different theoretical models used to analyze ultracold collisions.