Atomic Physics Division

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Laser Cooling and Trapping Group

Metastable Xenon Project

Optical Control of Ultra-cold Collisions

The study of ultra-cold collisions has attracted a great deal of interest in recent years, in large part because the low velocities (~6 cm/s is typical for metastable xenon, compared to ~300 m/s at room temperature) involved allow the atoms to absorb and re-emit photons during the collision process. This can dramatically change the dynamics of the collisions, allowing us to increase or decrease the rate of collisions through the application of appropriately tuned laser light. This optical control of the collisions can also be exploited to study collisions in time-resolved experiments.

The optical control experiment consists of collecting a sample of atoms in the MOT, extinguishing the MOT lasers, and applying a control laser pulse to the atoms. We measure the rate of ion production during the control pulse, and compare it to the rate of ion production from collisions in the absence of light. The excess collision rate produced by the control laser is shown below as a function of control laser detuning.

Figure 1

Figure 1. Laser-induced rate coefficient as a function of laser detuning for two different isotopes of xenon.

When the laser is to the red of atomic resonance (detuning < 0), colliding pairs of atoms are excited to an attractive molecular potential, and accelerated together. This gives rise to a large increase in the collision rate. To the blue of resonance (detuning > 0), the atoms are excited to a repulsive molecular potential, and prevented from reaching the region of small internuclear separation where Penning ionization occurs. This optical shielding suppresses the collision rate, and we are able to reduce the rate of ionizing collisions by a factor of 8 with this technique.

Figure 2
Figure 2. Measured suppression factor vs. laser detuning for collisions in the presence of MOT light. In the shaded region, the trap is distorted by the presence of the control laser.
  In the presence of the MOT light (comparing the collision rate with both MOT and control lasers to that with MOT laser only), we find a dramatic increase in the effectiveness of the shielding, with suppression factors greater than 30. As the rate in the presence of red-detuned MOT light is a factor of 20 higher than the rate with no light present, this gives an ionization rate in the presence of both MOT and control lasers that is 2/3 the rate with no light present. We have been able to increase the MOT density by 50%, and the total number of atoms by a factor of 2 using this technique.

Simple Landau-Zener theoretical models have been applied to the shielding and enhancement situations, and produce good agreement with the data. This optical control effect is also used in a pulsed experiment, to study the time dependence of the collision process.


  • M. Walhout, U. Sterr, C. Orzel, M. Hoogerland, and S.L. Rolston, Phys. Rev. Lett. 74, 506 (1995) (PDF Pre-print Get Acrobat Reader 130 kB).

  • K.-A. Suominen, K. Burnett, P.S. Julienne, M. Walhout, U. Sterr, C. Orzel, M. Hoogerland, and S.L. Rolston, Phys. Rev. A 53, 1678 (1996).
For more information, contact:
Steven Rolston
National Institute of Standards and Technology
Gaithersburg, MD 20899
(301) 975-6581

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