Laser Cooling and Trapping Group
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. 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.