Laser Cooling and Trapping Group

Metastable Xenon Project

Time-Resolved Studies of Ultra-Cold Collisions

Laser modification (optical control) of ultra-cold collisions takes advantage of the low velocities and long collision times involved in collisions at microKelvin temperatures to use photon scattering to increase or decrease the rate at which collisions occur. These effects present another intriguing possibility: using short laser pulses, we can watch interatomic collisions occuring in real time.

The time-resolved collisions experiment consists of applying a short (40 ns rms width) laser pulse tuned around the laser cooling transition at 882 nm to a sample of ultra-cold atoms collected in the MOT. Depending on the detuning, this pulse will give rise to a brief increase or decrease in the collision rate, which may be observed in the Penning ionization signal.

Figure 1. The observed collision enhancement or suppression from a 40 ns, ~750 mW/cm2 laser pulse applied at t = 0. The red points are for a red detuning of six natural linewidths (30 MHz), and show collisional enhancement. The initial peak is from Penning ionization, the second peak is from Xe2+ molecular ions formed through associative ionization. Blue points are for a blue detuning of 4 linewidths, and show collisional shielding. The solid line shows the time of flight for ions created in direct photoionization of the MOT by a 514 nm dye laser.

The pulse will excite colliding pairs of atoms to either attractive (for red detuning) or repulsive (for blue detuning) molecular states at a specific point (the Condon radius) determined by the detuning from resonance. For red-detuned light, the atoms are accelerated together, producing a brief increase in the collision rate; for blue-detuned light, the atoms are forced apart, producing a brief decrease in the collision rate. Ions are detected by a channel electron multiplier, and the ion counts are recorded with a multi-channel scaler, providing a histogram of ion production vs. time.

Figure 2. Collision times for shielding, Penning ionization and associative ionization for different laser detunings. Blue points are measured times for shielding experiments, black points are calculated times from a simple theoretical model. Blue lines represent linear fits to theory and exeriment, the green line is a fit to the detuning dependence calculated for assoicative ionization collisions.

Using this time-resolved technique, we distinguish between Penning ionization (Xe^* + Xe^*  Xe + Xe⁺ + e^-) and the formation of Xe₂⁺ molecular ions through associative ionization (Xe^* + Xe^*  Xe₂⁺ + e^-). We observe atoms colliding in the excited state (long thought to play an important role in collisional enhancement experiments), and establish the relative importance of both excited state survival and "flux enhancement" effects in the collision process. We also estimate the rate of molecule formation in excited state collisions, and find that simple theoretical models can be used to predict the dynamics of the collisional shielding process (see Figure 2 above).

Publications:

• C. Orzel, S.D. Bergeson, S. Kulin, and S.L. Rolston, Time-Resolved Studies of Ultracold Ionizing Collisions, Phys. Rev. Lett. 80, 5053 (1998).

For more information, contact:

Steven Rolston
National Institute of Standards and Technology
PHYS A168
Gaithersburg, MD 20899
(301) 975-6581
e-mail: steven.rolston@nist.gov

Inquiries or comments: steven.rolston@nist.gov
Online: May 1998   -   Last update: May 2003 (format only)