Laser Cooling and Trapping Group
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.
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.
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).
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