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Low-temperature physics

Low-temperature physics

Collisions feel the cold

30 Nov 2001

Laser cooling and trapping allow physicists to map the dynamics of every ion-atom encounter in a collision experiment.

For many years atomic-collision experiments basically involved measuring the deflection of fast projectiles that had passed through gaseous targets, or measuring the amount of light given off by atoms and molecules as they were bombarded with different projectiles. In recent years, however, the atomic-physics community has aspired to much more: we want to prepare a target of non-interacting atoms or molecules in a particular quantum state, strike it with projectiles of perfectly known speed, direction and internal state, and then record the time, frequency, speed, energy, direction, spin and internal state of every fragment that emerges from the collisions.

It is as if we have switched from three-ball billiards to 15-ball pool, and want to record and interpret not just the single impact of the cue ball on another ball but also the “break” shot that scatters all 15 balls over the entire table in a unique way. A further step in this direction has recently been taken by two teams of physicists that, for the first time, have combined the latest laser cooling and trapping techniques with a well established method for studying collisions.

This breakthrough has allowed the teams – one based at the KVI laboratory in Gröningen in the Netherlands, the other at Kansas State University in the US – to map out the complete kinematic details for each ion-atom encounter in collision experiments at keV energies.

In the December issue of Physics World, Keith B MacAdam of the University of Kentucky, USA, investigates the new technique.

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