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Cooling, Trapping, and Ultra-Cold Collisions

Ryan McMartin '05 next to the atomic beam apparatus
Atomic beam apparatus used for laser cooling and trapping experiments. From left to right, we see the trapping chamber, the Zeeman slowing magnet, part of the metastable beam source, and Ryan McMartin '05 (for scale). (Click for larger image).

I am in the process of building a lab at Union for laser cooling and trapping of metastable argon and krypton atoms. This work builds upon work that I did with metastable xenon as a graduate student.

Krypton energy level diagram

Figure 1: Energy levels and transitions for krypton. (Click for larger image.)

Argon and krypton (and rare gas atoms in general) cannot be laser cooled starting in their lowest energy (ground) states, as the wavelengths required are much too short for current laser technology. If they are excited into the first excited state, which has a lifetime of approximately 30 seconds, they can be cooled and trapped using light from infrared diode lasers, which are readily available. The cooling wavelengths for Ar* and Kr* are very similar (λ= 811.3 nm for Kr, λ= 811.5 nm for Kr), which allows both species to be trapped with the same optical system.

The energy levels for krypton are shown in Fig. 1 at left (the level structure in Ar is similar). The atoms may be excited to the metastable state either by collisions with electrons in a plasma discharge, or by a two-photon optical excitation. The system currently under construction will use a plasma discharge source, which has a low excitation efficiency, but is very robust, and will work for either species. In future work, I plan to investigate the optical excitation method, which is potentially more efficient, and will excite only Kr atoms.

Once we have trapped and cooled atoms, we plan to study ionizing collisions in these systems. Each metastable atom contains approximately 10 eV of internal energy, and when two metastable atoms collide, they have enough energy between them to ionize one of them. There are two types of ionizing collisions of interest to us: Penning ionization (PI), in which the two atoms collide to produce one ground state atom, one atomic ion, and a free electron

Kr* + Kr* → Kr + Kr+ + e-

and assocative ionization (AI), in which the two atoms stick together to form an ionized molecule

Kr* + Kr* → Kr2+

Both atomic ions and ionized molecules are easily detected, so the rate of collisions between atoms can be directly measured by measuring the rate of ion production.

These ionizing collisions are forbidden in spin-polarized systems, which leads to a large reduction in the ionization rate for metastable helium, and makes metastable helium BEC possible. In metastable xenon, the spin-polarization suppression is negligible, due to strong anisotropic interactions between the colliding atoms. The suppression has not yet been accurately measured in either Ar or Kr; measuring it is the first experiment planned for the Union College lab.