Cooling mechanisms in the sodium type-II magneto-optical trap

We present a rate equation model to account for the trapping mechanism in the sodium type-II magneto-optical trap (MOT). A key feature of the model is that excitation from the ground F=1 state to both excited states F^|IH=1 and F^|IH=0—possible in sodium because of the small excited state separation—is confirmed to be crucial in avoiding the optical pumping that otherwise inhibits steady-state trapping. Furthermore, we calculate that the mixed polarization typically present in a three-dimensional trapping arrangement gives substantially better one-dimensional trapping performance than pure circular polarization, and that the active role of repumping in the trapping process is important. The model is also applied to the type-I (F=2 to F^|IH=3) MOT and comparisons are made with experiment in both cases. The possibility of blue-detuned Sisyphus cooling in the F=1 to F^|IH=1 and 0 transition scheme is considered.