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 ${\mathrm{F}}^{|\mathrm{I}\mathrm{H}}$=1 and ${\mathrm{F}}^{|\mathrm{I}\mathrm{H}}$=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 ${\mathrm{F}}^{|\mathrm{I}\mathrm{H}}$=3) MOT and comparisons are made with experiment in both cases. The possibility of blue-detuned Sisyphus cooling in the F=1 to ${\mathrm{F}}^{|\mathrm{I}\mathrm{H}}$=1 and 0 transition scheme is considered.

• Received 31 October 1996

DOI:https://doi.org/10.1103/PhysRevA.55.4621