Dark region in fluorescence emitted by Rb atoms in a quadrupole magnetostatic field: observation and explanation

The quadrupole magnetostatic field (QMF), together with 3 pairs of laser beams, known as the magneto-optical trap, has been widely used for cooling and trapping atoms. This report concerns itself with some new phenomena observed in fluorescence emitted by Rb atoms in QMF under the circumstances of a single /spl sigma/ laser beam. A circularly polarized laser traveling-wave tuned to D/sub 2/-line is passed through a vacuum chamber along the central axis of two anti-Helmholtz coils. The fluorescence emitted by Rb atoms excited by the circularly polarized light in the direction at right angle to the direction of the incident laser beam is collected and imaged with all polarized directions taken into account. From the camera picture we can see that, around the zero-field point, there exists a dark region, the size of which is inversely proportional to the field gradient and is not sensitive to the detuning. The dark region corresponds to a dip in the profile of the fluorescence intensity versus the space. An attempt is made to explain the origin of this dark region.