Reversal of the direction of population transfer between Zeeman sublevels in optical pumping

We have observed a sudden reversal in the direction of population transfer between nearly degenerate Zeeman sublevels during optical pumping of the cesium 6${\mathit{S}}_{1/2}$(F=4)→6${\mathit{P}}_{3/2}$(F’=5) cycling transition. As a result of the reversal, the number of atoms in an atomic beam, with quantum numbers 6${\mathit{S}}_{1/2}$ (F=4; ${\mathrm{m}}_{\mathit{F}}$=-4), is a function of laser frequency and has a sharp dispersive shape. A rate-equation model indicates that similar dispersive shapes for the population can be expected in the optical pumping of any nondegenerate system with more than two levels, if the level spacing is much smaller than the laser linewidth.