We have observed a sudden reversal in the direction of population transfer between nearly degenerate Zeeman sublevels during optical pumping of the cesium 6S\n{F= s A)-* 6P3/2(F ,aB 5) cycling transition. As a result of the reversal, the number of atoms in an atomic beam, with quantum numbers 6SV2 (/ r=ss 4; mF = -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.PACS numbers: 32.80.Bx Optical pumping (OP) is a fundamental tool in laser spectroscopy, and plays a central role in many quantumoptics phenomena, including laser cooling and trapping, orientation of atoms possessing hyperfine structure, laser "shelving" and quantum jumps, and atomic parity violation. OP of the cesium hyperfine levels, in particular, has received recent attention in connection with laser cooling of atoms [1-3] and with efforts to improve cesium atomic clocks [4,5]. The importance of OP has been dramatically demonstrated recently in a new laser-cooling mechanism, which relies on laser-polarization-dependent OP between degenerate Zeeman sublevels [3]. In spite of the awareness of the importance of OP among nearly degenerate levels, it has been implicitly assumed in the extensive research dealing with the subject that if the linewidth of the laser and/or transition is much greater than the frequency difference between a group of nearly degenerate transitions, the direction of OP (increasing or decreasing population of a particular state) is independent of laser detuning and is completely determined by laser polarization and atomic selection rules.In this Letter, we show that such an approach is not correct in predicting the final population of the nearly degenerate Zeeman sublevels. The population of a Zeeman sublevel can be optically pumped or depleted, at the expense or gain of its nearly degenerate neighbors, with the direction of OP depending on the detuning of the laser from the center of the group of unresolved resonances. Most importantly, this behavior is present even if the degeneracy is lifted only slightly, as it is, for example, in almost all possible experiments, by ever-present miniscule residual magnetic fields.The observation of the sudden reversal in the direction of OP for a particular Zeeman sublevel was made for the F=4 ground state of cesium. The linewidths of the transitions between Zeeman sublevels in our experiments are nearly 3 orders of magnitude wider than the frequency separation between these transitions. In spite of this, OP is able to "resolve" a single magnetic sublevel of the F=4 hyperfine state and pump atoms into or out of this state depending on the detuning of the laser. The reversal is also predicted by a rate-equation model, which treats each Zeeman sublevel individually. The model, which is far from providing satisfactory quantitative agreement, n...

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