This paper deals with a theoretical and experimental study that describes the effect of a frequency detuning of the laser used to achieve optical pumping in a small optically pumped cesium beam resonator. The ground state Zeeman sublevels with opposite angular momentum are unequally populated, leading to an asymmetrical microwave spectrum. The relative population asymmetry as a function of the laser frequency has a dispersion-like shape. Its dependence on laser intensity, applied magnetic field, and laser linewidth is demonstrated for a laser at 852 nm and tuned to the ²S_1/2F=3→²P_3/2F'=3 transition of the Cs D₂ line. Finally, the effect of a slight laser detuning on the Rabi pulling frequency shift is discussed