Oscillation of the center of the Rabi pedestal in an optically pumped Cs beam standard

This paper is devoted to the study of the symmetry of the Rabi pedestal of the clock transition in an optically pumped Cs beam frequency standard. The frequency of the microwave oscillator is locked on the center of the Rabi pedestal using a square wave frequency modulation. We observe a modulation of the center frequency as a function of the modulation depth. Experimental investigations on this unexpected effect are reported.