Pump probe experiment in atomic fine structure levels: Observation of the oscillation of an angular wavepacket

Spin precession is investigated through a pump-probe technique which repres ents an alternative method to previous investigations by wavepacket interfe rometry. The excited wavepacket corresponds to a precession of spin and orb ital angular momentum around the total angular momentum. We discuss and com pare two equivalent descriptions of the phenomena, one given in the station ary states and the other in the bright state-dark state formalism. We show that this latter formalism is the most appropriate to describe the dynamics .