  Spin precession in Rubidium atoms is investigated through a pump-probe
technique. The excited wave packet corresponds to a precession of spin and
orbital angular momentum around the total angular momentum. We show that using
shaped laser pulses allows us to control this dynamics. With a Fourier
transform limited pulse, the wave packet is initially prepared in the bright
state (coupled to the initial state) whereas a pulse presenting a $\pi $ step
in the spectral phase prepares the wave packet in the dark state (uncoupled to
the initial state).
