Saturation of wave-packet interferences: Direct observation of spin precession in potassium atoms

A sequence of two identical ultrashort laser pulses creates quantum interfe rences in an atom. In the weak-field regime, the observed beats between int erferences arising from various transitions are not a conclusive signature of wave-packet creation. On the contrary, in the saturation regime, the app earance of slow modulation is. We report on the observation of such modulat ions in atomic potassium in this field regime. The results are explained wi thin the bright-state-dark-state framework. The wave packet corresponds to the precession of the electronic spin and orbital angular momentum. [S1050- 2947(99)50209-6].