The interaction of the sodium three-level system with a frequency-fixed or frequency-swept strong laser pulse is studied numerically. The validity of the secular-adiabatic approximation is analyzed. The failure of this approximation, caused by the LS decoupling, for the pulses with the peak Rabi frequency greater than the fine-structure splitting is shown.

• Received 7 July 1994

DOI:https://doi.org/10.1103/PhysRevA.51.1482