We report the observation of fine-structure and Zeeman quantum beats in highly excited sodium $D$ states prepared, for the first time in a quantum-beat experiment, by stepwise excitation using two synchronously pulsed dye lasers. We have used this new technique to measure the previously unknown fine-structure intervals of levels $9D$ and $10D$ of sodium, the values being, respectively, 124.5±1.5 and 91.5±1 MHz.

• Received 5 August 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.1063