Measurement of the Fine-Structure Splitting of the4FState in Atomic Sodium Using Two-Photon Spectroscopy with a Resonant Intermediate State

The fine-structure splitting of the $4F$ state of atomic sodium is determined using the technique of two-photon spectroscopy with a resonant intermediate state. The sensitivity of this Doppler-free technique allows observation of the electric-dipole-forbidden $3S\to 4F$ two-photon transition. The $4F$ fine-structure splitting is found to be 229±4 MHz. Its sign and magnitude are hydrogenic, in complete contrast to the $D$ levels of Na.