The $3S-5S$ transition of Na atoms is induced by absorption of two photons, one each from two circularly polarized beams traveling in opposite directions through the vapor. The beams are derived from a tunable, narrow-band pulsed dye laser. Doppler broadening is absent and the hyperfine structure is resolved. The hyperfine interaction constant $A_{5S}$ in the $5S$ state of the ${\mathrm{Na}}^{23}$ atom is determined to be 101±15 MHz.

• Received 4 February 1974

DOI:https://doi.org/10.1103/PhysRevLett.32.645