Observation of Two-Photon Absorption without Doppler Broadening on the3S5STransition in Sodium Vapor

Abstract
The 3S5S 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 A5S in the 5S state of the Na23 atom is determined to be 101±15 MHz.