We have observed a tunable 722 nm red emission in sodium vapor under a 578.7 nm two-photon resonance excitation. This tunable 722 nm emission has an intensity about two orders of magnitude smaller than that of the persistent strong 330.2 nm emission (the conversion efficiency of 330.2 nm emission is about 5 % as reported by Hartig in 1978). Tunability of this emission can be well described by the generating scheme omega(722nm) = 2 omega(Laser) - omega(4p-4s) + omega(4p-3d) - omega(Laser). The excitation function of the 722 nm emission has a typical multi-wave mixing pattern with a competing effect appearing at higher temperatures under a two-photon resonance excitation. The dependence of the 722 nm emission intensity on the laser power is a cubic relation which agrees with the above mentioned scheme. Numerical analysis indicates that the observation of this red emission can be attributed to the argon buffer gas resonance enhancement in matching phases between the pump and signal beams. Details will be discussed.
