Observation of Rydberg Series in Sodium Vapor by Two-Photon Resonant Nondegenerate Four-Wave Mixing

We apply two-photon resonant nondegenerate four-wave mixing
with a resonant intermediate state for the observation of Rydberg states in sodium vapor. The broadening and shift of the sodium 3S -11D transition perturbed by argon are investigated. This technique can achieve Doppler-free resolution of narrow spectral structures of Rydberg levels if lasers with narrow bandwidths are employed.