Direct observations on the dynamic Stark effect in two-photon absorption and parametric interaction

Measurements have been made on the energy shift in the $4s_{1/2}$ level for the sodium atom in linearly and circularly polarized beams in resonance with the $4s_{1/2} – 4p_{1/2,3/2}$ transition. Two-photon absorption and parametric mixing have been used in the probing. The shift, broadening, and splitting of the $4s_{1/2}$ level have been recorded.