Nonlinear interaction of Na Rydberg atoms (P series) with infrared and millimeter radiation

An experimental investigation was made of nonlinear resonant interaction of CO₂ laser radiation (λ = 10.6 μm) and of tunable millimeter radiation (λ = 3.8–5.6 mm) with Rydberg atoms of Na excited to the nP states with n = 11 and 36. These Na Rydberg atoms were formed in an atomic beam as a result of three-stage excitation in accordance with the scheme 3S_1/2→3P_3/2→4S_1/2→nP_1/2,3/2. Pulsed tunable organic dye lasers and a laser utilizing the F₂^– color centers in an LiF crystal were used. The saturation parameters were determined for one-photon 36P–37S (Δn = 1) and 11P–30D (Δn >> 1) transitions. The transition frequencies and the dipole moments were calculated, an analysis was made of the saturation mechanism, and the field broadening of the resonances was studied. The experimental results compared with theoretical calculations led to the conclusion that under experimental conditions an important role in the saturation effects was played by the Stark effect in an inhomogeneous field which reduced significantly the coherent interaction time. Saturation was observed and a study was made of the characteristics of two-photon absorption due to the 36P–37P transition.