MICROWAVE SPECTROSCOPY OF THE 2-PHOTON TRANSITIONS AND DOUBLE STARK RESONANCE IN SODIUM RYDBERG ATOMS

The experimental studies are given of the two-photon absorption and double resonance on the microwave nP-->(n+1)P transitions of Rydberg Na atoms in static electric field (double Stark resonance). The static polarizabilities of nP states with n=36 and 37 are measured in low electric fields. The deviations from the quadratic Stark effect due to the phenomena like avoided crossing are observed. The features and conditions for arising the double Stark resonance due to crossing the real intermediate and virtual levels of two-photon transition both in the mw frequency scale and electric field scale were investigated. The critical frequencies and fields are calculated and possibility to use the double Stark resonance for the absolute gauge of the electric field intensity in vacuum is demonstrated. The strong field effects are observed.