Two-Photon Ramsay Resonances

In Sect.10.6 we have seen that the first-order Doppler effect can be exactly cancelled for a two-photon transition if the two photons = ho)2 have opposite wave vectors = -j 2. A combination of Doppler-free two-photon absorp- 

The first two terms describe the conventional two-photon transitions in the first and second zones while the third term represents the interference leading to the Ramsay resonance. Due to the longitudinal thermal velocity distribution (v ), only the central maximum of the Ramsay resonance is observed with a theoretical halfwidth (for negligible natural width) of 

A quantitative description starts from the equations of motion (2.98) for the probability amplitudes a. which lead for the two-photon transitions between levels 1 and 2 to the equations [13.5,5a] 

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Under weak field conditions and for small detuning fl l/T l/T we obtain with the initial conditions a (0) = 1, a2(0) = 0 the probability amplitudes for molecules in the upper level (2) in the first and second zones 

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Figure 13.5 illustrates the narrowing of the two-photon resonance with two separated interaction fields. The technique of two-photon Ramsay resonances has been succesfully applied to the ultrahigh resolution spectroscopy of Rydberg levels in the rubidium atom using a well-stabilized cw dye laser [13.6]. At a separation of 4.2 mm between the two fields, Ramsay fringes with... 

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