Saturation Spectroscopy of Coupled Transitions

Assume that two laser fields are interacting simultaneously with a molecular system. If the two laser frequencies and 2 are tuned to two molecur lar transitions which share a common level (a), coupling phenomena occur due to the nonlinear interaction between the fields and the molecular system. The absorption of one of the laser waves is influenced by the presence of the other wave. This coupling is caused by several different effects. 

The first coupling effect is due to the selective saturation of the level population by each wave. When, for example, the frequency is tuned close to the molecular transition a b with o) = - E )/ fi, the wave can be 

This causes a Bennet hole in the population distribution n (v ) (see Fig. 10.17). When the second laser is tuned over the absorption profile of the transition a c, the Bennet hole causes a decrease in absorption of this laser. This can be monitored for instance by the corresponding decrease of the fluorescence intensity on the transition c m (Fig.10.31). 

This optical-optical double resonance has been already discussed in Sect.8.9 as a method of labelling molecular levels and identifying molecular transitions in complex spectra. There we had not, however, considered the line profiles of the double resonance limits, which will be the subject of this section. 

Since the two transitions are coupled only by those molecules within the velocity range Av which have been pumped by one of the lasers, the doubleresonance signals show similarly small homogeneous linewidths as in saturation spectroscopy with a single laser. However, for precise spectroscopy the common Bennet hole should be exactly at the oentev of the population distribution and not anywhere around v = 0. This can be achieved for instance by using the Lamb dip, produced in the standing wave of the pump field, to stabilize the pump laser frequency w to the center frequency 

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