A-type OODR

Fig. 5.15 Different schemes for OODR (a) V-type OODR (b) stepwise excitation (c) A-type OODR...

The last scheme in Fig. 5.15c, named A-type OODR, represents a stimulated resonant Raman process where the molecules are coherently transferred from level 11) to levels m) by absorption of the pump laser and stimulated emission by the probe laser. 

In the A-type OODR scheme (Fig. 5.32) the probe laser induces downward transitions from the upper level = 2) of the pump transition to lower levels / = m). This process, which is called stimulated emission pumping (SEP), may be regarded as a resonantly induced Raman-type transition. In case of monochromatic pump and probe lasers tuned to the frequencies co and co2, respectively, the resonance condition for a molecule moving with velocity v is... 

Fig. 5.33 A-type OODR spectroscopy illustrated for the example of the Cs2 molecule (a) term diagram (b) two OODR signals of the same transition measured with collinear and anticollinear propagation of pump and probe beams. For this example the homogeneous width of the upper (predissociating) level is y2 100 MHz [605]...

The optical pumping of the molecules is now performed with a separate laser beam, which is sent through the sample cell anticollinearly to the probe beam for V-type OODR, but collinearly for A-type OODR (Fig. 5.34). In order to keep the pump transition on the wanted selected transition, at first an ordinary Doppler-free polarization spectrum of the pump laser must be recorded. Therefore, the pump laser beam is split into a pump and a probe beam, and the spectrum is recorded while laser L2 is switched off. Now the pump laser is stabilized onto the wanted transition and the second (weak) probe laser L2 is simultaneously sent through the cell. 

For illustration. Fig. 5.35 displays a section of the polarization spectrum of Cs2 with V-type and A-type OODR signals of Cs2 transitions with a common upper or lower level, respectively [605, 606]. 

In a A-type OODR scheme a transition is induced between the ground-state levels 1) and 3) of the Cs2 molecule. The radiative lifetime of these levels is ti = T3 = 00. The upper level 2) has a lifetime of X2 = 10 ns. Calculate the linewidth of the OODR signal... 

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