OODR-polarization spectroscopy

Similar to polarization spectroscopy with a single tunable laser (Sect. 7.4) the sample cell is placed between two crossed polarizers and the transmitted probe-laser intensity Ix( 2) is measured as a function of the probe laser wavelength A2. 

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. 10.30). 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. There are several experimental tricks to separate the two probe beams of LI and L2 For A-type OODR these two probe beams travel into opposite directions and can therefore be detected by two different detectors. In case of V-type OODR the pump beam LI and the probe beam L2 are anti-collinear which means that the two probe beams are collinear. If their 

Easier with respect to the optical arrangement but with more involved electronics is the sum-frequency method. The pump beam LI and the probe beam L2 are chopped at two different frequencies with the same chopper (Fig.10.19). Both probe beams are detected by the same detector. The output signals are fed parallel into two lock-in detectors tuned to fj and fj+f2, respectively. While the lock-in at fj records the polarization spectrum of LI if A is tuned, the lock-in at (fx+f2) records the OODR spectrum induced by both lasers LI and L2 at a fixed wavelength Aj while Aj is tuned. 

For illustration. Fig. 10.31 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 [10.88,89]. 

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The downward transitions can be probed by the change in transmission of the probe wave by OODR-polarization spectroscopy or by ion-dip spectroscopy (Sect. 5.5.1). 

Doppler-free two-photon spectroscopy or with two-step excitation (Sect. 5.4). For illustration, Fig. 8.5 illustrates pressure broadening and shifts of a rotational transition to a Rydberg level of the Li2 molecule measured with Doppler-free OODR polarization spectroscopy (Sect. 5.5) in a lithium/argon heat pipe [980], where the intermediate level B(v, J ) was pumped optically by a circularly polarized pump laser. For the chosen temperature and pressure conditions the argon is confined to the cooled outer parts of the heat pipe, and the center of the heat pipe contains pure lithium vapor (98 % Li atoms and 2 % Li2 molecules) with a total vapor pressure p(Li) = p(Ar) up to argon pressures of 0.7 mbar. The observed pressure broadening and shift in this range p < 0.7 mbar are therefore caused by Li + Li collisions. 

Fig. 10.29. A-type OODR polarization 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. The homogeneous width of the upper (predissociating) level is for this example 2 MHz [10.88]...

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