Measurement of Recoil Shift

When the absorbing molecules with the resonant absorption frequency coq are placed inside the laser resonator, the standing laser wave of frequency co coq burns two Bennet holes into the population distribution Ni (Vz) (Fig. 9.2b and Sect. 2.2), which, according to (9.3), appear at the velocity components 

The corresponding peaks in the population distribution NkiVz) of molecules in the upper level A ) are shifted due to photon recoil (Fig. 9.2a). They show up, according to (9.5), at the velocity components 

The two holes in the ground-state population coincide for which gives oJ = 

The absorption of the laser is proportional to the population difference AN = Ni — Nk. This difference has two maxima at wi and W2- The laser output therefore exhibits two Lamb peaks inverse Lamb dips) (Fig. 9.2c) at the laser frequencies wi and (02, which are separated by twice the recoil energy 

Since such small splittings can only be observed if the width of the Lamb peaks is smaller than the recoil shift, all possible broadening effects, such as pressure broadening and transit-time broadening, must be carefully minimized. This can be achieved in experiments at low pressures and with expanded laser beam diameters [1117], An experimental example is displayed in Fig. 9.3. 

The two holes in the ground-state population coincide for v i = 0, which gives co = This happens, according to (14.8), for the laser frequency 

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