Measurements of the Recoil Shift

When the absorbing molecules with the resonant absorption frequency Wq are placed inside the laser resonator, the standing laser wave of frequency w burns two Bennet holes into the population distribution Nj(Vj) (Fig. 14.2b and Sect. 7.2) which, according to (14.3), appear at the velocity components 

The corresponding peaks in the population distribution Nij(Vj) of molecules in the upper level k) are shifted due to photon recoil (Fig. 14.2a). They show up, according to (14.5), at the velocity components 

The two holes in the ground-state population coincide for v j = 0, which gives w = ftc 2/2Mc2. This happens, according to (14.8), for the laser frequency 

The absorption of the laser is proportional to the population difference AN = Nj-Nj. This difference has two maxima at and Wj. The laser output will therefore exhibit two Lamb peaks (inverse Lamb dips) (Fig. 14.2c) at the laser frequencies and W2, 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, transit-time broadening, etc, must carefully be minimized. This can be achieved in experiments at low pressures and with expanded laser-beam diameters. An experimental example is displayed in Fig. 14.3. 

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