Optical pumping light shifts

Since extremely narrow resonance lines can be obtained in optical pumping experiments, frequency standards of comparatively simple design can be achieved. The hyperfine transitions used in the atomic-beam clock are also used in the optically pumped frequency standards. However, the resonance frequency is comparatively strongly dependent on the pressure of the buffer gas [7.18]. It is also dependent on the intensity of the pumping light ("light shifts") [7.16]. Thus, it would seem that an absolute frequency standard of maximal precision cannot be achieved. On the other hand, optically pumped systems have proven to be very suitable for relative measurements and as secondary standards. By observing sharp AF = 0, AMp = 1... 

The absorption and emission spectra of crystalline or amorphous solids can be varied within wide spectral ranges by doping them with atomic or molecular ions [5.121]. The strong interaction of these ions with the host lattice causes broadenings and shifts of the ionic energy levels. The absorption spectrum shown in Fig.5.72b for the example of alexandrit, depends on the polarization direction of the pump light. Optical pumping of excited states... 

We shall consider depopulation pumping for the case of light of non-uniform spectral distribution in section 17.9.7, where we discuss the problem of energy level shifts produced by optical pumping. 

Thus, in addition to the extrapolation to zero intensity of the pumping light which must be made to eliminate systematic errors due to the light shifts discussed in section 17.9.7, a further extrapolation of the measured hyperfine resonance frequencies to zero buffer-gas density must be made. Zero-field hyperfine intervals measured by the optical pumping method are listed in Table 18.3, together with a selection of the results obtained by the atomic beam technique. Generally the optical pumping results are more precise than the older atomic beam results. The table... 

In the typical setup, excitation light is provided by a pulsed (e.g., nanosecond) laser (emitting in the visible range, e.g., at 532 nm, if Mb is investigated), while the probe is delivered by a continuous-wave (cw) laser. The two beams are spatially overlapped in the sample, and the temporal changes in the optical properties (such as optical absorption or frequency shift) that follow the passage of the pump pulse are registered by a detector with short response time (relative to time scale of the processes monitored), such as a fast photodiode. 

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