Principles of optical pumping

In this chapter we shall be concerned mainly with the principles of the technique, the effect of relaxation processes, and magnetic resonance transitions between Zeeman sub-levels. We shall therefore initially describe the experiments in terms of the populations of the ground state sub-levels. The discussion of the effects of phase coherence (Hertzian coherence) and experiments involving transverse pumping is reserved until section 17.8. Moreover the application of optical pumping methods to the investigation of hyperfine intervals and the measurement of nuclear moments is postponed until Chapter 18, as are the applications of this technique in devices such as magnetometers, atomic clocks, and masers. 

In an optical pumping experiment, Fig.17.2, the light from a rubidium lamp is usually passed through a simple 

We now recall the selection rules for the Zeeman components of an electric dipole transition  

The transition probabilities for these two decay routes are in the ratio of 2 1. Thus after they have experienced one cycle of absorption and radiative decay, one third of the atoms which were originally in the j 7 state end up in the j = state of the ground level. 

This type of optical pumping which results in a sample possessing a non-thermal distribution of population among the sub-levels of the atomic ground state should be distinguished from the flash lamp pumping used in solid state and liquid lasers. In those systems the aim is to transfer a large fraction of the ions or molecules into a different electronic level. 

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