Saturation of Level Population by Optical Pumping

The effect of optical pumping on the saturation of population densities is illustrated by a two-level system with population densities N and A 2- The two levels are coupled to each other by absorption or emission and by relaxation processes, but have no transitions to other levels (Fig. 3.21). Such a true two-level system is realized by many atomic resonance transitions without hyperfine structure. 

With the probability 12 = B 2p (o) for a transition 11 2 by absorption of photons hco and the relaxation probability Ri for level /, the rate equation for the level population is 

When the pump rate P becomes much larger than the relaxation rates Ri, the population N approaches N/2, i.e., N = N2- This means that the absorption coefficient a = o N — N2) approaches zero for P - 00 (Fig. 3.22). The medium becomes completely transparent. 

Without a radiation field (P =0), the population densities at thermal equilibrium according to (3.59a, 3.59b) are 

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The population density of molecules in the absorbing level is decreased by optical pumping. This results in a nonlinear dependence of the absorbed radiation power on the incident power. Such techniques are therefore summarized as nonlinear spectroscopy, which also includes methods that are based on the simultaneous absorption of two or more photons dining an atomic or molecular transition. In the following sections the basic physics and the experimental realization of some important methods of nonlinear spectroscopy are discussed. At first we shall, however, treat the saturation of population densities by intense incident radiation. 

There are several different aspects of optical pumping that are related to a number of spectroscopic techniques based on optical pumping. The r.y aspect concerns the increase or decrease of the population in selected levels. At sufficiently high laser intensities the molecular transition can be saturated. This means that a maximum change AN = Nis — Nio of the population densities can be achieved, where A A is negative for the lower level and positive for the upper level of the transition (Sect. 2.1). In case of molecular transitions, where only a small fraction of all excited molecules returns back into the initial level /) by fluorescence, this level may be depleted rather completely. 

The analysis of molecular spectra is complicated because of the very large number of lines that is obtained simultaneously in normal excitation or absorption experiments. With narrow-band laser excitation an individual excited rotational-vibrational level can be populated selectively and only the decays originating in the excited state are observed. A similar simphfica-tion in absorption measurements is very desirable. Through the possibility of saturating optical transitions, a certain lower level can be labelled by depleting the population with a laser pump laser). If this laser is switched on and off repetitively, all absorption lines originating in the labelled level will be modulated when induced with a second (probe) laser [9.69, 9.70]. A number of schemes for modulation detection are indicated in Fig. 9.6. Several schemes can be used to ascertain that absorption has ocemred, as discussed... 

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