Optical Pumping and Double-Resonance Techniques

This change ANj of the population density can be probed by a second EM wave, which may be a radio-frequency (RF) field, a microwave or another laser beam. If this probe wave is tuned into resonance with a molecular transition sharing one of the two levels i) or k) with the pump transition, the pump laser and the probe wave are simultaneously in resonance with the coupled atomic or molecular transitions (Fig. 10.1). This situation is 

Even this double-resonance spectroscopy has already been applied to the study of atomic transitions before lasers were available. In these pre-laser-experiments incoherent atomic-resonance lamps served as pump sources and a radio-frequency field provided probe transitions between Zeeman levels of optically excited atomic states [10.5]. However, with tunable lasers as pump sources these techniques are no longer restricted to some special, favourable cases and the achievable signal-to-noise ratio of the double-resonance signals may be increased by several orders of magnitude [10.6]. 

In this chapter we shall treat the most important laser double-resonance techniques by illustration with several examples. While the pump transition is always induced by a pulsed or CW laser the probe field may be provided by any coherent source in the spectral range between the RF region and the UV. 

There are several different aspects of optical pumping which are related to a number of spectroscopic techniques based on optical pumping. The first 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 = Nj -Njo of the population densities can be achieved, where AN is negative for the lower level and positive for the upper level of the transition (Sect.7.1). In case of molecular transitions, where only a small fraction of all excited molecules returns back into the initial level i) by fluorescence, this level may be depleted rather completely. 

Since the fluorescent transitions have to obey certain selection rules, it is often possible to populate a selected level m) by fluorescence from the laser-pumped upper level (Fig. 10.1b). Even with a weak pump intensity large population densities in the level m) may be achieved. In the pre-laser area, the term optical pumping was used for this special case because this 

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Optical Pumping and Double-Resonance Techniques Fluorescence Dip 6 — 6,... 

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