Based on Double Resonance

The term double resonance as used in this section covers experiments where rf or microwave transitions within laser-optically excited short-lived (in contrast to metastable) states are detected via changes of the intensity, polarization, or wavelength of the reemitted fluorescence rather than via their influence on the population of the level to which the excited species decays. The short lifetime of the excited state of typically t = 10 sec excludes experimental schemes in which a certain excited state population has to survive for a longer period of time for example, probing the signal with another laser beam in a second interaction region is impossible as well as nonoptical registration methods. 

Part of the literature reviewed is closely related to that on optical pumping as contained in Section 3. 

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Other types of experiments based on double-resonance techniques provide alternate useful information as outlined in Table VIII. Moreover, double-resonance techniques have been used with optical and infrared radiation to observe microwave transitions from excited electronic states and excited vibrational states, respectively. 

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