High-Resolution Sub-Doppler Laser Spectroscopy

The spectral resolution of all methods discussed in Chap.8 was in principle limited by the Doppler width of lines in the molecular absorption-or emission-spectra, although the laser linewidth might have been much smaller. In this chapter we present several techniques which overcome this resolution limit and which allow one to resolve the natural linewidth even in the presence of a much larger Doppler width. These techniques, which have been developed recently, have already stimulated experimental and theoretical atomic physics in an outstanding way. These various Doppler-free methods certainly represent an important step towards a more detailed knowledge of molecular structure and of deeper details regarding the interaction of E.M. radiation and matter. 

While for most experiments in Doppler-limited spectroscopy-discussed in Chaps.8 and 9-wultimode lasers can be used (e.g., for optical pumping experiments, laser-induced fluorescence of atoms and simple molecules, or for Doppler-limited absorption spectroscopy) only some of the sub-Doppler methods, treated in this chapter, may be performed with pulsed or cw multimode lasers. Most of these techniques demand narrow-band tunable single ode lasers with a bandwidth which should be smaller than the desired spectral resolution. If the natural linewidth 6v has to be resolved, the laser frequency jitter should be smaller than 6v. This demands frequency stabilization techniques (see Sect.6.5) and there are many examples in this branch of high-resolution laser spectroscopy where the achieved resolution is- indeed limited by the stability of the laser. 

The basic principle of most Doppler-free techniques relies on a proper selection of a subgroup of molecules with velocity components v in the direction of the incident monochromatic wave, which fall into a small interval Av around v = 0. This selection can be achieved either by mechanical apertures which select a collimated molecular beam, or by selective saturation within the velocity distribution of absorbing molecules caused by a 

Strong pump wave, and a successive probing of this selective hole burning by a monochromatic tunable probe wave (see Sect.3.6). 

Another class of Doppler-free techniques is based on a coherent preparation of an atomic or molecular state and a detection scheme which is capable of detecting the phase relations of the wave functions in this state. 

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