Abstract
The spectroscopy of dilute vapours is ordinarily limited by Doppler broadening. By using two counterpropagating light beams from the same laser, it is possible to get over this limit, and to attain true natural widths. Some methods use the saturation of ordinary one-photon transitions, and permit the selection of the signal given by the particular class of atoms whose velocity is perpendicular to the light beams: Lamb-dip spectroscopy, self-saturated absorption or dispersion, polarization spectroscopy. The second possibility is to produce double-photon transitions (E 2 - E 1 = 2hv) in such a manner that the Doppler shifts of the two photons exactly cancel each other, and all the atoms undergo the transition together for the true value of the laser frequency: Doppler-free multiphoton spectroscopy.
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