Doppler-free multiphotonic spectroscopy

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Abstract
The review describes atomic transitions where two or more photons are simultaneously absorbed by an atom or a molecule (without any intermediate resonant level). The authors show theoretically and experimentally the possibilities of these transitions for numerous high-resolution spectroscopic studies. The Doppler broadening is suppressed in two-photon transitions if the atoms are irradiated by two laser waves travelling in opposite directions, because the Doppler shift in one wave cancels that in the other. A generalization is made for multiphotonic transitions. A typical experimental set-up with a CW dye laser is described in detail, but the possibilities of other light sources are also discussed.