Doppler-free resonant two-photon transition for isotope separation

The Doppler-free two-photon transition by the use of two opposing laser beams makes the highly selective excitation of isotopic molecules possible. Saturation and pressure dependence of Doppler-free two-photon excitation with arbitrary amplitudes of the optical fields have been calculated and compared with those of the two-step transition and the single-photon transition. The result at the Doppler limit for low pressure gas reveals that the probability of the selective two-photon excitation with a moderate laser power can be larger than that of the single-photon excitation as well as that of the two-step excitation.