Isotopically selective light-induced drift of atoms in a field of counterpropagating waves

Results are presented of a theoretical analysis of isotopically selective excitation of atoms in a field of counterpropagating waves with incomplete compensation for Doppler broadening. A density matrix formalism based on a strong collision model in a homogeneous steady-state case is used to obtain expressions for the total excitation probabilities and for the drift velocities of the resonant and nonresonant isotopes. An essentially new possibility for obtaining an asymmetric distribution function as a result of the dynamic two-photon Stark effect is noted whereby a light-induced drift (LID) can also be observed at exact resonance. Simple analytic expressions are derived for many cases and these can be used to analyze the dependence of the excitation and the drift velocity on the main characteristics of the process. Numerical estimates are given of the atomic parameters of alkali metals. The advantages of using this method for isotope separation compared with onephoton LID are discussed.