Abstract
A three-level model is used for an atom in which two transitions are driven by two laser beams, having constant amplitudes and detunings that are proportional to the time. Assuming that any two of the levels are unoccupied at large negative times, the authors obtain simple formulae for the occupation probabilities at large positive times. Previous results and the early calculation of Landau and Zerner (1932) are special cases of these general formulae.