Transitions from state /i to state /n of some two- and three-level systems under the influence of an electromagnetic wave and an additional field was investigated. Using the time-dependent perturbation theory, the transition amplitude cn(t) and probability density /cn(t)/2 of each system was derived. Subsequently, the average of the probability densities was obtained for a large value of the time interval t. Its dependence on the EM radiation frequency w and on the field strength y (or field frequency w\u27 was determined
