  Rabi frequencies for multiphoton absorption by atoms or molecules that can be
characterized as two- or three-level systems are obtained in the cases of
single and double laser perturbations. If the ambiguity in the origin of
absolute energy is conveniently gauged off in the three-level model, there
appears an additional non linear frequency which is originated from the
separate evolution of a relative-frequency component induced in the whole
system by the simultaneous action of the two perturbations.
