On multiphoton absorption by molecules
  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. 
Pedro F. Gonzalez-Diaz (IMAFF, CSIC, Spain)