Contributions of permanent dipole moments to molecular multiphoton excitation cross sections

The role of both permanent molecular dipoles and virtual molecular states, for two- and three-photon molecular excitation, is discussed in the context of the two- and three-photon excitation cross sections, with a ten-energy-level giant-dipole molecule as a model. Two types of excitation mechanisms are involved, that which requires permanent dipole moments and that which requires virtual molecular excited states. The results are relevant to the understanding of two- and three-photon excitation processes and for the design of fluorophores with large multiphoton absorption cross sections. (C) 2002 Optical Society of America.