Importance of initial and final states as intermediate states in two-photon spectroscopy of polar molecules

Zusammenfassung

Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. This mechanism may account for the largest contribution to the 2-photon cross section in polar mols. if the dipole moment strongly alters with excitation. Possible consequences for the 2-photon detection of forbidden states in polar polyene systems are discussed.