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

Two-photon transitions may proceed via a mechanism not involving virtually excited intermediate states. For some examples, it is shown numerically that this mechanism may account for the largest contribution to the two-photon cross section in polar molecules, if the dipole moment strongly alters with excitation. Possible consequences for the two-photon detection of ’’forbidden’’ states in polar polyene systems are discussed.