Excited State Symmetry Assignment Through Polarized Two-Photon Absorption Studies of Fluids

Abstract

We consider the effect of molecular symmetry on the two-photon absorption constants of a randomly oriented sample of molecules. It is shown that a complete polarization study of an allowed two-photon transition of a fluid generally yields enough information to permit an unequivocal identification of the symmetry species of the excited state. Ambiguities occur in only a few cases. Thus, oriented molecule studies for symmetry assignment will be largely unnecessary in two-photon molecular spectroscopy. Symmetry assignment rules are given for all of the 32 crystallographic point groups and for the two groups of linear molecules. It is shown that certain transitions, which are allowed for two photons of different energies, are forbidden for two photons of the same energy. The application of this formalism to Raman scattering problems is discussed.