Two-photon absorption spectroscopy of stilbene and phenanthrene: Excited-state analysis and comparison with ethylene and toluene.

Two-photon absorption (2PA) spectra of several prototypical molecules (ethylene, toluene, trans- and cis-stilbene, and phenanthrene) are computed using the equation-of-motion coupled-cluster method with single and double substitutions. The states giving rise to the largest 2PA cross sections are analyzed in terms of their orbital character and symmetry-based selection rules. The brightest 2PA transitions correspond to Rydberg-like states from fully symmetric irreducible representations. Symmetry selection rules dictate that totally symmetric transitions typically have the largest 2PA cross sections for an orientationally averaged sample when there is no resonance enhancement via one-photon accessible intermediate states. Transition dipole arguments suggest that the strongest transitions also involve the most delocalized orbitals, including Rydberg states, for which the relative transition intensities can be rationalized in terms of atomic selection rules. Analysis of the 2PA transitions provides a foundation for predicting relative 2PA cross sections of conjugated molecules based on simple symmetry and molecular orbital arguments.