Knowledge Bank

We have obtained the two photon excitation spectra of naphthalene and several related aromatics in solution using linearly and circularly polarized light. The polarization ratio $\Omega$, (defined as the intensity of absorption of circularly polarized light divided by that of linearly polarized light), depends on the symmetries of the ground and excited states. From our data, we have calculated the polarization ratios of several transitions. According to theory, for a molecule of $D2h$ symmetry where the ground state is totally symmetric, $\Omega =3/2$ for transitions to $Bg$ states and for transitions to $Ag$ $states.1$ We have not observed values of $\Omega$ as high as 3/2 in the systems we have examined. However, we have assigned the transitions by comparing the one photon absorbtion spectra and theoretical calculations to the two photon spectra. In all cases, we have observed $\Omega >1$ for upper states of $Bg$ symmetry and for upper states of $Ag$ symmetry.