Two-photon polarization measurements are shown to provide an incorrect symmetry assignment for the first excited state of phenanthrene, both in solution and in the vapor phase. Published assignments from one-photon, single-crystal measurements indicate a totally symmetric state, while the assignment from two-photon measurements indicates an antisymmetric state. Possible reasons for the failure of the two-photon symmetry assignment are low-frequency vibronic overlap, and interference among the diagonal tensor elements. Experimental evidence points to interference as the origin of the discrepancy.
A. C. Koskelo, D. L. Curtin, and M. J. Wirth, "Phase Interference in the Two-Photon Excitation of Phenanthrene," Appl. Spectrosc. 40, 486-489 (1986)