# Two-photon laser excitation of polycyclic aromatic molecules

## Abstract

The theory of two-photon absorption is extended to include vibronic mixing between electronic states. Theoretical expressions are derived which relate the possible transition pathways with measured quantities. The experimentally determined quantities are: the relative absorption cross sections of linearly versus circularly polarized exciting light, and the polarization of the fluorescence resulting from two-photon excitation by linearly polarized light. The theory is applied to measurements on two molecules of D2h symmetry, anthracene and 9,10 dichloroanthracene. Utilizing the equations developed, the relative contributions of the three pathways, (1) both photons absorbed along the short axis, (2) both photons absorbed along the long axis, and (3) one photon absorbed along the long axis and the other photon absorbed along the short axis, are determined. From the deduced values of the transition pathways the relative contributions of different final vibronic states to the absorption of two laser photons (ℏω=14 400 cm-1) are determined.