Electronic and vibronic contributions to two-photon absorption of molecules with multi-branched structures

The electronic and vibronic contributions to the two-photon absorption of a series molecules with multi-branched structures have been studied using ab initio response theory. The results indicate that the electronic coupling between different branches alone cannot explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the s ingle branch structure, whereas it is predicted that vibronic contributions can play an important role in this respect. It is shown that for multi-bra nched molecules the use of circularly polarized light can increase the two- photon absorption cross section by a factor of 1.5 over linearly polarized light excitation. (C) 2000 American Institute of Physics. [S0021-9606(00)31 041-8].