Resonant enhancement of two-photon absorption in substituted fluorene molecules.

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.