Two-photon absorption in organic dyes — Relation with the symmetry of the levels

We studied two-photon absorption in a number of organic dyes and found that the two-photon cross-section does not follow Schäfer-Schmidt's rule. We found that the ratio δ_ω/σ_2ω is a function of the difference between the peak absorption frequency and twice the laser frequency, ω_max - 2ω_L. We pointed out the importance of the symmetry of the energy levels by studying three families of dyes.