Two-photon absorption strength: a new tool for the quantification of two-photon absorption

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDOS and HF+CIS3-21G) two-photon absorption properties of the 2,2'-bi(9,9-dihexylfluorene) molecule are compared.