Justification of two-level approximation for description of two-photon absorption in Oxazine dyes

We measure and quantitatively analyze two-photon absorption (2PA) spectra and cross sections in a series of Oxazine molecules with varying chemical structures. We compare values of the permanent dipole moment difference (Delta mu) obtained by three different methods: (a) 2PA cross sections using two-level approximation; (b) Stark spectroscopy, and (c) solvatochromic shifts. The first two methods give coinciding Delta mu values, thus justifying two-level approximation, while solvatochromic shifts measurements yield systematically higher Delta mu values. Possible reasons for such a discrepancy are discussed.