Previously we have found that the two-photon absorption (2PA) spectra of red fluorescent proteins have enhanced vibronic transitions and are about twofold broader, compared to the corresponding one-photon absorption (1PA) spectra. We explained these observations by the dependence of the permanent dipole moment change (Deltamu) on a particular vibrational coordinate (in the Herzberg-Teller spirit). Here we demonstrate that this effect is responsible for the 2PA spectral shapes of many fluorescent dyes including rhodamines, ATTO dyes, fluorescein, oxazines, acridines, pyronines, resorufin, and thionine. By measuring the two-photon polarization ratio as a function of excitation wavelength in these dyes, we were able to separate the transitions with Deltamu perpendicular to the long molecular axis (“FC”-type) and parallel to it (“HT”-type). We have found the previously undetected “HT-ghost” transitions under the pure electronic envelope. These are the 0-0 transitions of the "HT" progression build on the BLA frequency that are non-vanishing if the corresponding Huang-Rhys factor is not zero.
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