Vibronic contributions to resonant NLO responses: two-photon absorption in push–pull chromophores

Two-photon absorption (TPA) spectra of push–pull chromophores are described in terms of a two-state model accounting for electron-vibration coupling. Vibrations have two main effects in TPA spectra. The most obvious one is the appearance of a vibrational structure in the spectrum; in this respect we find large Herzberg–Teller (HT) contributions. The second effect was not recognized so far: vibrational states contribute a new channel to TPA process, that shows up with a blue-shift and a distortion of the spectrum.