Vibronic contributions to resonant NLO responses: two-photon absorption inpush-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 th e appearance of a vibrational structure in the spectrum; in this respect we find large Herzberg-Teller (HT) contributions. The second effect was not r ecognized so far: vibrational states contribute a new channel to TPA proces s, that shows up with a blue-shift and a distortion of the spectrum. (C) 20 01 Elsevier Science B.V, All rights reserved.