  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 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. Vibrational-channel
contributions to other NLO responses are shortly discussed.
