Evidence for the Two-State-Two-Mode model in retinal protonated Schiff-bases from pump degenerate four-wave-mixing experiments.

We apply spectrally-resolved pump degenerate four-wave-mixing for the characterization of excited state low-frequency vibrational coherences during the initial events in excited state double-bond isomerization of retinal protonated Schiff-bases. A set of low-frequency coherences in the energetic range of 100-350 cm(-1) appears in the dynamics already for very early delays after initial excitation (<100 fs). The modulations are rapidly damped (<800 fs) and detectable only in a certain time window after initial excitation (<0.6 ps). Following the initial relaxation process, which leads the molecule to a stationary point in the S(1) state, it is not possible to re-excite the coherences in the excited state. Based on our observations, we conclude that the activation of the coherences is only possible to occur in a well-defined region of the excited state potential near the Franck-Condon region. Our results give direct experimental indication for the validity of the "Two-State-Two-Mode model", frequently applied for the interpretation of retinal isomerization dynamics.