Two-photon absorption spectrum of all-trans retinal

The nondegenerate two-photon absorption spectrum of all-trans retinal in a nonpolar solvent at room temperature has been measured for the first time. The multichannel detection utilizing the technique of femtosecond time-resolved pump–probe absorption spectroscopy has enabled us to obtain precise two-photon absorption spectra. It was found that the two-photon absorption maximum of all-trans retinal is located at 376 nm, which is 7 nm longer than the one-photon absorption peak. This peak wavelength difference indicates the 500-cm−1 energy gap between the S3 (Bu) state and the S2 (Ag) state. On the basis of the obtained two-photon absorption data and the available fluorescence data, we determined the 0–0 position of the S2 state to be 2.47 × 104 cm−1.