Degenerate Two-Photon Absorption in All-Trans Retinal: Nonlinear Spectrum and Theoretical Calculations

In this work we investigate the degenerate two-photon absorption spectrum of all-trans retinal in ethanol employing the Z-scan technique with femtosecond pulses. The two-photon absorption (2PA) spectrum presents a monotonous increase as the excitation wavelength approaches the one-photon absorption band and a peak at 790 nm. We attribute the 2PA band to the mixing of states 1Bu +-like and |S 1⟩, which are strongly allowed by one- and two-photon, respectively. We modeled the 2PA spectrum by using the sum-over-states approach and obtained spectroscopic parameters of the electronic transitions to |S 1⟩, |S 2⟩ (“1Bu +”), |S 3⟩, and |S 4⟩ singlet-excited states. The results were compared with theoretical predictions of one- and two-photon transition calculations using the response functions formalism within the density functional theory framework with the aid of the CAM-B3LYP functional.