Fourier Transform Multicolor Excitation Action Spectroscopy (FT-MEAS): A Model-Free Approach for Investigating...

We propose a model-free method, Fourier Transform Multicolor Excitation Action Spectroscopy (FT-MEAS), for analyzing multicolor, multistep photoreactions that proceed under photostationary conditions, such as photosynthesis and photocatalytic reactions. This method focuses on the photoreaction response induced by multicolor excitation and enables estimation of the first- and second-order partial derivatives of the response with respect to each excitation light as well as the two-color cross-partial derivatives. The validity of the method was examined through both numerical simulations and experiments. The results of numerical simulations on a three-color, three-level system, performed according to the proposed method, were compared with analytical results obtained under the steady-state approximation of the master equation. All partial derivative coefficients agreed within an error of less than 2%, and the effectiveness of the method was confirmed. The experimental evaluation of the usefulness of the method was carried out by investigating the three-color photoisomerization of p-aminoazobenzene, which has two isomers, using a home-built experimental setup. We successfully observed first- and second-order peaks, as well as cross term peaks, and estimated the values of the first- and second-order derivatives. These results indicate that FT-MEAS can estimate the features of the multicolor response under model-free conditions.