Methodology of Cross-Band Two-Dimensional Fourier Spectroscopy

Two-dimensional Fourier transform spectroscopy (2D Fourier transform spectroscopy, 2D FTS) is a nonstationary nonlinear optical method based on the process of four-wave mixing involving a sequence of three femtosecond pulses. The technique has become widely used for obtaining information about the structure and ultrafast transformation of complex organic and inorganic compounds with characteristic molecular vibrations in the mid-infrared range, and is referred to as two-dimensional Fourier infrared spectroscopy (2D Fourier Infrared Spectroscopy, 2D FTIR). In one of the possible configurations of the 2D FTS technique, the excitation and probing of the objects under study are carried out in significantly different spectral ranges. In the present work, an experimental implementation of such a cross-band variant of two-dimensional Fourier spectroscopy is demonstrated using the example of recording the vibrational levels of a solution of the organic dye DCM in the frequency range 1430–1820 cm ^-1 under electronic excitation of the sample by broadband visible-range radiation. It is shown that in a polar solvent, the relaxation of electronic excitation of the DCM dye is accompanied by activation of Franck–Condon vibrational modes, which ar visible in the two-dimensional Fourier spectrum at frequencies 1547 and 1576 cm ^-1. It is demonstrated that this technique allows for the recovery of the phase of the dye’s nonlinear absorption and the resolution of effects of spontaneous emission and absorption from the excited state. The presented technique of two-dimensional electron–vibrational Fourier spectroscopy can be applied to studies of the photooxidation process of marker proteins, in particular the EGFP protein. Cross-band two-dimensional spectroscopy could directly reveal the connection between electronic excitation and vibrational relaxation of the molecule, which directly affects the characteristic time of the photooxidation process.