Precise Electronic χ⁽²⁾ Spectra of Molecules Adsorbed at an Interface Measured by Multiplex Sum Frequency Generation

We have developed a new technique to obtain a second-order nonlinear (χ (2) ) electronic spectrum of molecules adsorbed at an interface. This technique is based on multiplex sum frequency generation that utilizes femtosecond near-infrared pulses and white light continuum. We obtained the |χ (2) | 2 spectra of a dye Nile Blue (NB) at the air/water interface with an unprecedented high signal-to-noise ratio and dense wavelength data points. Fitting analyses with an exact χ (2) expression were performed for the spectra, and detailed information about the electronic structure of NB at the interface was obtained.