Interface-Specific Vibrational Spectroscopy of Molecules with Visible Lights

Interface-specific observation of molecular vibrations was achieved with visible light by a fourth-order optical process. An aqueous solution of oxazine dye interfaced with air was irradiated with 16-fs pulses at 600 nm. A pump pulse excited Raman-active nuclear motions of the dye. A probe pulse interacted with the excited molecules to generate second harmonic light, the intensity of which was oscillated as a function of the pump−probe delay. Fourier transformation of the oscillation provided a vibrational spectrum of 0−1000 cm-1 for the dye located at the air−solution interface.