Molecular vibrations at a liquid-liquid interface observed by fourth-order Raman spectroscopy.

Interface-selective, Raman-based observation of molecular vibrations is demonstrated at a liquid-liquid interface. An aqueous solution of oxazine 170 dye interfaced with hexadecane is irradiated with pump and probe light pulses of 630-nm wavelengths in 17-fs width. The ultrashort pulses are broadened due to group velocity dispersion when traveling through the hexadecane layer. The dispersion is optically corrected to give an optimized instrumental response. The pump pulse induces a vibrational coherence of the dye via impulsive stimulated Raman scattering. The probe pulse generates second-harmonic light at the interface. The efficiency of the generation is modulated as a function of the pump-probe delay by the coherently excited molecules. Fourier transformation of the modulated efficiency presents the frequency spectrum of the vibrations. Five bands are recognized at 534, 557, 593, 619, and 683 cm(-1). The pump-and-probe process induces a fourth-order optical response that is forbidden in a centrosymmetric media. The contribution of an undesired, cascaded optical process is quantitatively considered and excluded.