Molecular Vibrations at a Liquid−Liquid Interface Observed by Fourth-Order Raman Spectroscopy

Phonon mode of TiO2 coupled with the electron transfer from N3 dye

Low frequency vibrational spectra of submonolayer N3 dye (Ru(4,4(')-dicarboxy-2,2(')-bipyridine)2(NCS)2) adsorbed on TiO2 (110) were reported by using fourth-order coherent Raman spectroscopy, which is interface-sensitive vibrational spectroscopy. Most of the peaks observed in the experiment were at the same frequency as that of Raman and infrared spectra of the dye and TiO2. Two interfacial modes at 640 and 100 cm(-1) and one resonantly enhanced phonon at 146 cm(-1) appeared in addition to the pure TiO2 and N3 spectra. Adsorption of N3 dye on TiO2 contributed to the enhancement of 100 and 146 cm(-1) mode. The results not only reported interfacial low-frequency vibrations of TiO2 (110) with N3 dye adsorption but also suggested the coupling between the surface vibrations of TiO2 and charge transfer between N3 dye and TiO2 on the surface.

Fourth-order coherent Raman spectroscopy of liquid-solid interfaces: near-surface phonons of TiO2 (110) in liquids

The fourth-order coherent Raman response of a TiO2 (110) surface covered by HCl aqueous solution, neat octanol, acetic acid, or carbon tetrachloride layers is acquired. Four fourth-order optical responses were identified at 837-826, 452-448, 371-362, and 184-183 cm(-1) and assigned to near-surface phonons of TiO2. A third-order response produced in the bulk liquid layer was superimposed on the fourth-order response, when coherent vibrations are efficiently excited in the layer.